Aeromedical Factors

The cones concentrated in the fovea centralis of the eye are primarily responsible for vision in bright light. [Figure 17-12] Center of vision Because of the high light level, rhodopsin, which is a biological pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light, is bleached out causing the rod Blind spot cells to become less effective. Pupil Mesopic Vision Retina Mesopic vision is achieved by a combination of rods and cones and is experienced at dawn, dusk, and during full moonlight. Visual acuity steadily decreases as available light Optic nerve decreases and color perception changes because the cones LEFT Right become less effective. Mesopic viewing period is considered the most dangerous period for viewing. As cone sensitivity Figure 17-14. Central blind spot. decreases, pilots should use off-center vision and proper scanning techniques to detect objects during low-light levels. has in each eye. Under normal binocular vision conditions (both eyes are used together), this is not a problem because Scotopic Vision an object cannot be in the blind spot of both eyes at the same Scotopic vision is experienced under low-light levels and time. On the other hand, where the field of vision of one eye the cones become ineffective, resulting in poor resolution of is obstructed by an object (windshield divider or another detail. Visual acuity decreases to 20/200 or less and enables aircraft), a visual target could fall in the blind spot of the a person to see only objects the size of or larger than the other eye and remain undetected. big “E” on visual acuity testing charts from 20 feet away. In other words, a person must stand at 20 feet to see what Figure 17-15 provides a dramatic example of the eye’s can normally be seen at 200 feet under daylight conditions. blind spot. When using scotopic vision, color perception is lost and a 1. Hold this page at an arm’s length. night blind spot in the central field of view appears at low light levels when the cone-cell sensitivity is lost. 2. Completely cover your left eye (without closing or pressing on it) using your hand or other flat object. Central Blind Spot 3. With your right eye, stare directly at the airplane on The area where the optic nerve connects to the retina in the the left side of the picture page. In your periphery, you back of each eye is known as the optic disk. There is a total will notice the black X on the right side of the picture. absence of cones and rods in this area, and consequently, 4. Slowly move the page closer to you while continuing each eye is completely blind in this spot. [Figure 17-14] to stare at the airplane. As a result, it is referred to as the blind spot that everyone Figure 17-15. The eye’s blind spot. 17-21