Two objects of the same brightness can be distinguished easily if there is a recognizable color contrast, even in low light. The clearest is black against white, the worse red against black or blue; however, few situations have such a clear definition. If the pilot is not sure what is being visualized, she will take more time than necessary to discern what the object or character really is.
These display characters and background displays can also be altered by the pilot within the cockpit, providing another opportunity to clarify the images.
It is obvious that such factors must be considered when designing and using instruments, gauges, maps, and checklists; however, since such design is not always the best or has already been constructed into the system, one must be aware that vision will be compromised under conditions of low light, poor contrast, and inefficient colors.
CORRECTION OF ACUITY
Several variations of the lens affect acuity; acuity means the sharpness, clar- ity, or “keenness” of vision, with perfect acuity meaning that the eye sees or perceives exactly what the object is, regardless of its distance from the eye. Acuity also refers to the ability to discriminate one small character from another when they are very close to each other. Poor vision will see these characters as one instead of several distinct ones. Because focusing and accommodation are relative to the status of the curvature of the lens, we can modify the curvature with artificial lenses (glasses or contacts).
Variations of acuity
Testing for acuity is accomplished by comparing what you can see clearly in specified conditions with what a normal eye with perfect visual acuity can see in the same conditions and distance from the characters. The eye doctor will have you distinguish various characters of different sizes. The bottom line of the eye chart (20/20 or 20/15, depending on the chart) can be read without difficulty by a normal eye; however, as the ability of the eye to see clearly diminishes, the characters must be larger, as in the second line (20/30).
This means that the poorer eye can only see a character size at 20 feet that a normal eye could see at 30 feet. These tests are also done under ideal condi- tions with bright light and black-and-white contrast. This doesn’t imply that an eye that can’t see 20/20 without correction is abnormal; it’s just unable to focus as well.
In terms of percentage decrement in acuity, and 100 percent being normal, consider that with 20/30 acuity there is a 10 percent loss in discrimination of characters and with 20/40 it’s 25 percent. Which means you’re likely to confusen and h,d and o,k and r,v and y, and many others.
Myopia (nearsightedness) is the most common acuity problem (Fig. 7-5). This means distant objects are out of focus without correction. Optically, the focal point of the lens at rest is in front of the retina.
Astigmatism is the unequal and variable curvature of the lens and the cornea that prevents an equal focus of varying distances; that is, light rays are not refractive equally through the lens. For example, a vertical light pole might be in focus, but the horizontal power lines might be out of focus. If one has a evenly spherical cornea and lens, there is no astigmatism.
Presbyopia, is an age related predictable change to the lens, causing a form of farsightedness which increases as we get older. The lens stiffens and is unable to accommodate effectively. The focal point at rest is behind the retina (Fig. 7-6). To be in focus, the object being visualized must be farther from the eye, which, in turn, makes the object smaller. It affects near vision and necessitates “reading glasses” while distant vision remains unchanged. Initial reading glasses are really just magnifiers. There is also less light passing through the older lens because of age-related yellowing, which further inter- feres with depth of field. It is common for the older pilot to have uncorrected distant vision remain at 20/20 but to need glasses to read.
Refractive error
The refractive powers of the lens can be measured to determine how well it can focus. This is accomplished by using a variety of different-strength artifi- cial lenses placed in front of your eye. You then tell the doctor which “supple- mental” lens makes a character clearer on a chart that you are visualizing.
Astigmatism can also be measured.
The measurement of these optical properties (measurements) is stated in diopters, which is the curvature of the lens in different planes. The entire def- inition of the lens’ ability to focus is called the refractive error.Plano is essen- tially perfect refraction. Corrective lenses can be ground using this refractive error when stated as a prescription.
108 Vision
Figure 7-5 Myopia (nearsightedness).
Figure 7-6 Hyperopia (farsightedness).
Correction of acuity 109 It is a good idea to get periodic checks of your vision by an eye doctor and not rely solely on FAA exams. Certainly the key ages where there are changes are in the early 20s and then progressively after age 40, when near vision will change. When it comes to vision in flight, it is crucial that you have strong 20/20, uncorrected or corrected, for both near and distant. The conditions of flight, as subsequently explained in this chapter, will impair vision and necessitate the best vision you can achieve before you begin the flight.
Figure 7-7 shows simulations of what is seen by persons with visual impairments.
Corrective lenses
Once the refractive error is defined, lenses can be ground to make up for the error of refraction, allowing the focal point to be on the retina and in focus.
This correction can be in the form of frame glasses or contact lenses. There are trade-offs. Frame glasses are generally more easily tolerated, whereas contact lenses allow for better peripheral vision. Also, as one gets older and near vision changes, there will be the need to wear glasses for near vision. If one already has “distant” glasses, then bifocals are needed.
Bifocal glasses (one part of the lens ground for distant vision and the other, lower part for near vision) are common, and one needs to get used to them.
Normal Vision Astigmatism
Presbyopia Myopia
Figure 7-7
Figure 7-7 Simulations of several visual impairments. Source: National Eye Institute, National Institutes of Health.
110 Vision Figure 7-7
Figure 7-7 (Continued)
Correction without glasses or contacts 111 Many pilots are reluctant to wear glasses, especially for reading; however, the sooner such glasses are worn as a habit, the easier it will be to adapt to the correction.
A common misconception is that not wearing prescription glasses strength- ens vision. The pilot therefore wears her glasses only when she feels she needs them. This is a poor policy. The eyes are not made stronger by not wearing glasses with the belief that the eyes are forced to maintain acuity.
Although this might be compared to building up muscles by using them more often, this is not the case with the eyes. In fact, eye strain and fatigue result from the eyes and the brain trying to focus unsuccessfully on an image without the help from correction; therefore, if you need glasses for correction, wear them.
There is also the technique of having one eye corrected for distant vision and correcting the other eye just for near vision (monovision correction). The brain, in time (a few hours), will adjust to these two separate sources of infor- mation and will process the two images together. However, the FAA require- ments are for both eyes to be corrected equally, primarily because of the adaptation time of the brain and the pilot not allowing enough time between the two variations of correction. Monovision, therefore, is not legal, and you must acknowledge that when you fill out the FAA medical form. Monocular vision, or using only one eye, corrected or uncorrected to 20/20, is legal (after a period of time of the pilot getting use to one eye) and reviewed by FAA.