Aeromedical Factors

The rods and cones (film) of Rods and the retina are cones the receptors which record the image and transmit it through the Fovea optic nerve to Fovea centralis (all cones) the brain for interpretation. Rod Lens Optic disk concentration (blind spot) Iris Optic nerve Retina PUPIL CORNEA The pupil (aperture) is the opening at Light passes through the cornea (the the center of the iris. The size of the transparent window on the front of the pupil is adjusted to control the amount eye) and then through the lens to of light entering the eye. focus on the retina. Figure 17-12. The human eye. light energy into electrical impulses that travel through nerves that a large amount of light overwhelms them, and they take to the brain. The brain interprets the electrical signals to form longer to “reset” and adapt to the dark again. There are so images. There are two kinds of light-sensitive cells in the many cones in the fovea that are at the very center of the eyes: rods and cones. [Figure 17-12] visual field but virtually has no rods at all. So in low light, the middle of the visual field is not very sensitive, but farther The cones are responsible for all color vision, from from the fovea, the rods are more numerous and provide the appreciating a glorious sunset to discerning the subtle shades major portion of night vision. in a fine painting. Cones are present throughout the retina, but are concentrated toward the center of the field of vision at the Vision Types back of the retina. There is a small pit called the fovea where There are three types of vision: photopic, mesopic, and almost all the light sensing cells are cones. This is the area scotopic. Each type functions under different sensory stimuli where most “looking” occurs (the center of the visual field or ambient light conditions. [Figure 17-13] where detail, color sensitivity, and resolution are highest). Photopic Vision While the cones and their associated nerves are well suited Photopic vision provides the capability for seeing color and to detecting fine detail and color in high light levels, the resolving fine detail (20/20 or better), but it functions only rods are better able to detect movement and provide vision in good illumination. Photopic vision is experienced during in dim light. The rods are unable to discern color but are daylight or when a high level of artificial illumination exists. very sensitive at low-light levels. The trouble with rods is Types of Vision Types of vision used Light level Technique of viewing Color perception Receptors used Acuity best Blind spot Photopic High Central Good Cones 20/20 Day Mesopic Medium/Low Both Some Cones/Rods Varies Day/Night Scotopic Low Scanning None Rods 20/200 Day/Night Figure 17-13. Types of vision. 17-20