Aeromedical Factors

Geometric Perspective make an obvious difference in the viewing angle of both eyes. An object may appear to have a different shape when viewed In the flight environment, most distances outside the cockpit at varying distances and from different angles. Geometric are so great that binocular cues are of little, if any, value. In perspective cues include linear perspective, apparent addition, binocular cues operate on a more subconscious foreshortening, and vertical position in the field. level than monocular cues and are performed automatically. • Linear perspective—parallel lines, such as runway Night Vision Illusions lights, power lines and railroad tracks, tend to There are many different types of visual illusions that converge as distance from the observer increases. commonly occur at night. Anticipating and maintaining [Figure 17-21A] awareness of them is usually the best way to avoid them. • Apparent foreshortening—the true shape of an object or a terrain feature appears elliptical when viewed Autokinesis from a distance. [Figure 17-21B] Autokinesis is caused by staring at a single point of light • Vertical position in the field—objects or terrain against a dark background for more than a few seconds. features farther away from the observer appear higher After a few moments, the light appears to move on its own. on the horizon than those closer to the observer. Apparent movement of the light source will begin in about [Figure 17-21C] 8 to 10 seconds. To prevent this illusion, focus the eyes on objects at varying distances and avoid fixating on one source Aerial Perspective of light. This illusion can be eliminated or reduced by visual The clarity of an object and the shadow cast by it are scanning, by increasing the number of lights, or by varying perceived by the brain and are cues for estimating distance. the light intensity. The most important of the three solutions Subtle variations in color or shade are clearer the closer the is visual scanning. A light or lights should not be stared at observer is to an object. However, as distance increases, for more than 10 seconds. these distinctions may become blurry. The same applies to an object detail or texture. As a person gets farther from an False Horizon object, its discrete details become less apparent. Another A false horizon can occur when the natural horizon is important fact to remember while flying at night is that every obscured or not readily apparent. It can be generated by object casts a shadow from a light source. The direction in confusing bright stars and city lights. It can also occur while which the shadow is cast depends on the position of the light flying toward the shore of an ocean or a large lake. Because source. If the shadow of an object is cast toward the observer, of the relative darkness of the water, the lights along the the object is closer than the light source is to the observer. shoreline can be mistaken for stars in the sky. [Figure 17-22] Binocular Cues Reversible Perspective Illusion Binocular cues of an object are dependent upon the slightly At night, an aircraft may appear to be moving away from different viewing angle of each eye of an object. Binocular a second aircraft when it is, in fact, approaching a second perception is useful only when the object is close enough to aircraft. This illusion often occurs when an aircraft is flying A B C Figure 17-21. Geometric perspective. 17-26