Eyes and Physiology of Vision

FAA Vision Standards

The visual acuity standards for medical certification by the FAA are very clear. First and Second Class medical certification require 20/20 vision with correction at distant and 20/40 at near. Third class certification only requires 20/40 corrected vision at distant and 20/40 at near. Distant vision is measured at the equivalent of 20 feet. Near vision is measured at 16 inches. For pilots aged 50 years and older, intermediate vision measured at 32 inches must correct to 20/40 or better. The previous uncorrected visual acuity standard at distant (20/100) was dropped in the September 1996 revision of Part 67 of the FARs. Currently, there are no uncorrected vision standards in the FARs. This means a pilot’s vision at distance could be 20/400, but as long as it corrects to 20/20 in each eye, the pilot meets the Part 67.103 vision standards for First Class medical certification.

With FAA order 3930.3B ATC vision standards were made similar to airman standards. With or without correction air traffic controllers must demonstrate 20/20 distant vision in each eye separately, 20/40 in each eye at 16 inches near vision, and 20/40 in each eye at 32 inches intermediate vision if they are 50 years of age or older. Glasses or contact lenses are permitted.

Anatomy of the Eye

The eye functions very much like a camera and the anatomy of the eye is similar to the camera’s structure. The cornea is a clear structure that covers the front of the eye, similar to a filter on a camera. Changes in the shape or curve of the cornea are responsible for many defects in visual acuity. Behind the cornea is the iris, the colored portion of the eye that changes size to accommodate changes in lighting conditions. This is similar to the diaphragm that sets the aperture in a camera. The empty center of the iris is the pupil. Immediately behind the pupil and iris is the lens. The lens can change shape to adjust the focus for near and distant objects, similar to the moving lens in a camera. As we age, the lens loses some of its ability to change shape and to focus on near objects. The back of the eye is lined by the retina, similar to film in a camera. The retina has rods and cones that function in various lighting conditions and perceive color. The central portion of the retina is called the macula or fovea, which is the area of sharpest vision. Nerves from the retina join at the back of the eye in the optic nerve. The optic nerves carry the information from the eye to the rear part of the brain, called the optical cortex. This is where images from both eyes are fused into a single image. Damage to any portion of this system will adversely affect vision or visual acuity.

How The Eye Focuses

Moving from the outer portion of the eye to inner, light passes through several structures before falling on the retina where the brain may process the light as a visual image. First, light passes through the cornea, the clear portion of the outer eye that contact lenses are placed on. This area is sensitive to touch. It usually remains clear unless there is an infection with scarring, trauma or vision correction surgery with complications. The normal cornea contributes approximately 45 diopters of focusing power to the eye. Diopters are a unit of measure describing the ability to bend light. The other structure to contribute to focusing light is the lens. The lens lies just under the colored iris and forms the back of the anterior chamber and the front of the posterior chamber. The lens is visible through the opening in the iris called the pupil. The lens of a young person can provide a range from one to up to 20 diopters of focusing power. This ability is termed accommodation. As people age and the lens stiffens, the focusing power of the lens falls off to a constant value near one diopter and the ability to focus at near without corrective lenses degrades. The inner lining of the back portion of the eye is the retina. Light must be focused on the surface of the retina for the brain to perceive a clear image.

Measuring Visual Acuity

Visual acuity is measured on the Snellen chart comparing the arc seconds (similar to degrees) of resolution the individual can discriminate compared to the “normal” individual. The tested individual is listed first and is expressed as a baseline distance of 20 units . The “normal” standard is listed second as the number of units away from the target a “normal ” individual would be able to read the same baseline target. If the visual acuity is normal, the reading is 20/20. Impaired acuity at the 20/80 level means the normal person can read at 80 feet what the tested individual can only read at 20 feet. For those individuals with better than normal vision, the visual acuity may read 20/15. In other words, they can read at 20 feet what most people can only read at 15 feet.

Refractive Errors – Myopia, Hyperopia, Astigmatism and Presbyopia

The inability to have “normal vision without correction may arise from many causes. The causes due to errors in refraction (focusing ability) are called myopia, hyperopia, astigmatism and presbyopia. Myopia is also known as “near-sightedness” and hyperopia is known as “far-sightedness.” Astigmatism is the distortion of an image due to irregularities in the curve of the cornea. Presbyopia is the progressive loss of the ability to focus at near due to stiffening of the lens with aging.

Presbyopia

Aging causes several effects in the eye. One is a stiffening of the lens and a progressive loss of the ability to change shapes. This stiffening of the lens with age is termed presbyopia. The lens is a convex disk that is held in place by ciliary muscles. The outer lining of the lens is called the capsule. The lens changes shape by contracting and relaxing the ciliary muscles to change the focal distance of the eye by making the lens thinner or thicker (accommodating). The younger lens can change shapes rather dramatically allowing reading at very close distances when the lens is thicker. As one ages, the individual finds that reading must be done at greater distances initially and then with the aid of reading glasses. The reading glasses add one half to several diopters of focusing power allowing more bending of light coming from close distances. Presbyopia is a universal phenomenon.

Implications of Presbyopia for Pilots & Controllers

Pilots who require reading glasses may note some additional problems in the cockpit. Traditional reading glasses “add” diopters in the lower visual field where most people hold reading material. Reading approach plates usually is aided by the traditional reading glasses. The instrument panel is at a further distance. This is the rationale behind the FAA intermediate vision standard measured at 32 inches. Some pilots may require a trifocal lens to see at near, at intermediate and at distant. Aircraft with overhead panels may necessitate a reading lenses put in the top of the glasses to clearly visualize all the overhead instruments. The term for this type of glasses is the “double D”, named for the two D shaped reading lenses in the upper and lower portions of the glasses. The central portion of the glasses may not need any correction. the glass in this portion of the spectacles will be flat or “plano” and will not add any diopters of correction. Controllers may also experience some difficulty finding the correct prescription to accommodate various work stations.

Monovision and Multifocal Contact Lenses

The FAA continues to prohibit the use of monovision contact lenses. With this technique, one contact lens is worn to focus at near for reading purposes and the other lens focuses at distant. Some eye specialists prescribe this combination to individuals who don’t want to wear reading glasses. This combination may degrade stereo acuity, binocular component to depth perception. The NTSB listed a pilot’s use of monovision contact lenses as a finding in a commercial carrier mishap, although there was considerable debate and disagreement from other eye specialists and aviation physicians about the validity of this finding.

More recently the FAA modified its stance on monovision correction slightly. After a six month adjustment period allowing the individual to adjust to new depth perception ques, the FAA has been willing to consider certification for those pilots that have monovision due to refractive surgery. Often this also requires a medical flight test. As noted, monovision correction with contacts is still prohibited and controllers have not typically been allowed monovision correction at all.

The FAA has a policy allowing pilots who receive monovision refractive surgery to fly with glasses that correct for distance and near for the first six months after surgery. After the six month adaptation period elapses, pilots may then fly without glasses, assuming one eye corrects to distant standards and the other eye corrects to near vision standards.

The FAA also approves the use of multifocal contact lenses which correct for near on the periphery and distance in the central portion for pilots. The protocol requires a one month adjustment period, no visual defects, and must meet FAA vision standards. Similarly, accommodative lenses replacement for cataracts was approved.

Myopia – Nearsightedness

Myopia, or nearsightedness, is the condition in which the eye has too much refractive power or the globe of the eyeball is too long. Light coming into the eye is bent to such a degree that the image focuses in front of the retina and begins to blur again by the time it strikes the retina. This excessive refractive power comes from a cornea that has a steeper curve than the normal eye. In essence, the eyeball is too long for the cornea. Because the lens adds refractive power when it thickens, it will not correct the situation. Distant objects that require little refraction to focus on the retina are thus blurred. Near objects which require more refractive power focus more easily on the retina.

Myopia may begin manifesting during adolescence. Concave lenses (negative diopters) correct the situation. Myopia may increase through the mid-20’s when the individual’s refractive prescription usually stabilizes. Because of the natural excess refractive power of the cornea, myopic individuals often do not need to use reading glasses for presbyopia until their late 40’s to mid 50’s. Contact lenses are frequently used by myopic individuals for a variety of reasons. These contact lenses are concave with thicker edges and a thinner middle. The greater the difference between the periphery and the middle, the greater the refraction correction. Many people with myopia are now considering surgical vision correction. Please see our article in the Information Resource section on Vision Correction Surgery for a full discussion.

Hyperopia – Farsightedness

Hyperopia is the condition where the cornea has too little refractive power or the eyeball is too short and images may be focused behind the retina. Only severe cases manifest themselves in early ages because of the ability of the lens to compensate. As the muscles of the lens relax, the lens thickens and ads refractive power. These individuals may see very well without glasses until the lens begins to lose its refractive power (accommodation) with age. They may require reading glasses by their early 40’s. They retain their ability to see at distance without correction with age.

Astigmatism

Astigmatism is the term used to describe uneven refractive power over different parts of the cornea. Essentially, the cornea is warped. In the absence of a significant refractive error (myopia or hyperopia), astigmatic individuals may see well in bright light when the pupil is small. Night vision is blurred as the pupil dilates and light passes through the warped periphery of the cornea. The degree of warping is measured in diopters versus a 360 degree axis with 0 degrees at the top of the cornea. Corrective lenses have a reciprocal warping to compensate.

Vision Correction Prescriptions

Prescriptions for corrective lenses may have up to four components. The first component is the refractive error. This measures the amount of curve in the corrective lenses to compensate for myopia or hyperopia. Since the myopic cornea has excessive refractive power or diopters, the prescription is written in terms of negative diopters of correction, e.g., -2.25. A correction for hyperopia might read +1.50 since additional diopters are needed to focus clearly.

The second and third components of a refractive prescription deal with astigmatism and are called cylindrical errors. Again the cylindrical error component of astigmatism is measured in diopters, but the location of the most significant warping is given in degrees, e.g., -0.75 x 145. This would be for simple astigmatism without hyperopia or myopia.

If either of these conditions coexist with astigmatism, the refractive error is listed first followed by the cylindrical error, e.g., -1.50 – 0.50 x 080. To confuse prescription writing, the same cylindrical error may be expressed in positive or negative numbers depending on the degrees of axis measured. In other words, is the cornea “warped in” on a North-South axis or is it “warped out” in an East-West axis. Both are the same but the underlying refractive error changes depending on whether the North-South or East-West axis is used for the baseline.

The final component of a corrective lens prescription is the accommodation or correction for presbyopia. This denotes the curve of the lens to be used in the bifocal portion of the lenses. It is usually termed an “add” since presbyopia always requires additional (positive diopters) refractive power. The Add is always listed last in the prescription.

Finally, the designation for the right eye is O.D. while the left eye is denoted by the abbreviation O.S. When discussing the vision using both eyes, the term O.U. is used. An example of an individual with 2.50 diopters of myopia, 0.75 additional negative diopters of astigmatism in a horizontal axis and presbyopia requiring 1.25 diopters of add for the bifocal in the right eye with only a 1.50 presbyopic add in the left eye (highly unlikely scenario) would read:

-2.50 -0.75 x 090o +1.25 O.D. Plano +1.50 O.S.

Phorias and Tropias

For best binocular vision and depth perception, both eyes should be able to focus simultaneously on an object at any given distance. The tendency for one eye to deviate away from the desired focus point, either by lateral of vertical movement compared to the other eye, is termed a phoria or a tropia. A phoria manifests when binocular vision is interrupted or with fatigue, while a tropia is a constant deviation of one eye away from the visual axis of the other eye. One example of a tropia manifesting occurs when an individual is tired and not focusing on a particular object. A double image may form, but the individual may quickly fuse the images with a blink or conscious effort to view the object.

The FAA has limits of phorias for first and Second class as well as ATC certification. No standard exists for Third Class certification. Phorias in which the eye tends to deviate inwards and outwards are called esophorias and exophorias, respectively. Six prism diopters is the limit for FAA certification of lateral phorias. One diopter of vertical phoria, termed hyperphoria, is allowed. AMEs may issue medical certificates to pilots who do not meet these standards, but an evaluation eye specialist is required to determine adequate bifoveal vision is present for binocular depth perception.

FAA Medical Certificate Vision Limitations

Limitations on FAA medical certificates are standardized depending of the type of vision correction an individual requires to meet the standards. For myopic pilots/controllers who cannot see 20/20 at distant in each eye without correction, the limitation on the FAA form 8500-8 airman’s Medical Certificate should read, “Must wear corrective lenses.” Corrective lenses can be either glasses or contact lenses. For presbyopic pilots/controllers with otherwise normal vision, the limitation should read, “Must possess corrective glasses for near vision.” In other words, the lenses must be available in the aircraft for reading but do not need to be worn continuously.

For those pilots/controllers with both near and distant corrective requirements, the limitation is “Must wear corrective lenses and must posses glasses for near vision.” Both bifocals or contact lenses for distant used with reading glasses for near meet this requirement. Again, use of multifocal contact lenses is allowed after 30 days adjustment, and monovision contacts are prohibited.

AMAS Aeromedical Assistance

For a more specific personal explanation to your questions or those concerning aeromedical certification, contact AMAS for a private consultation. For help in reporting treatment for and obtaining clearance from the FAA to fly or control with these conditions, refer to the AMAS FAA Confidential Questionnaire. If you are a AMAS Corporate Member, these services are FREE to you.