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INTRODUCTI ON

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OPENING REMARKS - Brigadier General F. Doppelt United States Air Force School of Aerospace Medicine First, I'd like to thank you for inviting me. It's a great privi- lege to be here. Vision has been a very key part of all of our military operations and has been a singular interest during my entire military career. On behalf of Colonel Davis, USAF/SAM Commander, I'd like to congratulate Colonel Tredici for doing an absolutely outstanding job in the world of aviation vision. Participation with the National Research Council's Committee on Vision has been a part of the activities of the Air Force and speci- fically the School of Aerospace Medicine for at least 40 years. The Armed Forces Vision Tester participation on significant committees that have worked on the use of drugs and their effect on vision, etc., are but a few singular accomplishments. In fact, we're all aware that when the School of Aviation Medicine, a laboratory in Long Island circa 1917, was first formed as one of their key leaders, Dr. Wilmer, an ophthalmol- ogist, was associated very prominently with aviation vision. As I look at the Aerospace Medical Division's many and diverse missions from aircrew health aspects to human centered technology and research development, a tremendous amount of what we do involves vision. Whether it's designing simulators, displays, night vision goggles, or working specific target identification tasks, vision is the key to almost everything the military operator does. We are visual animals for much of what we do is interpreted in that primary language of pictorial representations. If you shut your eyes, you can probably remember what this classroom looks like very, very well, because it's imprinted in a very "virtual" way in your mind. As engineers, we go out and put that information on knobs and dials and try to force the operator to relate the picture in his brain to some button, dial, or instrument. If you will, it goes through a double level of "virtual" information transformation. I remember back in 1965 when I first became associated with the space program, astronaut Gordon Cooper made some interesting visual observations in an early space flight which created some scientific controversy. As you may remember during a pass over Baja, he claimed to have seen a truck running down the center of the peninsula. We looked at the optics of the situation and concluded that he could not possibly have seen that truck running down the road. Well indeed, astronaut Cooper d id see the truck, for we later found out that there 17

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18 was such a truck traveling about thirty or forty miles an hour on a dirt road in the center of Baja. What astronaut Cooper saw was the cloud raised by the traveling vehicle. I'm sure he was able to compute its motion and properly concluded that it was a truck. So he used this visual process not only to see a piece of information but to interpret it and come up with what man does best--pattern recognition and inter- polation of that information into a meaningful identif ication We're in a world today where we know that our aircraft cannot oper- ate solely during the day. In fact, we understand very, very well that our adversaries are capable of operating at night, as demonstrated in Vietnam. We must develop technology which will make it difficult for our adversaries to assume that same nighttime safe harbor. We need to better understand how to marry sensor information, optics, man, and his machine in a way that will give us the same kind of daytime effectivity of performance at night that our operations need. ~ devoted to vision at night is really critical at this time. be busy at night, you need to keep us busy at night. We gain a tremen- dous advantage by being able to operate more credibly and more properly at night. You need to deal with how well does the eye see at night, how well have we selected the right crew person, and how well have we optimized the way in which he' s able to see and focus at night, through p roper equ ipment design. I congratulate Colonel Tredici and all the f ine people that are here today to put your best "eyes" together to develop night visual s tandards and perhaps a new night vision tester . Alto nave a conference He need to

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AMBIENT ILLUMINANCE DURING TWILIGHT AND FROM THE MOON Herschel W. Leibowitz The geometry of twilight, the transition between night and daylight and daylight and night, has been precisely defined in terms of the posi- tion of the sun with respect to the horizon. Sunrise occurs when the upper limb of the sun becomes visible. Sunset occurs when the upper limb of the sun becomes invisible. Twilight ends/begins when the cen- ter of the sun is 18 degrees below the horizon. During daylight, ambient illuminance is essentially independent of solar position until the sun falls to within 5 to 10 degrees of the hor- izon (Rozenberg, 1966~. The ambient illuminance at sunset/sunrise is about 30 footcandJes,* which is near the maximum of contrast-dependent functions such as visual resolution, so that variations in illuminance before sunset and after sunrise would not be expected to have behav- ioral consequences. Figure 1 presents ambient illuminance between sunrise/sunset and night as a function of the zenith distance of the center of the sun. During twilight ambient illuminance changes by almost 6 orders of magnitude. Between sunset/sunrise and a zenith distance of 103 degrees this function is approximately linear on a semi-logarithmic plot. Twilight is divided into three subperiods. Civil twilight is the period between sunset/sunrise and the time the center of the sun is 6 degrees below the horizon. It is generally assumed that "normal out- door activities" may be carried out during civil twilight. This is a reasonable expectation at sunset and sunrise, when ambient illuminance values are 30 footcandles. However, in view of the low illuminance levels encountered at the beginning of morning and the end of evening civil twilight, about 0.3 footcandle, one would expect significant Supported in part by grant EY 03276 from the National Eye Institute and by a grant from the U.S. Naval Development Center, Warminster, Pa. *All photometric values in this paper are given in terms of illumi- nance. The literature on visual performance is described in terms of luminance units, which specify the light useful for activating the vis- ual system. Luminance is the product of illuminance and the percent reflectance (albedo) of the object in question. 19

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20 degradation of visual resolution and other functions that are dependent on contrast sensitivity, as well as an increase in reaction time. How- ever, as a consequence of the selective degradation of recognition and orientation vision that occurs with a reduction in luminance, visual guidance and spatial orientation should be unaffected (Leibowitz and Owens, 1977~. Nautical twilight refers to the period when the center of the sun is between 6 and 12 degrees below the horizon. When the sun's center is 12 degrees below the horizon, marking the beginning of morning and the end of evening nautical twilight, ambient illuminance is scotopic, less than 0.003 footcandle. It is too dark to see the sea horizon precisely enough for determination of altitudes for navigation, but visually guided orientation should be possible under these conditions as well as some form perception in the peripheral visual fields. m e period when the sun is between 12 and 18 degrees below the hor- izon defines astronomical twilight. The sky is still light but there is very little ambient illuminance. When the sun is 18 degrees or more below the horizon, the indirect light from the sun is less than that provided by stars (0.0001 footcandle) and about the same as from airglow, zodiacal light, and the gegenschein. Illuminance levels are close to the absolute scotopic threshold of the dark-adapted human eye. 10 1 ,_ ' Lit In I I . ~ w~ ! Z l . . ok lo-2 l slow '' . _^ , lll 0~ 5~5eT . L = iCtVIL ~1 LIGHT _ j \ . I \ I \ I, 1 \ l \ \ \NA~ L l MALI HT ~I ~ S=ONOMIC)U ! l . ~, ,, 900 950 of. lose ~ lo ZENITH DISTA`E ~ SUN FIGURE 1 Ambient illuminance during twilight, measured on a horizon- tal surface, as a function of the zenith distance of the sun. Due to atmospheric refraction, the sun is visible at a zenith distance g reater than 9 ~ deg rees. Source: Explanatory Supplement.

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21 TABLE 1 Estimated Illuminance from the Moona E long at ion (degree s) 180 (full moon) 160 140 120 90 (half-illuminated moon) 60 40 20 Est imated I 1 luminance (footcandle) 0.02 0.013 0.008 0.005 0.002 0.0006 0.0002 0.00002 aBased on information from the Exploratory Supplement (1961~. Values overestimate ambient illuminance when the moon approaches the horizon. As a consequence of the changing inclination of the sun's apparent path through the sky and the horizon, the duration of twilight depends on both the latitude and the date. Near the equator, the sun's path is nearly perpendicular to the horizon. As a consequence, the sun's posi- tion changes rapidly with respect to the horizon and twilight is of relatively short duration. At 10 degrees north latitude (southern Philippines, Costa Rica, central Ethiopia), the duration of civil twi- light is between 21 and 23 min. At 40 degrees north latitude (New York, Rome, northern Japan), the range is from 27 to 32 min. At 60 degrees north latitude (Stockholm; Leningrad; Juneau, Alaska), the duration ranges from 40 to 106 min. The beginning and end of the various twi- light periods for any location on the earth's surface (altitude above sea level must also be taken into account) (Exploratory Supplement, 1961) can be interpolated to within 1-min accuracy by means of tables in The Astronomical Almanac published annually and jointly by the U.S. and British governments (Washington, D.C.: U.S. Government Printing Office; London: Her Majesty's Stationery Office). It is of interest to note that at 40 degrees north latitude, ambient illuminance from the sky during civil and nautical twilights changes by a factor of 2 every 4 to 5 min (depending on the date). Moonlight can have significant behavioral consequences during nautical and astronomical twilight and at night. For a full moon at the zenith, the illuminance on a horizontal surface is 0.02 footcandle, which approximates the ambient illuminance levels at the middle of nautical twilight. After the full moon, the illuminance from the moon falls off rapidly. Table 1 presents the estimated illuminance as a function of elongation (angular distance from the sun) based on infor- mation from the Exploratory Supplement (1961~. Moon phase, rising, and setting times can also be found In The Astronomical Almanac. -

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22 Meteorological and atmospheric conditions can have a significant effect on the quantity of light available during twilight and f rom the moon. Although the quantity of light f rom the sky during twilight and from the moon can be estimated accurately, the behavioral consequences of the changes in ambient illuminance have not been systematically docu- mented in relation to the positions of the sun and the moon. REFERENCES Explanatory Supplement to the Astronomical Ephemeris and the American , . Ephemeris and Nautical Almanac. 1961 London: Her Majesty' s Stationery Office, 1961. Leibowitz, H.W., and D.A. Owens 1977 Science 197: 4302. Rozenberg, Georgii V 1966 Twilight. . New York: Plenum.