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of the display problems of space flight can be solved by conventional techniques, there are certain novel problems remaining to be solved. Many of these will, of course, be worked out during early orbital flights. Additional Visual Problems Related to Space Flight There are several visual problems that do not fall conveniently into any of the preceding sectiona. 1. Monitoring and Vigilance The terms monitoring and vigilance often are used interchange- ably, whereas, in fact, they are really two different problems. In monitoring, the observer usually is dealing with several signals or inputs, and the job is one of continually assessing their state and noting any changes. Vigilance, on the other hand, has been defined as "a probability of signal detection (Jerison, 1959)." In sustained space flight these problems are basically the same as those in conventional craft, except that the responses may be more critical. Although monitoring and vigilance are intimately associated with such complex problems as fatigue, boredom, stress, confinement, isolation, etc., no attempt is made to examine these relationships here. In a space ship, the operator's task in regard to the display system consists chiefly of monitoring. It is essential, therefore, to have well-designed warning or attention-getting signals for purposes of alerting. Inasmuch as the visual system may be utilized continuously for routine operations, 24

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it probably will be desirable to employ various auditory warning signals in addition to the conventional lighting system. Auditory signals have the further advantage that the ear is not as susceptible to anoxia and noxious fumes as is the eye. Therefore, when the astronaut fails to respond to a visual warning signal, an auditory back-up signal could be utilized in extreme emergencies. In addition, it may be desirable to have occasional simulated emergencies which are either rare or of an unexpected nature. For a detailed review of pertinent literature on vigilance and monitoring, see McGrath, Harabedian, fc Buckner, 1959. 2 . Empty Field Myopia About ten years ago Whiteside (1953) determined that in the absence of visible detail in the field, the eye becomes myopic. Con- siderable work since has been performed in this area with respect to visibility and high-altitude flight (Brown, 1953; Miller, 1959; Miller 8t Ludvigh, 1961). "The absence of any other elements of detail in the visual field may prove to be no problem in space flight outside the earth's atmosphere. The lack of atmospheric dispersion will permit constant visibility of stars. These should afford ample cues for distance accommodation (Brown, 1961)." 3. High Contrast "In the absence of any diffusion as found in the earth's atmos- phere, it may be predicted that contrast within the visual field will be much greater than that to which we are normally accustomed. . . .[in 25

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addition to causing problems pertaining to interior illumination and fatigue, the high contrast] may influence judgments of size and distance outside of the space vehicle during landing on bodies having no atmos- phere or during rendezvous with other vehicles. [These problems will become particularly manifest when judgments of absolute size and distance are involved. Such judgments are generally acknowledged to be quite -poor.]" 4. Visual Detection "In searching for relatively small target vehicles in space, the lack of dispersion of light in the absence of an atmosphere!, although creating certain problems in regard to contrast,] may [ actually] prove . . .[to be beneficial.] The brightness of the background will be approxi- mately one-tenth the brightness of the sky on a moonlit night. The object for which the search is being made will be illuminated by re- flected light from the sun and will thus consist of a light spot on a dark field. Visual acuity is not a limiting factor in this case. Visibility will be limited solely by the amount of available energy in the visible range reflected from the target, [in the event, however, that the relative positions of the two vehicles are such that the object to be sighted is not markedly brighter, the problem may be extremely difficult.].. . The probability of detection in this kind of situation will depend upon the actual amount of light reflected from the target, the familiarity of the observer with the star pattern in which the target is to be detected, and the relative size of the visual field in which search must be conducted...." 26

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A recent study by Hall, Brown, Payne, and Rogers (1960) re- vealed that in a situation in which an observer was confronted with various patterns of dots having the same intensities, a large number of dots were seen periodically as being brighter than the others. In- asmuch as all such reports were falae, it suggests that the probability of detection of a space vehicle whose reflected light is similar to var- ious stars in the background will be very low unless high-intensity light sources or reflectors are employed. It would seem also that any such light sources or reflectors should be designed with respect to frequency and wave length so as to present a signal that is easily dis- tinguishable from the background. The problem of visual detection or rendezvous in space is further complicated by the fact that if the two vehicles are in orbit they will be going continually through a light-dark cycle. The existence of this constant fluctuation in illumination raises the question of whether over- haul (rendezvous) should be made in the light or dark phase. Studies should be performed to determine the important parameters involved in three-dimensional tracking in both illuminated and darkened visual fields. 5. Perception of Motion As suggested above, achieving a rendezvous in space with an object or orbiting vehicle is an exceedingly difficult task. Other problem areas which are involved are discussed in the next Section. If the visual capabilities of man are to be utilized in the terminal 27

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stages of such a task, it will be necessary for him to perform such functions as estimating closing rates, direction of movement, and other functions pertaining specifically to the problem of motion percep- tion. This is particularly true in the event that man attempts to navi- gate from one vehicle to another, using some type of self-contained propulsion system. Although motion perception has been studied extensively by numerous investigators, very little work has been done in which the test objects were systematically viewed against a homogeneous back- ground, either empty or uniformly patterned. Several pertinent ex- periments, however, were conducted by Duncker (1929) and later by Oppenheimer (1934). These workers investigated such problems aa the perception of the direction of motion and the relative motion of two targets in a totally dark room. More recently, similar experi- ments have been conducted by Miller and Hall (1961), Miller and Ludvigh (1961), and Baker (1960). Baker's work concerns the ability of an observer to judge whether an object in an empty visual field is approaching or receding as a function of velocity and visual angle, while the work of Miller and Ludvigh concerns the displacement threshold and the relative motion of two objects in the Ganzfeld. Much more research is needed in this area in order to determine the poten- tial capabilities of a man in tasks of this type. 28