The Eye
Of all our senses, vision is our most reliable sense. However, in conjunction with our other senses it does disrupt and affect the way we interpret what we think we are seeing. The eye's sense of up, down, left, right and level can come in conflict with these senses and cause vertigo.

Your one inch eyeball is more capable than any camera to make lens changes that focus and re-focus light arriving through the cornea, pupil and lens on the rear interior of the eyeball called the retina. All these adjustments are muscular. The functions of the eye are various; you have a central vision, a peripheral vision, coordination and balance information, and the sleep clock.

The convex lens inverts the image on the retina which consists of multiple layers (10) of sensors that make the conversion from light to electrical stimuli applied to both cones and rods. The cones electrically respond to both light and color input. The brighter the light the better the cone response. Cones exist mostly in the central fovea area and around the outer region. Rods are only light sensitive but are able to react to very little light but with no color. Rods are not at the fovea and are most numerous in an arc 20 degrees from it. Looking directly at an object at night in poor light will not allow you see it. The eyes see a 160-degree spherical arc with only a 5-degree cone of focus. The cone of 10-degrees; cannot see better than 20/100. We can only see three colors, red, green and blue. The brain mixes these to 'see' all other colors.

The eye has two lenses, one fixed and one variable. The outer part of the eye 'window' called the cornea is the fixed lens. Muscles that change its shape by making it thinner or thicker adjust the interior lens To focus as needed. There is no surgical correction for the effect age has on the ability of the eyes to focus.

Light adaptation occurs 90% in the first half-hour but will continue for an hour with the rods. Rods can detect low light levels down to that of a full moon. You cannot see using the center 5 degrees of focus. This adaptation can be destroyed in an instant by a bright light. Ability to keep one eye closed when using a light in the cockpit is a handy skill.

You have motion sensors in the ears and neck that stabilize the eyes to see in one place when you move your head sideways. These stabilizers do not work up and down. You cannot see while moving your eyes. All you will get is a gray blur.

There is a type of nearsighted deficiency that is common at night. Detection of this requires a special test that is worthwhile if continuous night flying is a project. Night vision depends on rhodopsin, commonly called visual purple. The production of rhodopsin in your body is strictly an individual matter dependent on genetics, health, diet, and age. Know your limitations.

Oxygen and the lack of it dramatically affect night vision. Altitude causes lose of retina efficiency from 5% to 40% from 4000' up to 16,000'. Smoking can add 7000' to these altitudes. Night vision is also subject to dramatic illusions. Bright lights and reflections will not only destroy your night adaptation but will give false impressions of proximity, speed, attitude, and changes.

Monocular Vision/ One-eye pilots
I've worked with a couple of students who had monocular vision - each of whom had to take a medical flight test with an FAA inspector. The inspector was actually very helpful and told us exactly what would be required on the test.
This included:
(1) Reading, identifying, and explaining airport signs and markings.
(2) Reading charts in the air and identifying landmarks.
(3) Tuning radios in flight. (4) Identifying objects on the ground (power lines, roads, railroads, etc.) and selecting areas suitable for emergency landings.
(5) Simulated emergency landings.
(6) Short-field landings (basically treated as an accuracy landing for the purpose of the test).
(7) Collision avoidance - being alert for other air traffic and following ATC instructions with respect to pattern operations ("follow the Bonanza turning final").

Note that the inspector will most likely be looking for performance at the Private Pilot level. It might be a good idea to review the Practical Test Standards (including the introduction) to get a general understanding of what will be expected of you.

Although both students reported that the test was relatively "painless", it was fairly lengthy - approximately 1.5 hours. I attribute this to the location (a very busy international airport in Class C airspace).

Interestingly enough, the students were allowed to log this flight as "PIC", even with the FAA guy on board (the inspector is neither a passenger nor an instructor).
E. Roberts, CFI

The Eye Reacts to Flight Conditions
--The smoker will have reduced vision, specifically peripheral vision. A smoker is more likely to miss seeing an aircraft approaching the side
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--The pilot who is under stress will tend to fixate on one thing. This may be the runway or the altimeter. Cockpit myopia will destroy the very importance of a variable scan and focus.

--Certain background conditions make it difficult to impossible to separate an aircraft from the background clutter of clouds, sun glare, or rooftops.

--Haze makes it very difficult to judge distance. This is especially true at night. The military is presently experimenting with using light to camouflage aircraft. Certain placements and intensities of lights on an aircraft in daylight can make an airplane harder to see.

--The eye can see only when it stops moving. Stopped the eye will focus on a 10 degree arc. To make a focused scan move your eyes in 10-degree jumps. To make a full scan move your head. Peripheral vision detects movement. Use peripheral vision at night for things you want to locate.

--Binocular vision is good for measuring distance up to about 20 feet. Beyond 20 feet we must depend on shadows, relative size of similar objects, overlapping, or other clues. Clouds are particularly difficult to judge for distance because there are no references.

--Empty field myopia is caused by haze or fog conditions that give the eyes nothing to focus on at a distance. The eye defaults to a distance of about 20 feet. To break this you must focus momentarily inside the cockpit and then outside. This process must be repeated frequently since there is no way of knowing when the default distance has kicked in.

--Red is a poor warning color. Green and yellow are much better.

--Red light in the cockpit should be avoided since it reduces the ability of the eyes to accommodate to reading. Military now uses blue cockpit lighting.

Light Color Test
On failing a day test you may take a night test and even if you pass your license will be restricted by a note. It is possible to improve red/green discrimination by wearing ruby colored contact lenses.
(Opinion)
The first thing to try if you flunk the color-dots test is to ask to immediately retake the test standing by the window, where the test sheets are illuminated by natural sunlight. Typical office fluorescent lighting distorts colors; if your color vision is marginal, looking at the test sheets in natural sunlight may be all that's needed to pass the test if you flunk it under fluorescence.--
Roy Smith, CFI-ASE-IA

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