Anatomically considered an extension of the brain, the eye is the biological camera for the human body. Focusing light energy and translating it into electrical energy then transmitting that data to the brain via the optic nerve is the complex process known more commonly as eye-sight.
Light first passes through the outer layer of the eyeball called the cornea. Working as a “lens cap” for the eyeball it also has refractive (bending) properties that help focus the light entering the eye. Next, light rays entering the eye are refracted by the lens and focused onto the retina. The retina is comparable to the film in a camera. Focus can be changed by ciliary muscles that surround the lens and with contraction or relaxation can change the shape of the lens which affects focus.
The colored membrane that determines eye color is the iris and the center portion of the iris is the pupil. By modulating the size of the pupil the eye can control how much light is allowed to enter and changes the depth of field. Allowing more light into the eye by increasing the size of the pupil, such as in low light situations, the depth of field is decreased. This means the nearest and farthest distances within which everything is in focus is shortened. This modulation is used primarily to make vision easier in high or low light situations.
Once allowed to enter the eye and focused, the light energy meets the retina, or the “film” of the eye. The retina contains cells that translate light energy into electrical energy. Using cone cells to sense bright lights and colors and rod cells for low light or night vision, this data is transmitted to the brain via the optic nerve. Rods are used primarily as peripheral vision and are considered to be 10,000 times more sensitive to light than the cones (which is the reason peripheral vision, or off-set viewing, is recommended for night time traffic scanning). Where the optic nerve is formed on the retina is devoid of cones and rods which results in a blind spot. Each eye compensates for the other eye’s blind spot.
There are several visual impairments that can either develop or be genetically inherited. Most common is myopia, or nearsightedness. This is caused by the focal point of the lens being in front of the retina. Conversely, presbyopia is a form of farsightedness caused by the stiffening of the lens makes accommodation (the ability to change the focal point) difficult. This requires objects to be farther away from the eye to be in focus which could result in poor vision up close yet still yield good distant vision. Correction for this involves reading glasses which magnifies close up objects allowing them to be focused.
An additional impairment to visual acuity is called astigmatism, or an unequal and variable curvature of the lens and the cornea that prevents an equal focus at varying distances. This causes light rays to refract unequally through the lens creating varying levels of focus in either eye. This condition can be mediated or completely rectified with glasses, contact lenses or a vision correction procedure.
Along with refraction and focus irregularities, vision can be impaired by lens clarity. In some cases the lens can become opaque resulting in a cataract. Cataracts may form by age alone or extended exposure to ultra-violet rays absorbed while flying at high altitudes for long periods of time. In addition, less light may pass through the lens due to age-related yellowing which interferes with the depth of field. Prevention is the best method by wearing UV filtering sunglasses that block the full ultraviolet range. Once developed cataracts can be removed and replaced with surgery.
Finally, the eye can even be affected temporarily by factors that are known to affect the brain in the same manner. Hypoxia, fatigue, carbon monoxide, or toxins can severely impair the eyes ability to see clearly. Hypoxia increases time required for adaptation to night vision and can occur at altitudes as low as 5,000 feet. Carbon monoxide build up in the blood affects the pilot in the same way hypoxia does because carbon monoxide blocks the hemoglobin’s ability to attach to and carry oxygen. Fatigue can affect mental alertness and visual recognition, which could impair a pilot’s ability to scan, maintain focus, or even impair judgment. Toxins, such as alcohol, create a state of histotoxic hypoxia that can reduce visual acuity well past when the last drink was consumed.
These various components that work together, mostly unconsciously, allow us to operate visually in the cockpit and on a day-to-day basis. Maintaining a healthy lifestyle as well as taking precautions to protect and preserve your vision will ensure a long and safe career in the cockpit.