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Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium (1958)

Chapter: Session IV - New Techniques Under Development

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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 143
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 144
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 145
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 146
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 147
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 148
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 149
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 150
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 151
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 152
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 153
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 154
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 155
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 156
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 157
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 158
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 159
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 160
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 166
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 170
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 192
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 193
Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Suggested Citation:"Session IV - New Techniques Under Development." National Research Council. 1958. Illumination and Visibility of Radar and Sonar Displays: Proceedings of a Symposium. Washington, DC: The National Academies Press. doi: 10.17226/9554.
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Page 196

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SESSION IV: NEW TECHNIQUES UNDER DEVELOPMENT CHAIRMEN Cal. Charles S. Gersoni MSC, and Col. Charles W. Hill, MS' THE PRESENTATION OF ALPHANUMERIC INFORMATION S. H. Boyd and C. W. Johnson, Stromberg-Carlson Company, San Diego TARGET INTENSITY ON A RADAR INDICATOR AS A FUNCTION OF THE RADAR PARAMETERS J. W. Ogland, Air Arm Division, Westinghouse Electric Corporation BRIGHT DISPLAYS VIA SCAN CONVERSION Thomas S. Wonnell and William E. Miller, Technical Development Center, Civil Aeronautics Administration THE ARMY-NAVY AIRCRAFT INSTRUMENTATION PROGRAM George W. Hoover, Office of Naval Research TRANSPARENT PHOSPHORS FOR CATHODE-RAY TUBES Charles Feldman, U. S. Naval Research Laboratory DIRECT-VIEW STORAGE TUBES George F. Smith, Hughes Research Laboratories BRIGHT-STEADY ELECTROLUMINESCENT DISPLAYS UNDER DEVELOPMENT W. L. Gardner, Lincoln Laboratory, Massachusetts Institute of Technology 141

The Presentation of Alphanumeric Information S. H. BOYD and C. W. JOHNSON, Stromberg-Carlson, San DIego Summary-Presentation o f letters numbers and symbols by means of cathode-ray tubes imposes problems whether characters are generated by raster scan, Lissajous, or shaped beam. In the Charactron shaped-beam tube, the electron beam imposes a char- acter from a matrix on the phosphor screen. Several important parameters interact with the light output from the tube and the number of characters presented. Operation of these factors in relation to room illumination is illustrated by surveillance systems. Not too long ago in surveillance systems, it was sufficient to present raw radar and supplementary data to observers. The systems performed satisfactorily, since the observer had sufficient time to calculate the necessary information for his task manually from the raw data. It is becoming increasingly important to provide the observer with sufficient quantity and quality of information to make decisions more accurately and rapidly. The problem is being continually mag- nified because of the increased speed and number of objects under surveillance. As a result, raw radar data and manual calculation are no longer adequate. Therefore' electronic data-processing equipment and character-generating cathode-ray tubes are now being incorporated. The data-processing equipment converts the raw data into digital form, after which it is presented to the ob- server as letters, numbers, and symbols by the cathode-ray tube. Presentation of alphanumeric data by means of cathode-ray tubes has im- posed new conditions and problems upon the display designer. It is the intent of this paper to discuss methods of character generation and some of the problems involved. Character generation Character-type information on cathode-ray tubes is generally generated by one of three methods: the raster scan' Lissajous, or the shaped beam. In the raster- scan technique, the intensity of the electron beam during the sweep is controlled. The characters are generated in segments in much the same manner employed in facsimile recording. In the Lissajous method, the electron beam is used as one would use a pencil; a como~na~on of intensification and deflection is used to obtain the desired share. In the sha~ed-beam approach' a series of stencil-like ~. ~.~ ~. 1 . 1 1 . 1 . 1 1 ~1_ ~1 ~ _ ~ ~ ~ openings within the tube are used to shape the electron beam into Ine prop~- configuration. The following will be confined to the shaped-beam method of character display. Figure 1 is a sketch of a Charactron shaped-beam tube and a typical matrix format. The tube has a 19-in. diameter screen and is used for visual displays. The operation of the Charactron shaped-beam tube is essentially the same as 142

that of any cathode-ray tube. An electron beam is generated by a conventional, high-resolution, electron gun. This beam is accelerated to the phosphor screen, where its energy is transformed into light. The major difference in the shaped- beam approach is that, instead of a spot striking the screen, a beam having a prescribed cross section is used. Formation of this shaped beam and the need to retain the shape at the screen make this tube different from standard cathode- ray tubes. The heart of the tube is the beam-forming matrix. As indicated in Fig. 1, there are 64 characters arranged in an 8 x 8 format.) The choice of char- acters is essentially unlimited. MU-METAL SH I ELD COI | ~ ADJUSTERS HELICAL ACCELERATOR7 ELEcTRON/ S ~ act _~ ////~/~ ~ CONVERGENCE / / ~ \ COIL / DEFLECTION \ REFERENCE YOKE \ PLATES DISPLAYED\ \\ CHARACTER ~ {DOTTED LINE INDICATES PATH \ OF ELECTRON BEAM) \ Fig. 1. Charactron shaped-beam tube: C1 9K tube and coil assembly; typical matrix format. ., In operation, an electron beam is generated by the electron gun. The size of the beam is adjusted to cover only one of the available characters in the matrix. Voltages applied to the horizontal and vertical selection plates direct the electron beam to the desired location on the matrix. The matrix intercepts this beam in much the same way that a stencil intercepts a light beam. The beam on passing through the aperture, takes on the cross- section of a character. The electron beam is usually deflected off-axis in the process of character selection. Since this would give poor registration at the screen, a convergence coil and set of reference plates are used to return the beam to the tube axis. The deflection yoke is used to deflect the beam to the desired location on the tube face. After deflection, the beam is accelerated to a high energy by means of a helical-type accelerator. The shaped electron beam then strikes the phosphor screen where the Here is converted to light. 7 - - - ~- -- - - _ _ _ _ Only one character is generated at a time. Hence, any character may be positioned at any point on the screen. In most applications a display rate of approximately 10,000 characters per see is used. Depending upon the system HA greater or lesser number of characters may be used. [Iowever, 64 have been found adequate for most applications. 143

parameters, a rate up to 20~000 characters per see is attainable. At higher speeds, component limitations destroy the display usefulness.2 At present, there are two types of Charactron shaped-beam tubes in quan- tity production: the C19K shown in Fig. 1, and the C7C which has a 7-in. di- ameter screen and presents characters 0.035 in. high for use in photographic recording and with projection equipment. In addition, a new tube type, the C19Q, has been developed to serve as a combined display. This tube, which has the capability of presenting high-resolution data in typical radar form ir1 con- junction with characters which can be varied in size, will be used in future air-traffic-control installations. Display design considerations A problem encountered in any CRT display is obtaining sufficient light output. In a character-generating system, the maximum beam-current density is less than that obtainable when conventional radar data are presented because of "space charge" effects. The influence of "space charge" becomes more noticeable ir1 character presentations since identification has been added to the problem of detection. A degree of blooming which is not considered harmful in radar displays may become serious when presenting characters such as a B and an 8. Blooming in character displays may be considered analogous to noise in radar displays. The total light output obtainable when using the shaped-beam principle cannot be compared to other types of displays by merely comparing the ratios of beam-current densities employed. A proper evaluation requires a comparison 2This statement holds true only for magnetically deflected displays. With electostatic deflec- tion, display rates of 50,000 characters per see or more are possible. This method is not practical in large screen displays at present because of deflection distortions. _ n) C;Pf1T WRITING >< - - - - - - \~ _b) SHAPED BEAM WRITING - I Tl ME Fig. 2. Comparative light output curves for spot writing and shaped-beam writing of an elemental area. Only one pulse is shown. - 144_

of coulombs per unit area per unit time. This can best be recognized by reference to Fig. 2, which gives curares of light output as function of time for an elemental area. In radar or other types of displays employing a sweep, the time a given point on the screen is excited depends upon the sweep speed and spot size This time is generally of the order of 0.01 to 1 parsec. Hence, curve a represents the light output for spot writing. In the shapecl-beam display, a given elemental area is generally excited for 10 to 100 Alec, yielding a curve of the type shown in b. Total light output is obtained from the area under the curve. Therefore, the excitation time becomes an important parameter when analyzing a system em- ploying cathode-ray tubes. A second important condition that must be considered in designing character-generating displays is the number of characters to be presented. Let us suppose that the generation of a character requires, on the average, 100 ,usec. The consequent presentation rate of 10,000 characters per sec is a fundamental restriction. As the quantity of information increases the number of times per second that it can be presented decreases. If it is desired to present 1,000 char- acters, then the maximum frame rate is 10 per sec. If 2,000 characters are to be presented, then the maximum frame rate is 5 per sec. In addition, if several different displays are being generated from the same source (for instance, the storage drum of a computer), and if it is desired to display different information serially on different consoles then the frame rate is determined by the total data to be presented and not merely by the data which happen to be utilized. This relation can be expressed as 1 Fn~a.r ~ X to where Flax is the maximum frame rate in see -1, to is the average time in see necessary to present a character, and N is the total number of characters being considered. This fundamental relation influences many of the parameters in- volved in the display, such as integrated light output from a given character, flicker, and legibility, as well as the choice of phosphor and ambient illumination. In most character-generating displays only a fraction of to is used for actual character generation. The remainder of to allows the components to stabilize. The actual print time per character varies from 10 to 100 ,usec, depending upon various factors such as the component stabilization period and the output speed of the data-processing equipment. Systems applications The Charactron shaped-beam tube has been used in several applications having different operating parameters. In addition, systems being developed introduce new conditions which must be considered in system design. As examples of different applications, a Navy surveillance system and the SAGE displays will be compared. The display portion of the Navy surveillance system is cur- rently being manufactured at Stromberg-Carlson, and many of the problems concerning its ambient lighting are still unsolved. In the Navy surveillance 145

- D lo. . I ~ >.ol tool \ \ a! HI 1 ~ AMBIENT ILLUMINATION 0 3 6 10 IS 20 2S 3S 4$ S5 65 75 am_ ~I~3 Cam) _ w ~ 0.1 HI ! . I ~ _~ .~_ ooo S - ~ {ma) _ . * ~ooo ! : I 30~ ~ 2~0 _ m 10= / w ... . ~ / / HI a. / ~ / ~^~aiENT lLLu~'NATION ~ _ ~ 0 ~ 6 uNGLANK[NG :~PLlTuOE (VOLTS) LOW SCREEN CURRENT HIGH SCREEN CURRENT ! lava / 1 _... O. ~5 (ma) _ h fig. 3. Measurements taken on the screen of a C1 9K tube. 146

system, it is required to display as many as 1356 characters. This requirement, coupled with the output speed of the computer storage drum, results in a frame rate of 15 per see, or a total available time per character of approximately 50 ,usec. However, the actual printing time per character is 10 Basic, giving a total of 150 user per see per character. Figure 3 shows experimental data taken on a P14 phosphor screen of a C19K tube. The data were recorded under operating conditions comparable to those which will be employed with the Navy surveillance display. Included in the figure are curves of mean character brightness character-background con ~ . . . . .. ... . ~ ~ . ~ ~ , trast, apparent flicker, and 1eglblllty plotted as function of screen current yu~l- blanking amplitude). Since it is generally desirable to maximize light output without destroying character legibility, the tube is operated just below the blooming point. Analysis of the figure shows that at this point. the pulsating brightness flicker is high, ~ · ~ · ~ - · ~ ~ · ~ ~ ~ · 1 . . . 1 1 ~ ~ _ _ ~ ~ ~ the apparent legibility is only fair, and the brightness contrast has been reclucea. It would appear that the overall character presentation could be improved by reducing the mean character brightness. It can be seen that a reduction in mean brightness can reduce the apparent flicker considerably and thereby increase apparent legibility. This can be accomplished by reducing the screen current, using a filter over the screen to remove a portion of the character brightness' or 0 1 ~ 1 · · ~ ~ ~1 1 · · ~ · ~ . · 1 . ~ 1 ~ ~ 1_ _ _ t _ ~ by a combination ot the two. lne choice is lalrly stralgntIorwara 1I Ine OL)~-V~1 is located in an isolated room and if his only task is that of viewing the character presentation. In this case, the screen current can be reduced to a point at which .1 1 . 1 1 1 1 · 1 . tne cnarac~er-oac~rouncr Mess is high, the flicker is just noticeable, and O O lo, · · the legibility is good. Unfortunately, this situation rarely presents itself. Usually, a compromise must be reached such that adequate illumination is available for the observer to carry out a secondary task without destroying character legibility. To achieve the need for adequate illumination, two techniques appear to be feasible: selective spectrum and polarization. Each technique employs a filter over the display screen to reduce the mean character brightness to a level of just noticeable flicker, high background contrast, and good legibility. Appropriate lighting sources, when coupled with these filters, apparently provide satisfactory levels of illumination in the working area, but do not wash out the character- background brightness. This simplified analysis cannot be expected to solve all the problems associ- ated with the design of the Navy surveillance (or any other) lighting nor does it choose between those schemes that appear to be feasible. However, this type of analysis, as a minimum, should be used before the display parameters, such as frame rate and print time' have been fixed, in order that satisfactory illumi- nation can be provided. In contrast to this example, let us consider the SAGE display. Because of the quantity of data to be presented, the frame rate is limited to a 2~/2-sec cycle. The actual printing time per character is 25 ,usec. Under these conditions, an on-off type flicker is present, and the total light output is low. Because of the 147

operating conditions imposed on the tube, a low-level ambient lighting is required. These problems, coupled with others described in a previous paper, have appar- ently limited the choice to a selective spectrum lighting system, the broad-band blue. In conclusion, character displays are playing a more important part in surveillance situations, and the outlook for the future is that the applications will continue to grow. Because of this growth it is important that systems designers become familiar with the variables involved in this type of display. Only in this manner can they hope to improve the position of the observer who, in the end, is faced with making accurate decisions and making them quickly it is this task that determines the worth of any display system. -148

Target Intensity on a Raclar Inclicator as a Function of the Raclar Parameters J. W. OGLAND, Westinghouse Electric Corporation Summary With a fighter aircraft radar, detection is determined by the intensity impulse. For a typical radar CRT, maximum brightness is reached long before the grid voltage reaches zero, and at about ]/4 maximum beam current. Brightness saturation zs not serious for target ctetectzon because the intensity continues to increase uniformly up to zero grid voltage. When the numbers of overlapping excitations is increased, the intensity also is in- creased, but not proportionately. A maximum value is reached asymptoticaZZy, and more sZowZy the longer is the persistence of the phosphor. Although the intensity as a function of the num- ber of excitations saturates, the intensity impulse increases Zine- arZy. The spot intensity increases with increasing prf. It is essen- tiaZZy a negative exponential relationship to the interval between pulses. The intensity increases with increasing puZse-width, but less than proportionately. The intensity increases essentiaZZy ZinearZy with with screen voltage. The spot brightness increases nearly pro- portionateZy to the square of the screen voltage. In the direct-view storage tube, the brightness or intensity attozned zs not higher than in a conventional CRT, but the persistence, which is con- troZZed eZectricaZZy, is much longer. The intensity-time integral therefore is much larger, which gives the impression of much higher brightness, target detectability is better. Transparent phos- phors reduce the background brightness and improve the contrast with a given signal strength. Presently avaiZabZe transparent phosphors are efficient for low beam currents. If research in progress leads to good efficiency also at high beam currents, good contrast and detectability wiZZ be attained. At high altitudes, the radar indicator of fighter airplanes is subject to in- tense environmental illumination which tends to wash out the radar information presented on the screen. To attain good detectability, the radar designer should choose the electrical parameters in such a way that the target spots have the best light output possible. Very little information is available on the behavior of the cathode-ray tube pertaining to this situation. To provide some data which may assist the designer in making the best choice of parameters, some measurements have been made of the CRT under simulated radar operating conditions. Vision scientists have already provided data by means of which the visual requirements for detection of a small bright spot on a background of varying brightness can be predicted. These investigations were made by varying the spo brightness' sizes duration and the background brightness. and registering the ,_, , ~ ~ 7 ~ response of a crew of observers. . ~, ~- -~ ~ ~ --- - ~ At, -- For investigating the performance of radar indicators, the same technique may be employed by varying the electrical parameters. It appears, however, that -149-

with the above information on human vision available, this method is unneces- sarily time consuming for a study of this kind. Furthermore, since the radar indicator tube does not have an instantaneous respone and is nonlinear, valu- able information on its behavior will be lost if this technique is used. For these reasons, CRT operated under simulated radar conditions has been investigated by recording the output in photometric units when varying the parameters. These investigations will, thus, utilize existing knowledge on human . . vision. Human requirements for target detection Before discussing some of the measurements made on the CRT, it appears desirable to draw attention to some requirements for detection. Figure 1 gives Am m a He o Cut C) - a to .01 = :: ~ 10 100 1000 1 09000 BACKGROUND BRIGHTNESS (FT-L) Fig. 1. Contrast required for detection as a function of background brightness, for various spot sizes. The spot sizes are given in mm diameter at a viewing dis- tance of 20 in. Exposure 30 msec {Graham and Bartlettl). the results of an investigation by Graham and Bartlett. Similar investigations have been made by Blackwell.~ The individual curves for different sizes of spot show the contrast ratio needed for detection as a function of background bright- ness. The larger spots need less contrast ratio than the smaller. Since the intensity of a bright spot is equal to the product of its area and brightness (as long as spot radius is small compared with the viewing distance), Fig. 1 may be replotted showing spot intensity required for detection as a function of background bright- ness. This is shown in Fig. 2. The ordinate is called incremental intensity because it is the product of spot area and ~ B. the difference between spot brightness and background brightness. This is justified since any brightness ~ B produced by the CRT electron beam will add to, or float on top of, and background brightness produced. by ambient light. At high brightness, the curves for medium-sized spots overlap. These are the spots that occur in a radar presentation' where de- tection is determined by the spot intensity rather than the brightness alone. The brightness may be reduced if the spot area is increased in proportion, or vice -150

loo 10 L J Cl z C' - __ Z .10 zl~ clot 10 100 1000 BACKGROUND BRIGHTNESS (FT-L) 10,000 Fig. 2. Incremental spot intensity required for detection as a function of back- ground brightness. ~ Derived from Fig.1.) versa. For low' background brightness, these curves do not overlap' but this is without importance since our problem of detection is at high brightness. The measurements of Graham and Bartlett were made with a duration of the bright spots of 30 msec. For shorter durations a higher contrast is needed for detection. It is known that up to about 0.1 sec intensity and duration are reciprocal, i.e., the product is constant.3 The vertical scale on Fig. 2 could therefore be multiplied by 30 and expressed ire candle-milliseconds. This product may be called intensity impulse. The significance of this term will be seen below. iGraham, C. H. and Bartlett, N. R. The relation of size of stimulus and intensity in th human eye: III. The influence of area on foveal intensity discrimination. J. Exp. Psychol., 1940 27, 149-1 59. 2Blackwell, H. R. Contrast thresholds of the human eye. J. Opt. Soc. Amer., 1946, 36, 624-643. 3Stevens, S. S. (Ed.) Handbook on Experimental Psychology, Wiley, 1951, 964. 151

Photometric measurements The photometric measurements were made with a 6-in. electrostatic cathode- ray tube, type 6DP, and a Spectra photometer' type UB1/2, covering an angle of i/2 a. The tube was operated essentially as in a radar B-scope, but all parameters could be varied and measured. These included d-c operating voltages, grid pulse amplitude and width' prf, and number of hits per scan. The beam could either move as or1 a B-scope or be made stationary, in which case the excitation of the target spot was controlled by an electronic counter. When the grid drive is varied, the brightness follows the curve shown in Fig. 3, which is a nearly straight line for low grid drive values. Since the plot 1000 - z ~ 300 ID > A: Cal - ~r - ~ 100 A: o 30 / / / / 7 , / BEAM CURRENT / - 1 00 l, ~ -80 -60 -40 -20 0 OR ID VOLTAGE AT PULSE TOPS Fig. 3. Spot brightness in ft-l, and peak beam current in pa for one CRT, type 6DP7, as a function of grid voltage at pulse tops. Between pulses, the beam is cut off. Sixteen square pulses at a prf of 1200 excite a stationary spot each second. Screen voltage 12 kv. The brightness shown is the integrated value during the period of 16 excitations. An eye response filter is included in the photometer. 152-

is or semilog scale, this means that the brightness increases nearly exponentially with grid drive in the beginning. Long before zero grid voltage is reached how- ever, the brightness reaches an asymptote. This is not some kind of an absolute ceiling of brightness set by the phosphor. With d-c voltages on the grids much higher brightness values have been measured. This plot was taken with 16 one-,usec pulses occurring at a prf of 1200 once per second. The grid values given are the voltages during the pulse. Between pulses, the beam is cut off. The brightness asymptote is not due to beam-current saturation. This is seen from the lower curve, which is nearly a straight line up to zero grid, i.e., nearly an exponential function. If the spot intensity is plotted' the curve shown in Fig. 4 is obtained. No saturation occurs, as exhibited by the brightness curve. The reason for this differ- ence lies in the spot size. When the grid is driven harder, the spot size increases as seen on the lower curve in Fig. 4. If the spot brightness were? the only or 10 an 3 J C1 Ad IS J 1 -100 -80 -60 -40 -20 0 GRID VOLTAGE AT PULSE TOPS 1 Fig. 4. Spot intensity in millicandes, and spot diameter in mm as ~ function of grid voltage at pulse tops. Same CRT and operating conditions as in Fig. 3. 153

main determining factor for detection, the asymptote found in the brightness curve would be a great disadvantage. Since detectability is determined by spot intensity, the occurrence of the asymptote seems to be without importance for detectability because the intensity curve does not show this effect. If spot defi- nition were a major consideration, the increase in spot size naturally would be a disadvantage, but this is not the case in this type of equipment. ~. . . The curves of Figs. 3 and 4 were obtained with a P7 phosphor. They would look the same for a P2 phosphor except that values would be higher. If the num- ber of pulses in each sequence is increased, higher brightness and intensity values are obtained. The increase is, however, not porportional to the number of pulses, or hits per scan. The number of hits per scan is increased if the sweep speed of the antenna is reduced or the antenna beam width is increased. The brightness of the long persistence P7 phosphor builds up more slowly than that of the P2 phosphor, and the pulsations are smaller. A difference of this kind might be expected due to the difference in persistence. With a long-persistence phosphor, the buildup will continue for a longer time until equilibrium is reached between electron energy fed in and light energy emitted. The final brightness level reached is higher for P2 than for P7. The final brightness reached for a given number of hits for the radar system is, however, not the determining factor for detection. As seen above, detectability of a target on a given background brightness is determined by its intensity im- pulse. The intensity impulse is given by the area under the curve up to the pulse number chosen. Since the intensity attains a higher value for P2 and rises faster, P2 will give a higher intensity impulse than P7, and should therefore give better target detection in this type of radar. The matter of buildup has another implication due to the inherent noise voltages in all electronic equipments. The weakest targets set the limitations to the range of the radar system. These weak targets are more or less mixed with noise. Noise occurs with a random distribution, whereas the targets are repeti- tive. It might, therefore' be expected that a slow intensity buildup would im- prove the visual signal-to-noise ratio. The intensity impulse, however does not show the same relationship to number of pulses. A further study of this aspect IS necessary before making a choice. In other words, the visual signal-to-noise ratio should be compared with the electrical signal-to-noise ratio as affected by the phosphor buildup function. A discussion of the noise problem is outside the scope of this paper. It shall only be mentioned that in the systems discussed the ad ^^ ~ ~ . , verse ettect ot the noise Does not seem to be due to its increasing the background brightness. Because of the grainy structure of the noise as it appears on the screen, it is rather a matter of making distinctions difficult. Any means that will make distinctions easier will improve the system. Instead of varying the number of hits, prf may be varied as shown in Fig. 5. Above a certain prf' brightness and intensity appear to increase proportional to the logarithm of prf. -154-

2200 2000 1800 In ~ 1600 m 1400 J o 1200- o 11 t', 1000 CO He I a, 400 200 / / i . -22 - 20 - 1 8 1 ~/ 1 ~, 7 ~ BRIG HTN ESS/ / _ /INTENSITY 7~ -6 O LL 16 c, An ~5 -14 ~ J J - - -10 >~ z - 8 LLJ Z 4 2 00 1000 2000 4000 200 400 PRF Fig. 5. Spot brightness and intensity as a function of prf. Pulse width = 1 parsec, 16 pulses per sequence, one sequence per sec. Grid voltage at pulse top =-20 v. In Fig. 6 the intensity is plotted against the interval between pulses' or the reciprocal of the prf. The logarithm of the intensity is also shown. It is seen that there are two regions in which the logarithm of the intensity is linear, i.e., the intensity follows first one negative exponential curve with respect to the pulse interval and then goes over into another negative exponential. The measurements of buildup as well as relationship to prf, would be more useful if the intensity impulse had been plotted. Equipment for such measure- ments has very recently been built. Preliminary measurements show that the intensity impulse increases linearly with hits per scan, even though the intensity buildup goes into saturation or equilibrium. When increasing the number of hits, one does not experience any region of diminishing return. If the pulse width is varied and the other parameters kept constant, bright- ness and intensity will not increase proportionately. The phosphor efficiency appears to decrease with increased pulse width. This may be seen in Fig. 7, where phosphor efficiency expressed as intensity per unit beam current is plotted as a function of average beam current for three different pulse widths and for dc, and for the two phosphors, P2 and P7. If the screen voltage is increased, the brightness will increase essentially as the square of the voltage. The intensity, however, increases only linearly, except for the low-voltage region. This change in relationship to voltage is presumably because the spot decreases with higher voltage. Higher screen voltage naturally 155

requires higher output for the deflection drive circuits' which may not be easily obtainable. Other means for enhancing detectability Since the required spot intensity is determined by the background brightness, detectability may be improved either by increasing the spot intensity by the best 4000 2000 1000 500 8~ l 1 16 14 12 U, o ,~ 10 E - , e of z 6 _ _~ _ ~N 0.46t \ 0.035/e W\; INTENSITY- _ 4- _ O P R F 400 300 250 1 1 200 _ -1.3 ~11, _ ,0. . 2 3 INTERVAL BETWEEN PULSES (milliseconds) 2 ~ 4 s .2 .0 0.9 -0.8 z Z . -0.7 o . -0.6 -0.4 Fig. 6. Spot intensity as ~ function of interval between pulses t 1 /prf ) . Same operating conditions as Fig. 5. 3 0()3 0.1 0.3 AVERAGE BEAM CURRENT (bra) Fig. 7. Efficiency of P2 and P7 phosphors in relation to pulse width, expressed as intensity per unit beam current with prf of 1200. -156 2~ 1.0

choice of electrical parameters or by reducing the background brightness. The latter may be done either by means of restricting outside illumination of the screens such as by a hood, or by absorbing the incident light' such as by using a transparent phosphor followed by a black surface. The transparent phosphor looks very promising when excited by a d-c beam. It is efficient at low beam current. In this type of radar application, however, very high instantaneous beam current must be used because of the small duty cycle. Due to its intimate con- tact with the glass or substrate, the transparent phosphor has high resistance to burning. It is not yet known whether transparent phosphors can be made efficient at high current densities; they are still in the experimental stage. Further re- search is needed and is in progress. If the efficiency demonstrated at low current densities can be obtained at high current density, high target intensities will be produced on greatly reduced background brightness. The contrast will be greatly increased and the target detectability improved. The direct-view storage tube should also be mentioned. This has a very desirable feature in its long and controllable persistence. High intensity is ob- tained, but not higher than the instantaneous intensity in a normal CRT. Con- sequently, the tube gives a very high intensity impulse. Also, a more-or-less con- tinuous picture is retained on the screen. A nearly linear intensity buildup can be provided to enhance visual signal-to-noise ratio. A drawback is that the tube is more complicated and more expensive than a conventional CRT. Comments following Mr. Ogland's paper: Francis: Several times this morning I heard mention of photometric meas- urements on the cathode-ray tubes. We and others have been trying for some time to make good field measurements on scopes. It sounds to me as if you have come the closest of any of us. I wonder if you could give us a little idea of the equipment you've been using. Ogled: The equipment we have been using is the Spectra Spot Photomete made by Photo Research Corporation of Burbank, California. This instrument comes in two models, one with 1~/2° visual angle and the other faith it. We have chosen the latter, which is model UBi/2. With an auxiliary close-up lens the reticle covers a spot of less than i/2-mm diameter. This was used for the brightness measurements. The intensity measurements were made by focusing on the target spot with the photometer positioned at a greater distance, so that the reticle covers an area larger than the spot. The reading gives the average brightness of this area. Knowing the size of the area it is easy to convert the ft-l reading to give the spot intensity expressed in candles. Francis: We've been using the same gadget, but some of my electronic friends tell me that it's not fast enough. Have you modified it at all to get a faster response? Ogland: The instruments is actually designed for measuring constant brightness, which is read on a moving coil meter calibrated in foot-lamberts. When measuring brightness pulses on a CRT screen simulating radar targets, 157

we therefore hook an oscilloscope across the moving coil meter and take photo- graphs of the produced deflection. The scope deflection is calibrated against the meter reading or by means of a 100 ft-l brightness standard. The instrument in- corporates a d-c amplifier which falls off at fairly low frequencies, especially for the high sensitivity ranges. We have made a rough check of the instrument with light pulses produced by a small window in a rotating disk and also by an approx- imately sinusoidally varying light source. The instrument in combination with the scope was found satisfactory except at the highest sensitivity range. It should be kept in mind that with these measurements, simulating a radar target' we were not interested in the response to the individual excitations or grid pulses, but rather in the average brightness attained over the pulse train consisting of sev- eral pulses spaced according to the given prf. The measurements were made for detectability studies, and the observer's eye will not respond to brightness varia- tions between the individual pulses in the train. An idea of the response of the equipment can be obtained by comparing the build-up curves for the two phos- phors P2 and PI shown in Fig. 5. For other investigations, recording of the brightness produced by the in- dividual excitations is of interest. For that purpose a photometer was made con- sisting of the same optics as the Spectra and a photomultiplier tube lP21 followed by a video amplifier with a bandwidth of 8 me. The output is presented on an oscilloscope and photographed as before. The output can also be fed to an inte- grator and the integrated light output, or intensity impulse, can be measured for different operating conditions and phosphors. 158

Brig hi Displays via Scan Conversion THOMAS S. WONNELL and WILLIAM E. MILLER CAA Technical Development Center Summary Increasing use of radar arid the need for retention of information by air-traffic-control centers have resulted in studies of projection storage tubes, rapid-processing film equipment, and scan conversion. A radar PPI presentation may be changed into a television picture by scan conversion. The CAA Technical De- velopment Center (TRC) has evaluated two equipments utilizing scan-conversion tubes; results were favorable for one. After oper- ational evaluation of the reliable equipment, which has good resolution and excellent storage, a number of additional features will be considered for optimal utilization of the scan-converted picture. Radar has been an integral part of the CAA air-traffic-control system for a number of years. Thus far, the primary utilization of radar has been to expedite air traffic flow within many terminal areas. At present, the CAA is procuring and installing long-range radars to augment the en-route air-traffic-control system. With more and more emphasis being placed on radar in present-day air-traffic control, one of the items of vital concern is the development of an improved air-traffic-control radar display. For traffic control, information other than the aircraft range and azimuth data provided by a radar indicator is required. Dependent upon the type of con- trol, such additional information includes aircraft identity, altitude, route, and destination. A further requirement is that the desired radar display system in- corporates some means of coordinating all of this information with specific radar targets. The posting of this information now and for the immediate future is accom- plished by writing in longhand and is subject to repetitive reading. The coordina- tion of written materials with radar targets requires a large radar presentation in an environment of high ambient light. As in all radar usage, the control of air traffic can utilize target storage for the obvious advantages to be gained by storage, such as differentiation between moving and stationary targets, heading, and a coarse speed indication. With these requirements, the basic specifications for a desired radar display system for immediate use become a large, bright presentation of radar information and adequate target storage control. In the Fall of 1956, TDC conducted a detailed study of the "state of the art" to provide for an immediate need for a large, bright radar display for use in air-traffic-control centers. As a result of. this study, the three most promising techniques of producing such a display were determined to be: projection storage tubes, rapid processing film equipment, and scan con- version. The purpose of this paper is to discuss, in some detail, the theory of scan conversion and the results of the evaluation of two different types of scan-con . version equipments. 159-

Scan conversion is a technique whereby a radar PPI presentation is con- verted to a television picture. This conversion process provides a radar signal in the form wherein the many developments of the vast television industry can be utilized to advantage for air-traffic control. Some of these advantages include availability of highly engineered' reliable equipment, priced in a competitive market. Such equipment includes monitors, mixers, video switchers, distribution equipment, remoting equipment, etc., all of which can be used as building blocks for attaining maximum efficiency in meeting the specific requirements of in- dividual control centers. In the transmission of picture information it is frequently necessary to change from one type of scanning to another, such as radial scanning (PPI) to rectilinear scanning (raster), or to change the frame frequency of transmitted electrical information, as in coordinating one television system with another. For these purposes the scan conversion or frequency converter type of storage tube is very useful. Considerable developmental work has already been accomplished on several of these devices. Two equipments have been evaluated at TDC which employ scan-conversion storage tubes. The first of these equipments was the SRD-1 bright display, manu- factured by DuMont under a TDC development contract, which used the RCA Graphechon scan conversion storage tube. This tube, represented by Fig. 1, em- ploys a writing gun mounted at one end of a tube, a reading gun mounted at the opposite end, a thin insulating target between them, and a collector cylinder surrounding the target. The target consists of a sheet of thin insulating material, such as magnesium fluoride, approximately 0.5-,u thick, evaporated onto one side of a thin aluminum foil which is supported on a fine metallic mesh having high electron transmission. The writing gun, facing the aluminum-covered side of the target, is operated with a high negative potential so that electrons from it have sufficient energy to pass through the foil and cause induced conductivity in the insulator. The input signal is applied to the control grid of the writing gun. The output signal is obtained from the backplate as a voltage variation across the GRI D NO. 2 GRID NO. I CATHODE / I HEATER l ,: I _ Jew ii Ii Il ~ BEAM iota ' DEfLECTING COILS VIDEO SIGNAL TUNED SIGNAL SOUR CE TO RAD 10 FREO UE N CY OUTPU T . T OUT WRITING SECTION TARGET READING SECTION I ANTI- SHADING ELECTRODES CONDUCTING INSULATING SIDE ~ SIDE GRID NO. 4 GRID NO. 4 ~ ~ T ~[GRID NO. 3 GRID NO. 31 Ixxx~ocxx~; . . _ , 1 / 1 . r 7 ~ e ~ SECONDARIES - -~- , ~ ~ ~ . . ~ SCA N N I N G E LOON PENETRATING ELECTRONS ARID NO. 2 I GRID NO. I I ~ CATHODE OR SCANNING ELECTRON I ll t ~ BEAM-I~RF PULSES , '' It I I 'A :`SECONDARIES All 1 l he DEFLECTING COlL5 1 | LO' D | | ISOLATING | IMPEDANCE IMPEDANCE l l _ ,_ _ ~ NEGATIVE POSITIVE VOLTAGE VOLTAGE Fig. 1. I mproved type of Gra phechon. 160 RADI O FREQUENCY _ OSCI LLATOR _ TO GRID NO. I

output resistor. Simultaneous van rising and reading can be accomplished by means of the two guns. In the writing function, as a result of the scanning process during previous readings, the insulating surface of the target is assumed to be approximately at collector potential. Since the aluminum foil on the opposite side of the target is always maintained at about --50 v, a potential difference of 50 or is established between the front surface of the insulating target and the back. Writing is accom- plished by scanning the target with the writing beam, whose current is modu- lated by the input signal. Since the cathode potential of the writing gun is suffi- ciently negative (10,000 v), primary electrons from the writing beam penetrate the aluminum foil and the insulation layer. The resulting bombardment-induced conductivity in the insulating target elements lowers the potential of their front surfaces by varying degrees to that of the 50-v aluminum backplate. The front surface of the insulating target will thus acquire a pattern of potential variations between O and 50 v. The reading function is accomplished by scanning the target surface with an unmodulated reading beam. Since the cathode of the reading gun is operated at approximately 1000 v, there is little effect from bombardment-induced con- ductivity, and all the target elements shift their potentials, acquired during writing, toward that of the collector by secondary emission. As the elements are sequentially charged positive during this process, a corresponding capacity cur- rent to the backplate arises producing a signal voltage across the output resistor. By adjusting the current in the reading beam, the reading time can be varied from a few seconds to about a minute. The output polarity of the Graphechon is positive in the sense that a more positive input signal voltage causes a more posi- tive voltage in the output from the corresponding storage element. In this mode of operation, poor or no halftones are observed in the output. When the Graphechon is employed for simultaneous writing and reading, it is necessary to prevent the writing-beam current modulations from generating a signal directly in the output. This is accomplished by intensity-modulating the reading beam at a frequency of 30 me, well above the maximum frequency con- tained in the input writing signal. As a result, the desired output will be an amplitude-modulated 30-mc signal, which can be separated from the lower- frequency components produced by the writing beam and rectified to produce the desired reading signal. The evaluation test results of SRD-1 bright-display equipment as a means of producing the desired radar display were negative. Characteristics of the Graphechon scan conversion storage tube as operated in the SOD-! equipment may be listed as follows: (1) adequate target storage was variable from 1 to a maximum of 30 see, depending upon adjustment of circuit parameters; (2) the fundamental tube design and mode of operation of the Graphechon prevented the rendition of half-tones in the target trail; (3) the requirement of a 30-mc radio-frequency signal for simultaneous reading and writing introduced a noise problem which had a deteriorating effect on the radar picture; and (4) maxim 161

mum system resolution was approximately 450 lines. In summary, the Graphe- chon scan-converted radar picture was not as good in definition and in ability to "read" weak targets as a standard PI CRT indicator. The Technical Development Center had practically abandoned the scan- conversion approach to the desired air-traffic-control radar display when the ~,, A 1 J French 11-44u scan conversion equipment appeared upon the scene. The Inter- continental Electronics Corporation of Mineola, L. I., New York, proposed that TDC evaluate a product, for which they are the U. S. outlet, manufactured by Compagnie General de Telegraphic sans Fil (CSF) of Paris, France. Preliminary evaluation of the TI-440 revealed startling performance characteristics, which resulted in prompt purchase of the equipment and subsequent detailed evaluation testing. The heart of the TI-440 equipment is a scan convertor tube (type TMA 403X). This tube employs a writing gun mounted at one end of a tube, a reading gun mounted at the opposite ends a thin insulating target between the two guns' and a collector cylinder surrounding the target. Although the construction of the tube is similar to that of the Graphechon, the physical size of the tube is larger. Figure 2 represents the French tube. The writing beam is magnetically deflected and electrostatically focused, while the reading beam uses electrostatic deflection and focus. The target, collector, and correction ring are quite similar to those of the Graphechon with respect to position and function. The target size is approxi- mately 2~/4 in. in diameter. COLLECTOR CORRECTION RING VOLTAGE CONTROL VOLTAGE CONTROL 1 .- . SCREEN G2 (-1200 V) CATHO D E ( - 15 0 0 V ) | O UTPUT TARGET OUTPUT 4 ~--FO 5 G ODE DEFLECTION PLATES GUN = ( I 5 00 V ELUDED . _ _, _. _ .. . ISOLATION ~ p 1 FILAMENT-CATHODE CONTROL GRID G. / CONNECTION / INPUT SIGNA L AND D-C SIAS Fl LAMENT-CATHODE CONNECTION L~1 8¢ 6.3 V ~V TARGET DEFLECTION YOKE / CC RRECTION RING 2.05 MINIMUM COLLECTOR DIAMETER Fig. 2. Scan-conversion storage tube (TMA 403X). I ISOLATION) CATHODE (-10 KV MAXIMUM) Evaluation tests revealed the TI-440 to be a reliable, well-packaged equip- ment with good resolution and excellent storage control characteristics. System horizontal resolution was measured in excess of 600 lines. Storage was adjustable from O.l see to 20 min plus. The first two TMA 403X tubes were noted to have considerably different storage characteristics. Information was received that varying storage characteristics were a function of the composition of the target insulation material and that tubes could be manufactured to comply with specific storage specifications. Subsequent to the time that storage specifications were pro- vided, some six tubes have been purchased and checked for storage characteris- tics. Each of these tubes was tested for compliance with the storage specifica - 162

lions and determined to be satisfactory. The manufacturer's control of tube characteristics have thus far been very reliable. The TI-440 equipment is capable of halftone rendition. Evaluation tests show three to four steps of grey in written information. During image decay, the target trail exhibits all shades of grey. The greatly improved operational char- acteristics of the TI-440 over the SRD-1 appear to be the result of a more ad- vantageous mode of operation and application of a conversion storage tube. The signal out from the tube is derived from both the target back plate and the col- lector. These two signals' which are 180° out of phase, are applied to the grid and cathode in a preamplifier stage' resulting in a greater output signal level than is available from the Graphechon. At the same time, the French have incorporated circuitry which provides for the cancellation of cross-talk signals. The undesired writing signal in the tube output appears on the target backplate and collector in phase. A cathode-follower stage and balancing control, inserted in the target signal line, provides for complete cross-talk elimination in the preamplifier without the use of the 30-mc cross-talk separation system of the SRD-1. The TI-440 equipment is at present being purchased by CAA for operational in service evaluation at 13 centers for air traffic control. Some of these installa- tions will be made in towers and others at en-route centers. The equipment, as manufactured in France, will be used for these tests without modification. A number of features available today in radar indicators should be incorpo- rated in the TI-440 scan conversion equipment to tailor the system to the specific needs of CAA air traffic control. These features are mainly concerned with required changes in the writing circuitry to provide for off-centering and time- sharing. Such features are well within the state of the art and should become items of specifications when and if the CAA engages in large-scale procurement of scan conversion equipment. The display of a scan-converted radar picture is relatively simple. As long as the TV scan is standard, off-the-shelf TV monitors of sufficient quality may be used. The brightness of the display is governed by the same conditions that control the brightness of the home TV receiver. A more useful presentation is required for the en-route air traffic controller than a simple scan-converted radar picture on a 17- or 21-inch TV monitor. Methods of using the scan-converted picture along with aircraft identifying markers have been explored. An early approach was to project the picture from overhead on to a horizontal plotting surface on which a map of the area was drawn. So-called "shrimp-boats," or markers containing essential control infor- mation for given aircraft, were moved along by hand in such manner that the radar return of the aircraft fell on its corresponding marker. This method of dis- play appears to have some merit for use in en-route traffic control; however, the available TV projectors lacked the brightness required for operating under suf- ficient ambient light. Enough ambient light is required to permit controllers to read penciled notations on the shrimp boats. This particular technique, known as the panoramic operational radar display, or Panop display, is receiving further 163

evaluation with a Kelvin-Hughes, rapid-process-film, radar-projection system. Should the technique prove operationally desirable, it is proposed to have suitable TV projection equipment developed for use with the scan-converted picture. The Spanrad display' or superimposed panoramic radar display, evolved from efforts to utilize the scan-converted picture in a plotting technique with "shrimp-boat" markers under high ambient light levels. A 27-in. TNT monitor is mounted alongside a plotting table. The plotting table is viewed by a vidicon camera, synchronized to the scan-conversion system. Thus, the information on the Blotting table is electronically superimposed on the scan-converted radar · 1 . . ~ _ . . my. . ~ ~ . . ~ picture on the ~/-~n. monitor. 1 he m~xect camera anct scan-conver~ea picture Clay be transmitted to any point where it is needed. This type of en-route radar dis- play has been used exclusively in TDC simulation for the past year. Tests have been conducted to determine the value of presenting the different v~cleo inputs to Spanrad display in color. For one test, the radar information was presented in green, plotting surface information in red, and video mapping in blue. One advantage derived by the color display is the creation of greater contrast between the shrimp boat marker and the radar target trail. The present limitation on the Spanrad system is the horizontal and vertical resolution, which permits reading of characters no smaller than i/2 in. on a 40-in. diameter plotting surface. The TV standard in use in this program is 625 lines, interlace 2:1, with a 30-c frame rate. The t/2-in. characters are too large to permit much information to be carried on the marker. The marker size is restricted to approximately 13/4 in. by 3/4 in. to permit use in a normal density area without crowding. The resolution of the system however, does not limit the use of the Spanrad display when it is used as the point of control. For this technique, it is not necessary for the camera to read information on the markers. It must merely superimpose the outline of the marker on the radar blips. With a map or the plotting surface, the controller is able to make control decisions form the position of the markers, presuming, of course, that they have been positioned accurately with respect to the radar returns in the monitor. Another method of display receiving consideration is the use of a 22-in. flat-face Kinescope in a horizontal plotting console. With this display' small clear plastic markers are used. Notation is made on the marker with a grease pencil. Of the methods of utilizing the scan-converted picture, the most attractive to the engineer is the Spanrad configuration. To make it more useful to the con- troller, we feel that it is necessary to go to a higher-resolution system, and an effort in that direction has been started. The first step is to obtain a Kinescope capable of resolution in the order of a thousand lines. The next step is to decide on lines and frame standards. In order to avoid extreme video amplifier band- width requirements, it might be worthwhile to consider a frame rate below 30 per see and reduce flicker by use of a Kinescope with a screen persistence. These avenues are being explored at present for better methods of presenting the scan- converted picture. 164

The Army-Navy Aircraft Instrumentation Program GEORGE W. HOOVER, Office of Naval Research Summary The inadequacies of aircraft instrumentation led the Navy in 1953 to initiate development of an integrated presenta- tion of flight data. With participation by the Army, the program has since been extended to include rotary-wing aircraft, as we,! as fixed-wing airplanes. Operational analysis indicated informa- tional requirements for displays in terms of the phases of a com- bat mission. A contact analogue was developed to provide orien- tation information relative to the pilot's need to know his position and motion in space. A situation display was developed to pro- vide command information relative to what he should do. In addition, requirements for quantitative information were con- sidered. The problem of aircraft instrumentation may be divided into four major areas' each of which must be completely adequate if all-weather operation is to become a reality. These areas are: all-weather instrumentation, performance envelope data, escape capability, and integration of all components to provide a reliable, nonredundant system. Present instruments are not adequate to provide the pilot with an essential, integrated display. Information relating to the air- craft performance envelope, when present, is extremely difficult to assimilate and requires a great deal of training and experience. The problem of escape has been approached, but the present solution is still insufficient for the pilot's safety. When operating under high-performance conditions, he receives too little information too late. As for redundancy, the fact that the average aircraft system is made up of many groups of subsystems is evidence that there is too much repetition of gyros, amplifiers, computers, and displays. The Navy, re- cently joined by the Army, has been carrying out research and development to correct these deficiencies. This paper describes the work and accomplishments of the Army-Navy Instrumentation Program. At the request of the Navy in 1953, 250 representatives of the aircraft and instrument-manufacturing industries attended a conference in Washingon, D.C., along with representatives from the Services. It was disclosed that the Office of Naval Research and the Bureau of Aeronautics had initiated an industry-wide program aimed at the development of an integrated presentation of flight data. It was believed by the Navy that the necessary coordination could be best per- formed by an agency with design responsibility for the total system. The Doug- las Aircraft Company's El Segundo Division was selected as coordinator for the fixed-wing portion of the program. Shortly after this, the Army joined the Navy in sponsoring the program, and the rotary-wing phase of the program was initi- ated with Bell Helicopter Corporation as coordinator. At the outset, it was recognized that man has basic limitations. Man-made machines will never duplicate man's brain and his ability to assume command -165

in emergencies, yet today's high performance vehicles confront the operator with complex mechanisms and instrumentation. His display panel presents an array of instruments which cannot be read as an entire unit. The operator must scan, choose, and interpret numerous bits of information before he ca initiate the appropriate control responses in the performance of his mission. Un- necessary interpretation and integration by the operator results frequently in errors' disorientation' accidents, aborted missions, and deterioration of morale. Attempts have been made to align instruments to reduce the pilot's task, and to combine instruments. Still the problem has persisted. An entirely new con- cept of instrument presentation had to be conceived since it was felt that the presentation should give the pilot direct responses to his questions. Operational analysis showed that the majority of combat missions Carl be broken down into a number of phases: take-off and climb rendezvous, cruise and navigation strike' traffic control' and landing. Although all of these phases have the same basic requirements it was necessary to question pilots on each phase to establish whether significant differences existed. To uncover require- ments in detail, an unusual method of interrogation was utilized. It was neces- sary to continue to ask "why" after each statement by the pilot' until the interrogator's "why" questioned the validity of the objective, or until the answer could stand unchallenged. Through repeated questions, and not without great difficulty in getting the pilot to clarify what he really needed in the way of basic information, it was found that among the pilot's primary needs was neither a compass nor a vertical gyro, but his orientation with respect to the ground plane, i.e., spatial orientation. This method of interrogation was employed with a sizable number of pilots to determine the basic information required for each flight phase, until the in- formation requirements were unequivocally established. Since the program's inception, no major change has been found necessary in the statement of re- quirements for information. A list of information requirements was distributed to a group of human engineers, each having been assigned specific phases of flight to study. Their task was to determine the methods by which the informa- tion could best be displayed for the pilot to operate his aircraft with maximum efficiency and minimum mental integration. To determine the effectiveness of the display, it was necessary to establish a yardstick for comparison. Initially, no attempt was made to design an opti- mum system, but rather to design a system which would be adequate to permit the pilot to operate his aircraft under instrument conditions as effectively as under ideal contact conditions. This approach gave a point-of-reference from which to continue the optimum display and control system determinations. Since pilots are generally capable of operating quite effectively under ideal con- tact conditions, the human engineers had to determine how the visual world provides answers to information requirements: vi%., what factors in the Visual world enable the pilot to operate his aircraft? 166

It was established early in the study that there are three basic categories of information required throughout all phases of flight: orientation information, command or director information, and quantitative information. A pilot judges his relationship to space and time, under contact conditions, primarily by refer- ence to visual cues. One important visual cue was found to be an internal reference, which permits the pilot to regard himself and his aircraft as a single unit. Such a reference is normally available to the pilot in the form of a wind- shield. A second important visual cue is external reference. To the pilot, one of the most common external references is the horizon. It enables him to determine the relationship of his aircraft to external objects, The horizon, by itself, quite often gives rise to misinterpretations. Apparently, converging parallel lines help the pilot to judge angular and altitude changes as in a dive. The texture of a reference surface is used by the pilot to determine slant of the surface, altitude, and distance. This powerful visual cue is sufficient in itself to establish orienta- tion without reference to the horizon. In air-to-air orientation, size is also an important cue. Movement over the surface results in an apparent distortion of the visual field, known as motion parallax. Motion parallax provides a com- pelling cue to distance, speed, and direction of motion. By combining the visual cues abstracted from the real world, an artificial model was created which, for purposes of orientation, is perceptibly equivalent to the real world. A display developed from this model may be called a contact analogue. It is not necessary for the contact analogue to represent a surface ruled into a regular grid. For some cases, the optimum pattern might take var- ious forms, such as randomly located regular shapes, randomly located random shapes, or regularly located random shapes. The real world does not, in general, contain command or director informa- tion. However, since mission objectives require operation about all three axes, such commands may be expressed in a manner compatible with cues for basic orientation. In addition to the contact analogue, a display was required to fulfill navigation, cruise, and tactical planning. Information concerning local geog- raphy, present position and heading, targets, destinations, flight plan, fuel range remaining, and other points of tactical interest, were integrated into a display. The requirements for the tactical situation display and additional quantitative information have yet to be fully determined, but work is in progress in these areas. The detailed display requirements for the contact analogue and the situa- tion display were distributed in the form of specifications to a number of com- panies with broad experience in the field of systems and component develop- ment. Their task was not to design specific equipment, but rather to determine technical requirements for display, data processing, and sensing. The technical requirements group determined the need for development of a thin, transparent, display medium. Additional examination of system and aircraft requirements indicated that one possible solution might be the development of a thin, trans- parent, cathode-ray tube. Studies established the various physical phenomena which had to be sensed, the computer requirements, and the techniques applicable - 167

for the generation of the display. Ir~vestlgation established the possibility of developing specific sensors. The results of the feasibility studies indicated the need for development of circuit concepts and the need for research in materials and fabrication processes necessary to meet the requirements of the total system. Contracts for the development of sensors, computing elements, and display media were awarded as these requirements were established in the fixed- and rotary . . wing aircraft program. These have been the concepts and the methods employed in the continuing effort to improve the relationship between man and the machine he controls. The Army-Navy aircraft instrument program, combining the techniques fa- cilities, and investigative skills of the aircraft and instrument manufacturers, is moving toward the satisfactory completion of this program by adhering to the team approach method. The program embodies the following: an optimum presentation and controls for the aircraft man-machine system; industry-wide participation; a completely integrated system reducing weight, size' mainte- nance, and training time; additional reliability; and development proceeding from logical requirements derived from fundamental considerations rather than from undirected invention. Editor's note: The motion-picture film which Cdr. George W. Hoover showed may be obtained on a short-term loan basis by writing to him at Code 461, Office of Naval Research, Navy Department, Washington 25, D.C. Those interested should request Army-Navy In- strumentation Program film MC 8833. 168

Transparent Phosphors for Cathocle-Ray Tubes CHARLES FELDMAN, U. S. Naval Research Laboratory Summary-Transparent phosphors are formed by evaporation in a vacuum. The method, applicable to most common phosphors, yields luminescent screens having the same properties as these phosphors. The many advantages of transparent screens over the conventional powdered screens include high resolution and corz trast. The transparent screen has been applied to a flat cathode ray tube, a daylight viewing tube, and a laminated color tube. The powdered phosphor luminescent screens employed today in cathode- ray tubes are composed of layers of small luminescent grains whose diameters are between 3 and 8 a. This grain structure causes the screens to be white and opaque, and to scatter laterally the luminescent light produced by an electron beam. As a result, there is halation and limited resolution and definition of the image. Ambient light falling on the screen is scattered back to the observer, making the screen appear bright and, thereby, limiting the contrast of the Image. It is quite desirable to form luminescent screens free of grains that scatter light. Such screens will be transparent. One could, in principle, accomplish this by growing a single crystal of the phosphor having the dimensions of the screen desired. However, this is quite impossible with the present crystal-growing technique. Fortunately, one can accomplish the same thing by forming the screen of grains which are too small to scatter light. To accomplish this, the grains must be approximately 100 times smaller than those currently employed. The technique, to be described here, of thermal evaporation in a vacuum has proved to be singularly successful in producing luminescent screens of sufficiently small grain size to be transparent.) The luminescent screens formed by this method are thinner (0.1 to 1 A) than the diameter of a single grain in the conventional powdered material. It must be pointed out that the image on the conventional powdered screens will be brighter than that on the corresponding transparent screen. On the powdered screens, the light from the image is scattered to the halo and central spot as illustrated in Fig. 1. On the transparent screen, a large fraction of the luminescent light is trapped by internal reflection and escapes only at the edges of the screen. Figure 2 is a photograph of the electron images produced on con- ventional and transparent screens. The fraction of total light reaching the ob- server from a completely transparent screen may be easily estimated as follows. Assuming a nonabsorbing film, the ratio of the light emitted from the face of the screen in the direction of the viewer to the total light output is: solid angle subtended by the angle of internal reflection (! _ Cos bc) total solid angle = 2 169-

ELECTRON BEAM ~1 sac OBSERVER ' i> a Aft 0~9=,~a oD __-- ~ _N Fig. 1. Schematic diagram illustrating {a) transparent screens and (b) powdered screens. (A) (B) (a) (D) Fig. 2. Photograph of electron-beam trace on a conventional powdered screen (A&C) and a transparent screen {B&D), illustrating the difference in definition between the two types of screens. -170

where 6c is the critical angle of internal reflection defined by sin be-try, where n is the refractive index of luminescent layer. ~ ~ v ~^ vie ~t~ ~ ~ v ~^ The critical angles and per cent of light reaching the observer for a few common phosphors are listed in Table 1. Referring to this table one sees that twist ~c much fight r~rh~c the nh.~erver from a Zn~SiO4 screen as from a ZnS screen. Even though the Arts screens are more efficient than the Z~oSiO~ screens in the powdered form they may have equal efficiency in the trans- parent form. Table 1. Light Loss by Ir~ternal Reflection. Material Index of refraction Critical Angle degrees ) 25 32 36 46 ZaS CaWO4 Zn.,SiO4 CaF 2.4 1.9 1.7 1.4 Amount of light reaching observer ( per cent) 5 8 10 15 Table 1 assumes a maximum amount of light lost. Any absorption, non- uniformity of screen thickness, or slight fogging of the film will reduce this value. Measurements by Studer and Cusano2 and the author) indicate that in actual practice no more than 50 per cent of the light is lost by this mechanism. By aluminizing the transparent screens, brightness may be approximately doubled. However, contrast and transparency are lost at the same time. The light loss due to internal reflection may be easily recovered by oper- ating the cathode-ray tubes at higher power levels. Due to the greater adhesion of the transparent films to the glass substrate and due to the thinness of the films' the heat generated in the phosphor by the electron beam is dissipated rapidly. The electron beam power may consequently be raised by a factor of 2 to 3 without danger of producing screen burn. Formation of transparent luminescent screens :Formation of transparent screens involves essentially two distinct steps. The first step is evaporation of the commercial phosphor material onto a glass substrate. During this evaporation, the phosphor decomposes into its more stable molecular components and condenses on the substrate as a film com- posed of a mixture of the components. Zn~SiO4 EMn], for example, decomposes into the reduced oxides of ZnO, SiO2 or SiO, and MnO. Some phosphors such as ZnS fMn], decompose- only into ZnS and MnS. The second step involves the decomposition of these components into the crystalline phosphor on the substrate. This step is accomplished by firing the layer in the proper atmosphere. - iFeldman, C. and O'1Iara, M. Formation of luminescent films by evaporation. J. Opt. SOC. Am. 1957, 47, 300. 2Studer, F. J. and Cusano, D. A. Transparent phosphor coatings. J. Opt. Soc. Am. 1955, 45, 493. 171

The evaporation is performed from the best obtainable commercial phos- phors in a vacuum-bell-jar system under a pressure of about 5 x 1O-5 mm Hg. The size of the screen iS7 thus, limited only by the dimensions of the vacuum chamber. The usual procedures for forming films by evaporation are applicable to phosphor films. However the requirements for purity are more stringent ~ ~ , .~ ~ 1 · .~ r ~ ~ .. . 1 . 1 . _ 1 ___' tnan m other types ot evaporations, and precautions must oe laden IO ellml- nate all sources of impurities. The substrates are cleaned and outgassed thoroughly. A removable screen is kept between the filament and substrates until the material in the filament begins to evaporate. A liquid-air trap is used to prevent oil from the diffusion pump entering the system. Table 2 summarizes details of the procedures used in forming the screens. The last column of the table lists firing conditions used in forming the lumines- cent screens. Freshly evaporated films generally exhibit little or no lumines- cence. However' when films are fired in a proper atmosphere, good lumines Table 2. Conditions Used in Forming Luminescent Films. Firing Conditions ~ ° E E Q ~ O ~O G o' ° " ~ E 0 , m._ ~ 4, ._ ·- 4, , ~0 ZnS [Mn] tantalum 1050° 60 N.B.S vacuum 750-800 boat special for 5-15 min glass CaF2 [Mn] tantalum 1550° 5 silica vacuum 600-700 boat glass ford min (Zn:,Mg) Fat [Mn] tantalum 1500° 5 glass vacuum 600 ford min (P-12) boat Zn~(PO4~3 Irvin] tungsten 1525° 20 glass air 850 for 10 min (P-22) basket CeWO4 [w] tungsten 2000 ° 3 glass air 550 for 120 min (P-5 ~straight wire Zn2SiO4 Irvin] tantalum 1300° 30 silica air 1100 for 15 min (P-1 ~boat Vycor MgSiO3 [Mn] tantalum 1350° 15 silica air 1100 for 15 min (P-13) boat Vycor (Ca,Mg`) (SiO3~)2 tungsten 2000 ° 5 Vycor air 950 for 15 min ETi] basket 172

cence results. The activator impurity, essential to luminescence, is not lost in the evaporation process, as is generally assumed' but is incorporated in the film as a separate phase. Firing the film causes the activator to diffuse into the host crystallites of the film and, at the same time, causes good crystallite formation. In the case of the phosphors containing oxygen, CaWO~, Zn.SiO4 and Zn,~P04~, etc., the firing also serves to oxidize or add oxygen to the partially reduced compounds. When these phosphors are evaporated, some of the oxygen is lost, and the resulting films are dark in color. The dark color disappears at relatively low temperatures (~300° C), but no luminescence results at these temperatures. The best firing temperature for forming luminescent films appears to be about the same as that used in forming the powdered phosphor material. If the films are overfired, the Grystallites grow rather large and the screen becomes fogged. It has been found that the usual phosphor chemistry is applicable to thin films, e.g., the condition for forming the various crystal phases found in the bulk powder appear to be the same in the films, and the amount of activator in the film necessary for maximum luminescent efficiency appears to be about the same as in the luminescent powdered material. Due to the high firing temperatures (500°C to 1100°C) necessary, the choice of glass substrate material is quite important. In addition to being able to withstand high temperatures, it has been found desirable in some cases to have the thermal expansion coefficients of the film and substrate similar. This match- ing of expansion coefficients helps to prevent very thick films from cracking as they are fired. In the case of ZnS film, a glass meeting these requirements was made at the National Bureau of Standards; this substrate is designated in Table 1 by "N.B.S. special glass." The relationship between phosphor and glass substrate is further compli- cated by the diffusion of impurities from the glass into the screen as it is fired and by the possible chemical reactions that can occur between glass and phos- phor at the high temperature used. These phenomena are currently being studied. The luminescent screens formed on the proper substrates are extremely rugged, and the adherence to the glass is excellent. Screens may be cleaned and polished without fear of destruction. This is quite a contrast to the delicate nature of the settled powder screens. The luminescent properties of the screens, with the exceptions of a loss of brightness of about 50 per cent, are identical to those of conventional pow- dered screens. The luminescent emission peaks correspond to those observed in the powdered material. The voltage and current relationships conform with general phosphor theory. It has been found essential to place a conductive layer on top of the luminescent film in order to prevent charges from building up on the surface. -1 73

This charging of the screen surface by electrons appears to be greater in evap- orated than powdered-type screens. Thin conducting layers of stannic oxide, cuprous iodide, and aluminum have been used for this purpose. The aluminum layer is used, of course, only when non-transparent screens are desired. All of these layers may easily be deposited on the screen surface. Due to the high firing temperature used in forming luminescent screens on substrates' it is not practical to deposit the luminescent screen on glass already coated with a conducting layer. Applications In general, transparent screens may be used to advantage wherever im- proved conventional screens are used. Absence of halation and improved reso- lution, as illustrated in Fig. 3, bring about a great improvement in systems where resolution is important, as in computer readout and flying spot scanning. There are a number of applications, however, which require the use of trans- parent screens and which would be impossible with the present opaque powdered screens. Three of these applications, transparent flat tubes, daylight Viewing tubes, and laminated color tubes, are described below. For many years the Office of Naval Research has been involved in a long- range program of making basic improvements in aircraft instrumentation. This program, being carried out in cooperation with the Army, is known as the Army-Navy Instrumentation Program. The goal of this work is to simplify the operation of airplanes, reduce the training time of pilots, and help them get the maximum performance from their aircraft. As part of this program, it is im- portant to obtain transparent luminescent screens for use in conjunction with the Aiken, flat cathode-ray tube. The tube is mounted just inside the windscreen of the cockpit. The pilot receives visual and electronic information at the same time and in the same place. Windshield-sized, transparent screens have been inserted inside the Aiken tube and are being tested in aircraft. The transparent screens are used as inserts because the NTycor plates on which they are formed are not compatible to the soft glass used in the tube. The problem of glass com- patibility is currently being studied. An important application of the transparent screen, in both military and commercial fields, is the daylight viewing tube. In powdered screens, ambient light is scattered back to the observer. As the brightness of surrounding light in creases, a point is soon reached where the image on the tube is no longer visible, i.e.' luminance of the ambient light scattered to the observer's eyes is equal to that reaching the observer from the image itself. For television sets, this lum . ~ . inance level occurs under daylight conditions. For normal radar scopes, the level is considerably lower. If, on the other hand, light falling on the tube face passes through a trans- parent screen into the tube where it is trapped, the tube face appears dark, re- gardless of the intensity of the light. The trace on the tube need not be extremely :~Aiken, W. R. A thin cathode-ray tube. IRE Proc. 1957, 45, ~1599. 174-

bright since the contrast remains good. This phenomenon is easily observed by looking down a long tube or pipe which is closed on the opposite end. The in- side of the tube looks black regardless of the lighting conditions. Since cathode- ray tubes and pipes do not have the same geometry' one must cause the light to be absorbed by other means such as coating the inside walls of the cathode-ray tube or the back of the transparent screen itself with a light-absorbing medium such as carbon blacl;. Figure 3 shows a military application of this principle. A tube, containing a Zn~,SiO; transparent screen and coated on the inside with colloidal carbon, was mounted in a simulated cockpit. A 3-ft searchlight was focussed on the tube. The cathode-ray trace could be easily seen although the luminance of the trace was only about 40 ft-l and the luminance of white paper in the searchlight beam was about 10,000 ft-l. The same principle, applied to television, proved to be equally successful. Fig. 3. Daylight viewing tubes mounted in a simulated cockpit and illuminated with a 3-ft. searchlight. Note the black background on the tube face. Electron image consists of two concentric circles and a straight line. The thin, transparent phosphor layers described above make possible a cathode-ray tube that has been contemplated for some time.4 The tube uses a screen consisting of layers of different phosphors deposited on top of one another. The general principle of the laminated screen may best be described with the aid of Fig. 4. Electrons enter the screen and strike phosphor No. 1, exciting it to luminesce. The luminescence is viewed through the remaining phosphors and glass substrate. If the electron energy is low and the electron is stopped in this 4Feldman, C. Bilay-er Dichromatic cathode screen. J. Opt. Soc. Am. 1957, 47, 790. 175

first layer, the color viewed will be only that due to phosphor No. 1. As the electron energy is increased, electrons begin to penetrate phosphor No. 1 ant; excite phosphor No. 2, which luminesces a different color than phosphor No. 1. The color seen by the observer is a combination of these two colors. At still higher energies, the electrons pass through phosphor No. 1 without exciting it, and the color viewed is only that of phosphor No. 2. This general process can be continued for three or more layers. E LECTR ONS /////////// C O N D U C T I NG C OA Tl NGy / / / 1 PHOSPHOR # I PHO S P HOR ~ 2 PHOSPHOR # 3 | ~ ~ GLASS ~ ~7 V O B S E RVE R Fig. 4. Schematic drawing of a laminated color screen. 176-

The electron energies, or voltages, at which the above events occur, depend only on the thickness of the layers. By choosing the correct combination of phosphor layer thickness and non-luminescent spacers between layers, the pure color of each phosphor can be obtained. For color television, a luminescent layer of each of three primary colors is required. For other uses such as radar, two layers may be sufficient. Figure 5 shows the emission, at different voltages, from a two-layered screen consisting of: CaWO.; (blue) /ZnSEMn] (yellow) /glass. The corresponding colors of each of these voltages is shown on the chromaticity diagram in Fig. 6. In this illustration, the transition from blue to yellow re- quires a voltage change of 15 kv. The color change from 8 to 16 TV appears to the eye to be about the same as the change from 5 to 20 kv. 100 ~~ //\ 80 i 60 an 8 KV \ ~ r \ \ 100 ~ 80 Z 60 llJ > ,5 4 0 LL a: 20 o ~''a. ~ 1 'I 1 1 ~: 12 KV ) ~ r I I I I I ! i i 1 1 1 1 400 500 600 WAVELENGTH ( m`,) _, 700 400 I; ~ K V _ 11 \ 500 600 700 WAVELENGTH ( me ) Fig. 5. Luminescent emission of a bilayer screen at various electronaccelerating Volta ges. Other bilayer phosphor screens such as: Mg,SiO4 (red ~ /Zn ,SiOi (green) /glass have been examined, as represented by the dotted line in Fig. 6. The red layer in this particular screen was sufficiently thin that the transition from red to green, passing through yellow and orange, required a voltage change of only -1 77

approximately 4 kv. Tricolor screens consisting of more complex layers have been made with equal success and will be reported on in the future. Transparent laminated color screens have the same advantages of contrast and resolution as the single-layer monochrome screens. Transparent luminescent screens are only in their early stage of develop- ment. Work on many other phosphors, on the reactions between glass substrate and phosphor and between phosphor layers themselves, and on a multitude of other details, needs still to be carried out. It is quite obvious' however, that these screens will play an important role in future developments by the cathode-ray- tube and television industries. flood my\. .80 .70 .60 r ~ :' o .50 ~ g.40 _\ .30 - \ .20 - \ .10 - 1~0~'' ~\ ~ \ ./V ,' 5 KV G . ,~ \ \ \ \ \ ~0~ DOW, ~` '\, \~ O .10 .20 .30 .40 .50 .60 .70 VALUES OF X Fig. 6. Chromaticity diagram indicating colors resulting from two different laminated color screens. 178

Direct-View Storage Tubes GEORGE F. SMITH, Hughes Research Laboratories Summary Five-in. direct view storage tubes are currently available in a variety of models. The bistabZe tube is well suited for transient oscillograph applications. Because of their ability to store shades of gray, the halftone tubes are better suited for many radar and picture applications. Developmental samples of 21-zn. storage tubes have been constructed, with spot-writing and character-writing gurus. The large tubes provide substan- tially greater resolution and ease of viewing. Experimental models of a multicolor storage tube have been made. As the name implies, a direct-view storage tube is a storage tube in which the output is visual. The primary function of such a device is to provide a bright and persistent display of nonrecurrent pictorial information. Storage tubes are well suited for the persistent display of such types of information as radar or sonar data, processed alphanumeric information transient wave- forms7 and narrow-bard television pictures. On the exterior, storage tubes closely resemble ordinary cathode-ray tubes. Several companies are currently manu- facturing 5-in. tubes. Larger storage tubes, having diameters up to 21 in., and specialized tubes such as a multicolor storage tube, are currently being de- veloped ir1 order to increase the capacity for the storage and display of infor- mation, or to afford more convenient viewing. There are two kinds of direct-view storage tubes: bistable and halftone. The bistable or two-tone tube presents two shades of grey, namely, black and white' and incorporates regeneration to provide an indefinitely long retention time. The halftone tube provides a continuous range of grey shades but has a limited retention time. The external and internal appearance of both tubes is similar; the schematic drawing in Fig. 1 will serve to illustrate both types' each ,( *.: :.:.~''.~? :.,:~: * ):.:i'~ *,:: :,:,,j.b,,, .::,::,::,::,ri,~ FLOOD GUS ( Ov CATHODE ) ~,,,,_ ~ nl IHR_-=-=---- DEFLECTI oN YOKE WR'T'~IG GUN ( - 2 . s kv CA=~ooE ) --COLLECTOR MESH (+150v) STORAGE MESH (ABOUT Ov ) -VlEWI~G SCREEN (+ 10 kv ) . ~ . . 1 1 -- ~ I==.== COLLIMATOR ELECTRODE Fig. 1. Schematic of a direct-view storage tube. 179-

of which consists of four essential elements: a storage mesh' an adjacent phos- phor viewing screens a flood gun, and a writing gun. The storage mesh is a fine metal screen with several hundred holes per linear inch. A thin layer of insulating dielectric is deposited on the side of the mesh facing the flood guru. The flood gun covers the storage surface with a uniform, broad beam of slow electrons. A charge pattern corresponding lo the picture to be viewed may be deposited on the surface of the dielectric. Each elemental area of the charge pattern controls the transmission of electrons from the flood gun in the same way as the control grid of an ordinary receiving tube controls the plate current. In areas where the charge pattern is relatively more positive, flood electrons can perpetrate the storage mesh; in areas where the pattern is negative, the flood electrons are repelled and cannot penetrate the mesh. Those electrons which succeed in passing through the storage mesh are then accelerated so that they may strike the phosphor viewing screen at a high energy and produce a light pattern corresponding to the charge pattern on the insulating storage surface. As long as the charge pattern remains undisturbed, the picture to be viewed persists. The writing gun, which may be a conventional cathode-ray gun, serves to deposit the charge pattern. If a uniform negative "black" pattern is assumed' the writing gun can deposit a positive "white" pattern on the dielectric surface. The dielectric material is chosen to have a high secondary-electron-emission ratio at the bombarding energy of the writing electrons. Several secondary elec- trons are emitted from the insulating surface for every incident primary elec- tron, so that the surface is rapidly charged in the positive direction by the writ- ing guru. Erasure is usually accomplished by bombarding the entire target with low-energy electrons from the flood gun. The modes of erasure are different for the bistable and halftone tubes and will-be discussed later. Bistable tubes The bistable mode of storage is well suited for the display of processed data or transient waveforms, such as alphanumeric information from a computer or an A-scan radar presentation. This type of storage tube was first proposed by A. V. Haeff~ ~ in 1947. The Memotron@ and Typotron~ :3 tubes incorporate bi- stable storage; production models of these two tubes have been available for the past three years. A typical bistable-storage-characteristic curve is shown in Fig. 2, where display brightness is plotted against storage-surface potential. Some of the elec- trons from the flood gun provide the display and others are responsible for the regenerative action. There are two equilibrium dielectric potentials: (a) flood- gun cathode potential, or black, and (b) collector-mesh potential, or white. In the absence of a writing beam, the black and the white areas of the dielectric are maintained at their respective equilibrium potentials by the low-energy ~Haeff, A V. A memory tube. Electronics, 1947, 90, 80-83. 'Smith, S. T., and Brown, H. E. Direct-viewing memory tube. Proc. IRE, 1953, 41, 1167-1171. 3Smith, H. M. The Typotron, a novel character-display storage tube. IRE Convention Record, 1955, 3 (Part 4), 129 134. 180-

flood electrons. In the white areas, where the dielectric is charged to the col- lector potential, the flood electrons strike at a velocity sufficient to produce a secondary-emission ratio greater than unity, i.e., more than one secondary elec- tron is released for every incident flood electron. As long as the collector re- mains more positive than the dielectric surface, it will attract and collect the secondary electrons so that a net positive charging current will exist. Any ten- dency of the surface potential to fall below the collector potential is thus com- pensated for by the positive charging action of the secondary emission. In th black areas, near zero-volt potential, the flood electrons strike the surface with almost zero velocity and produce virtually no secondary electrons. Hence, any tendency of the surface potential to rise above zero volts is compensated for by the negative charging action of the extremely low-energy flood electrons. Sur- face areas with a potential having any value between zero and that of the collector potential will be driven in either the negative or the positive direction until they reach one or the other of the stable points. e R IGHThJ ENS ( PERCENT) oo 1 ~1 ~1 Al I - 60 1; 1 1 1 7~ VO VCOLL . - WHITE STABLE POINT BLACK STABLE POINT I I ~ 1 50 100 150 200 -50 STORAGE SURFACE POTENTIAL (VOLTS) Fig. 2. Typical storage characteristics, showing brightness as a function of storage dielectric potential. Curves are shown for both the bistable mode and the halftone mode. A critical potential, labeled V`) in Fig. 2, divides the two regions of negative and positive regeneration. In order to write a blacl: surface to white, the writing beam must raise the storage-surface potential to a value just beyond this critical potential, so that positive regeneration will carry the surface the rest of the way 181

to the collector potential. As discussed in the introduction, the writing beam deposits a positive charge patterns by using the high secondary-electron-emis- sion ratio of the dielectric at the writing-beam energy. A stored pattern can be erased by momentarily decreasing the collector potential in order to eliminate the bistable regeneration. If the collector potential is decreased to a value slightly below the critical potentials id,, positive regeneration is eliminated and the po- tential of the written area will be reduced to zero volts, that is, to black. The 5-in. Memotron tube, which incorporates a cathode-ray-type spot- writing gun, has a linear writing speed of 100~000 in. per see, with a resolution of 60 white lines per in. The Typotron tube utilizes the same storage assembly, but is equipped with a character-writing gun for writing alphanumeric informa- tion directly. A schematic diagram of the Typotron tube is shown in Fig. 3. The beam of electrons from the writing gun is formed into the shape of a char- acter by passing it through the character matrix, a stencil containing 64 letters, each approximately 0.016 in. high. The beam is returned to the axis by a mag- netic lens and is then directed to any chosen spot on the storage target by means of deflection plates. The 5-in. Typotron tube can write 25,000 letters per see; thus it can operate as a direct-output device for a high-speed digital computer. Figure 4 shows the face of a Typotron tube on which all the characters of the stencil have been stored. Both Memotron and Typotron tubes can present only two shades, black and white, but both provide indefinitely long retention times. A'/A~ CON\ `~v HA R At _ .~ ~ ^.~- ~: ~ W 177~(i__L,£ ~:~ DE FL EC LION PL ARES ~ ~;~'~ Fig. 3. Schematic of the character-writing Typotron storage tube. / l Halftone tubes The first work on a direct-view halftone storage tube was begun by Max Knoll and others about 1949.4-0 Tubes of this type, such as the Hughes Tono- tron tube, have been in production for nearly two years. The principal advan 182-

tage to the half tone storage tube is that, unlike the bistable tube, it can store and display a continuous range of grey between black and white. For this rea- ~;~n it is much better suited for raw radar displays, where discrimination of targets in noise is desired. Halftone pictures can be stored at constant luminance (up to several thousand ft-l) for periods as long as 1 min' or the display can be made to fade toward black with an arbitrary time constant, just as in a long- persistence cathode-ray tube. The continuous grey scale is obtained only with . . . a Ante retention time, but the retention times available are adequate for most ~ . . applications. Fig. 4. Stored characters on a 5-in. Typotron tube. Figure 2 shows both a typical halftone storage characteristic and a bistable characteristic. The interesting features of the halftone characteristic are shown in more detail in Fig. 5. The entire halftone potential range is negative with respect to the flood-gun cathode. This means that none of the electrons from the flood gun can strike the storage surface and hence that the charge pattern should be undisturbed during viewing, even without regeneration. Unfortu- nately, this is not strictly true since positive ions are formed by the flood beam from the residual gas in the tube. Some of these ions are attracted toward the neg- ative storage surface and slowly charge it positive toward white. The retention 4Knoll, M., Hook, H. O., and Stone, R. P. Characteristics of a transmission control viewing storage tube with halftone display. Proc. IRE, 1954, 42, 1496-1504. 5Knoll, M., and Kazan, B. Viewing storage tubes. Advances in Electronics, 1956, S. ~Hergenrother, R. C., and Gardner, B. C. The recording storage tube. Proc. IRE, 1950, 38, 740-747. 183-

time is determined by the gas pressure in the envelope. Values of the order of minute can be achieved. a As in the case of the bistable tube, writing is accomplished by means of a well-focused beam of high-erzergy electrons from the writing gun. By virtue of its high secondary-emission ratio, the dielectric surface is charged positively when bombarded by these electrons. / CUTOFF \ - | HALF-TONE | RANGE ~1 l _1 - 8 - 6 - 4 - 2 0 STORAGE SURFACE POTENTIAL (VOLTS) ~ 100 80 60 - 40 `~, of 20 m o Fig. 5. Halftone storage characteristic of a 5-in. Tonotron tube. The flood gun is commonly used for erasure. It is clear from Fig. 5 that, in order to erase, the storage-surface potential must be lowered to its cutoff value, in this case about 5.5 v. This is done by momentarily raising the metal-mesh potential by 5.5 v. The storage-surface potential is then capacitively increased by an equal increment. Thus, any part of the storage surface having a potential on the useful part of the storage characteristic is elevated slightly above the cathode potential. Flood electrons strike the storage surface, and, since they have only a few volts of energy, for which the secondary-emission ratio is less than 184

one' they charge the surface negatively down to the flood-cathode potential. After this happens, the storage mesh is lowered by the same 5.5-v increment, capacitively lowering the entire dielectric surface to its cutoff potential. This process can be carried out all at once with a single pulse, or a pulse train con- sisting of widely spaced short pulses can be used; in this case, the erasure is spread out over a period of time. If the latter mode of operation is used, the stored pattern will fade out with a time constant that depends only on the duty cycle of the erase pulse train. Halftone tubes also have been made with a separate low-energy erase gun to be used for spot-by-spot or line erasure, al- though the performance of such a low-energy gun cannot provide high erasing speed or resolution. When operated properly, a 5-in. halftone storage tube provides a display quite adequate for the tonal range and resolution of a typical standard television picture. However, particular care must be taken to ensure that the video input is matched to the dynamic range of the storage tube. An ordinary cathode-ray tube has a very large dynamic range since the phosphor brightness is directly proportional to the writing-beam current for any current that the gun will provide. It can be seen ire Fig. 5 that the dynamic range of the halftone storage tube is definitely limited. In order to make full use of the range, the erase-pulse amplitude and the video signal must be carefully matched to the storage char- acteristic. If the erase pulse is too large' the storage-surface potential will be reduced beyond the cutoff point, and, thus, a threshold for writing (eliminating low-level signals) is introduced. On the other hand, if the amplitudes of the video peaks are excessive, the tube may be driven to saturation and the large- signal detail will thereby be clipped. The problem of matching the video to the storage characteristic can become particularly severe if a fast storage tube is used for a long-range radar display or for certain types of slow-scan television. In such applications' the writing beam may scan the storage target so slowly that it is difficult to keep the beam current low enough to prevent saturation in the highlights. Five-in. halftone tubes have full-brightness writing speeds of a few hundred thousand in. per see, with written resolutions of from 60 to 80 lines per in. The written resolution is generally somewhat better for shades of grey than for full-brightness writing. Five or six halftone shades are available, with full-tone brightnesses up to several thousand ft-l. Controlled-fade times of from 2 to 30 see are feasible, with maximum retention times of the order of one min. Figure 6 is a photograph of a picture stored on a 5-in. Tonotron tube. One field of a commercial television picture was used as the input signal. The individual scan lines in the picture are clearly visible in the original photograph. Large-screen and multicolor storage tubes More over-all resolution and the convenience of a larger display are de- sirable in indicator tubes for air-traffic control, ground radar' and processed data 185

systems. A 21-in. halftone tuber is under development for such applications. This tube makes use of curved' self-supporting screens and an envelope with a curved faceplate. Because of mechanical considerations, neither the meshes nor the faceplate can be flat when the tube diameter becomes this large. The thick, self-supporting target material, which cannot be fabricated with as many holes per in. as thin, flat material, still provides several times as many holes in the entire target and a substantially increased total resolution. Current develop- mental tubes provide resolutions in excess of 750 distinguishable white lines. The spot-writing version of the 21-in. tube can write at least 40,000 in. per see, and the light output is in excess of 50 ft-l, an output quite adequate for use is normally lighted areas. A letter-writing version of the 21-in. tube can write 40~000 letters per sec with a displayed letter size of 0.15 in. Halftone uniform- ity comparable to that of the 5-in. storage tube has been achieved. fig. 6. One field of a television picture stored on a 5-in. Tonotron tube. Also in the advanced development stage is a multicolor storage tube'89 which incorporates shadow-mask color selection together with halftone storage in a 10-in. envelope, as shown in Fig. 7. A storage screen is inserted between the shadow mask and the color-dot plate of a conventional three-gun color tube. A ring-type flood gun has been added to provide the low-energy viewing beam. 7Koda, N. I;, Lehrer, N. H., and Ketchpel, R. D. Twenty-one-inch direct-view-storage tube. IRE WESCON Convention Record, 19577 1 (Part 7), 78. SBeintema, C. D., Smith, S. T., and Vant-lIull, L. L. Multicolor storage tube. IRE Trans- actions PGED, 1957, ED-4, 303 309. ~Vant-Hull, L. L., and Beintema, C. D. Multicolor storage tube. Wright Air Development Center, 1957, Technical Report 57-400. 186

As shown in Fig. 8' the storage target has one hole for every phosphor color dot. The writing beam from one gun, say the red gun' can strike the dielectric around only those storage-surface holes that are aligned with red phosphor dots. The flood beam coming through the shadow mask from the extended- source flood gun illuminates the entire storage surface but penetrates it only where the charge pattern is positive. Excellent color purity has been achieved over the entire 6-in. useful screen diameter of the tube and at least seven distinct colors can be distinguished. A color storage tube could be used for any applica- tion in which another display dimension is required, e.g., for the display of identification, speed, or altitude, in addition to the usual radar parameters of range and azimuth. _ VIEW PLATE STORAGE M ASK \ SHADOW MASK \ \ SHIELD GRID\\ \ \. 0~ . li ~1 l l , I ll ll ~1 1 I:,,= ~ THREE- BEAM WRITING GUN ool l: ~ Rl NO- TYPE FLOOD GUNK 1~ 1-\ . ~ ~ 7~ ~ COLL IMATOR -18 INCHES Fig. 7. Schematic of the multicolor storage tube. 187 , ~ 1 9 1/4 I NCHES

SHADOW MASK APERTURE STORAGE MASK APERTURE PHOSPHOR COLOR DOT r ~ ~ ~ I ~ ~ V ~Y I ~ ,_ _~/ / Fig. 8. Alignment of shadow mask, storage mask, and phosphor dots in the multicolor store ge tube. 188

BrighI-Steady Electroluminescent Displays Uncler Development* W. L. GARDNER, MIT Lincoln Laboratory Summary-Displays for presenting the output of computers can be improved by utilizing the solid-state processes of electro- luminescence and photoconductivity. Persistent-emission panels and storage panels are of particular interest. Persistent-emission panels, the construction of which is described, have adequate resolution and are superior in their light output and persistence to the phosphors of conventional cathode-ray tubes. The con- struction of storage panels is similar to that of persistent-emission panels, and methods for triggering the pane, are of particular interest. Our group at the Laboratory, working on display techniques, has in the past been associated with display components such as the Charactron tube, the Typotron direct-view storage tube, and high-speed film-processing systems. For some time now' we have been investigating improvements to computer output displays that might be made possible by utilizing the solid-state processes of electroluminescence and photoconductivity. The work can be divided into two areas: (1) the application of electro- luminescent and photoconductive processes to display panels with persistent light-emission characteristics; (2) the application of these processes to storage or memory panels. Persistent-emission panels These panels are distinctly different from the class of memory-display panels (bistable storage cells) in that they exhibit a characteristic steady emis- sion pulse for a controlled period, with a sharp fall-off of emitted light at the end of this period. For applications where repetitive information in short-pulse form is displayed, the overall advantages potentially available are a steady and high light output and automatic cut-off at the cessation of input pulses. Figure 1 shows a comparison of this light-emission characteristic with that of conven- tional phosphors. It can be seen that the light output is higher and holds rela- tively steady for a period of 5 to 7 sec. This period can be varied by panel con- struction and applied-voltage variation. Comparison emission values for PI and P19 phosphors currently used are also shown. The difference in the area under each curve is a measure of the integrated useful light, and this difference shows a possible improvement factor of 50 to 100. :Figure 2 shows the construction of a persistent-emission panel. The ma- terials utilized are capable of withstanding high temperatures and can be fabricated in large, flat-panel construction. The first layer is transparent glass, e.g., the faceplate glass of a cathode-ray tube. The second layer is a transparent electrode of tin oxide. The next layer is an electroluminescent layer, 0.002-in. *The research in this document was supported jointly by the Army, Navy, and Air Force under contract with the Massachusetts Institute of Technology. 189

In a fired glass medium. The next thick, consisting of the phosphor supported in OA~ layer is a 0.001-in. webbing of optically-opaque and electrically-insulating black glass. On this is deposited a 0.005-in. layer of sintered powder cadmium sul- phide. This is baked at 535° C for 15 min. Finally, a transparent top electrode is applied. This is indicated by the metallic strip electrode. 10 us 4 - 4 a 0 1 . 1 \ 200ClS 600 v 7 0 1 2 3 4 5 6 \ Time ( seconds ) Fig. 1. Light emission from a persistent panel. 8 9 With application of 500 c at 300 ~ and with pulse radiation incident on the metallic electrode side, the light-emission characteristic from the corresponding electroluminescent pillar is of the general form shown in Fig. 1. Pulse radiation can be in the form of either an incident electron beam or light emission from an intermediate phosphor. At present' the phosphor method is the most efficient in terms of utilization of beam energy. Figure 3 is a photograph of light emission from an operating panel with a specific light pattern incident. This is a 4-in. X 4-in. panel with cell spacing of 25 cells per in. As Fig. 3 indicates, with this resolution the delineation is still sharp, and the information pattern does not diffuse into adjacent cells. Panels with 40 cells per in. have been made, and it is estimated that 80 cells per in. can be attained without changing present panel-fabrication methods. Photometer traces of the light emission from a persistent panel have been re- corded when a series of four input pulses were spaced over approximately 2~/2 sec and then terminated. Because of the logarithmic response of the eye the fall-off in light during the 2~/2 sec cycle was not significant, and the emission 190-

appeared steady. However, at the cessation of the input pulses the light emission fell quite rapidly, and within 1 see was at the. background level. - O I. ~ Hi. - ~... .~. Fig. 2. Panel construction for a persistent-emission display. Fig. 3. Photograph of operating panel. 191

Figure 4 is a plot of light emission as a function of time for various applied voltages at the fixed frequency. The family of curves demonstrates that light output is increased, and that the shape of the emission curve changes, as the voltage is increased. a 0 4 600 v. us 6 so Q 50~ ~ 1 i, 2 · - 500 c/s 40~\\ \\ \N ~ _ ~ ~ '- --I- - - . 0 3 6 9 Time ~ seconds ~ Fig. 4. Insight emission vs. time for various voltages. Figure 5 shows a similar family of curves for various frequencies at a fixed applied voltage. It is of interest here that the shape of the plateau, i.e., its height and length, can be significantly changed by varying the frequency. On the other hand, the integrated light output, i.e., the total area under each curve, is approximately constant. In general, the voltage and frequency stability re- quired of the power supplies is not at all critical. However, an ability to vary these over a range would be useful for some applications. Storage panels Large, flat displays utilizing electroluminescent and photoconductive components have also been under investigation at the laboratory. These con- sist of an optical storage panel (a flat array of optically bistable cells) with a flat X-Y trigger panel. There are other combinations of light-emitting and voltage-control elements which might constitute a display, e.g., electrolumines- cent cells with ferro-electric or ferro-magnetic control elements. However, we will discuss here only the type of work our group has been concerned with. 192

8e, u] 4 ~ - 4 o o _ 4 4~ o .= 2 :~2000c/s · o 500 v.A.C. 1000 c/s \ \\ 500 c/s Time ( seconds ) Fig. 5. Light emission vs. time for various frequencies. ~200 c/s 9 The optical storage panel resembles the Persistron panel (cf. Fig. 2~. The various layers are as follows: First, there is a supporting glass substrate, and then a transparent tin oxide conductor continuous over the whole panel. The . ~. . ~· · . next layer consists of isolated tin oxide conductors; this is produced by a photo- etch technique. Next is a layer of thin black glass which is optically and elec- trically insulating. ~. The next layer IS 01 sistered powder But, VVI11~11 Ib ~11 :^ of ^;~^Y.^A ~r`~7`rA^~ CA~ `xrh;~h ;~ rare J _ _ , ·. . ~ , veniently formed by a mechanical mask and air abrasive techniques. Finally, a top electrode is applied which is continuous and does not go down into the hole, or cavity. The inside wall of the hole represents a photoconductive element. The square cell of electroluminescent material at the bottom of the hole is a capaci- tive component. The electrical circuit, then, is a resistive component in series with a capacitive component. In the dark, most of the voltage appears across the resistor. When trigger light is incident, the resistance drops and the electro- luminescent cells light up. A portion of the emitted light feeds back on the inside wall of the hole and maintains the resistance in the low state so that the cell remains on. To extinguish the cell, the applied voltage is cut, allowing the dark resistance to be restored, and' when the voltage is reapplied, the cell 193

remains off until triggered again by incident light. Present photoconductive material and available light processes are such that a 1-msec trigger time per cell is apparently feasible. Figure 6 shows one method of triggering an optical memory panel. Trans- parent electrodes' consisting of parallel strips, are crossed with respect to each other on the top and bottom of a thin (0.002-in.) electroluminescent panel. When a voltage wave is applied to the one electrode on each axis, the inter- secting point lights up. Light levels of 100 to 1000 ft-l are achievable with 500 pulses to 50,000 c. A 1-msec envelope of these pulses is sufficient to trigger a corresponding storage cell when the storage panel is adjacent to the trigger panel. . ~ . ::~: Fig. 6. Trigger panel. Unfortunately, with this crossed electrode structure a i/2-voltage wave form appears along the line, and the whole line also lights up with decreased lumi- nance. The contrast between the intersecting point and the line is an important consideration, and' with certain modes of driving, this contrast can be as high as 100 to 1. The driving circuitry, shown in Fig. 67 consists of a saturable core which performs the double function of a voltage transformer and switch. There is one core per line. The core is of the order of 3/8-in. diameter. The transistor driver network is so arranged that only one core at any one time is in the unsaturated state and, therefore, can act as a transformer of the single high- frequency driver. Our present developmental goal is an 8-in. X 8-in. array with 16 cells per in. The array should have 128 cores for each axis. However' because of the binary logic in the transistor drivers considerably fewer transistors than this will be required for each axis. The complete assembly will work from low-level binary computer output signals. The light emission should fall in the 5- to 10 194

ft-l range with good contrast. At this emission level' components have yielded 4000 hr of operation at 100 per cent duty cycle. There are several applications for a display panel with this resolution and writing speed. The combination of trigger panel and storage panel represents a complete display unit. It is an initial step away from the standard CRT display. ., 195

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