Pilot Handbook of Aeronautical Knowledge, The Human Eye
Light enters through the cornea at the front of the eye
The light then continues through the pupil which adjusts the amount of light which is allowed to enter the eye
It travels through the lens where it is focused onto the retina
The retina takes the light and through cells, converts light energy into electrical impulses for the brain
A blind spot is located where the fovea attaches to the retina
Beware of parallax error when attempting to read needles or trim the ball, etc...
Pilot Handbook of Aeronautical Knowledge, The Human Eye
Vision in Flight:
Major factors that determine how effectively vision can be used are the level of illumination and the technique of scanning the sky for other aircraft
Vision Under Dim and Bright Illumination:
Under conditions of dim illumination, small print and colors on aeronautical charts and aircraft instruments become unreadable unless adequate cockpit lighting is available. Moreover, another aircraft must be much closer to be seen unless its navigation lights are on
In darkness, vision becomes more sensitive to light, a process called dark adaptation. Although exposure to total darkness for at least 30 minutes is required for complete dark adaptation, a pilot can achieve a moderate degree of dark adaptation within 20 minutes under dim red cockpit lighting. Since red light severely distorts colors, especially on aeronautical charts, and can cause serious difficulty in focusing the eyes on objects inside the aircraft, its use is advisable only where optimum outside night vision capability is necessary. Even so, white cockpit lighting must be available when needed for map and instrument reading, especially under IFR conditions. Dark adaptation is impaired by exposure to cabin pressure altitudes above 5,000 feet, carbon monoxide inhaled in smoking and from exhaust fumes, deficiency of Vitamin A in the diet, and by prolonged exposure to bright sunlight. Since any degree of dark adaptation is lost within a few seconds of viewing a bright light, a pilot should close one eye when using a light to preserve some degree of night vision
Excessive illumination, especially from light reflected off the canopy, surfaces inside the aircraft, clouds, water, snow, and desert terrain, can produce glare, with uncomfortable squinting, watering of the eyes, and even temporary blindness. Sunglasses for protection from glare should absorb at least 85 percent of visible light (15 percent transmittance) and all colors equally (neutral transmittance), with negligible image distortion from refractive and prismatic errors
Visual Scanning:
The eye can better see motion
Keep the eye constantly scanning where you believe an object is located
The difference between night and day vision are cones and rods [Figure 2]
Both are located at the back of the eye or retina, a layer upon which all images are focused
These nerves connect to the cells of the optic nerve, which transmits messages directly to the brain
Both rods and cones are used for vision during daylight and while there is not a clear-cut division of function, the rods are what makes night vision possible
Rods:
The rods on the retina are concentrated in a ring around the cones
There are virtually no rods at all in the center of the eye
Because rods are mostly located away from the fovea, they are responsible for peripheral vision
This requires a pilot to look off center to see an object at night
Unable to discern color but are sensitive on low lights
In the absence of normal light, the process of night vision is placed almost entirely on the rods
They detect objects (shapes), particularly those that are moving, but do not give detail or color-only shades of gray
Contain rhodopsin, called visual purple, which is especially sensitive to light
Once fully adapted to darkness the rods are 10,000 times more sensitive to light
Low acuity
Cones:
The cones on the retina are responsible for all colors, fine detail, and far away objects
Cones are most concentrated toward the center of the field of vision (focal)
Function in day and night (moonlight)
Unlike at night, objects can be seen best by looking directly at it
High acuity
Dark Adaptation:
Dark adaptation allows the eyes to adjust to low light levels
Example: entering a darkened movie theater you can't see anything initially, but as time goes on your eyes adjust to the point features on the walls, or seats start to stand out
This process is entirely reversed when you step outside and you have to shade your eyes due to the bright light
Process:
The pupils of the eyes first enlarge to receive as much of the available light as possible
After approximately 5 to 10 minutes, the cones become adjusted to the dim light and the eyes become 100 times more sensitive to the light than they were before the dark room was entered
The rods take about 30 minutes to fully adapt to darkness, but when they do adjust, they are about 100,000 times more sensitive to light than they were in the lighted area
Any bright light, such as a room or headlight, can completely destroy night adaptation in only 10 seconds
Pilots should close one eye when using a light to preserve some degree of night vision
Temporary blindness, caused by an unusually bright light, may result in illusions or after images until the eyes recover from the brightness
This results in misjudging or incorrectly identifying objects, such as mistaking slanted clouds for the horizon or populated areas for a landing field
Vertigo is experienced as a feeling of dizziness and imbalance that can create or increase illusions
The illusions seem very real and pilots at every level of experience and skill can be affected
Under conditions of dim illumination, aeronautical charts and aircraft instruments can become unreadable unless adequately lit
Conversely, too much light can hinder an outside scan for other traffic and landmarks
During night flights in the vicinity of lighting, flight deck lights should be turned up to help prevent loss of night vision due to the bright flashes
Using a white lens flashlight can ruin adaptation while using a red lens or green lens flashlight can wash out those colors on a map
You must use lighting conservatively
Dark adaptation is also impaired by exposure to cabin pressure altitudes above 5,000', carbon monoxide inhalation through smoking, deficiency of Vitamin A in the diet, and by prolonged exposure to bright sunlight
Additional Notes:
Adapt the eyes to darkness prior to flight and keep them adapted
If oxygen is available, use it during night flying
Do not wear sunglasses after sunset
Move the eyes slower than in daylight
Blink the eyes if they become blurred
Concentrate on seeing objects
Force the eyes to view off center
Maintain good physical condition
Avoid smoking, drinking, and using drugs that may be harmful
Avoid keeping the interior lights turned up beyond what is required
If the eyes become blurry, blinking may refocus the lens
Blind Zones:
The day blind spot is on the optic nerve where there are no light receptors
The night blind spot is due to the concentration of cones in an area surrounding the fovea on the retina
Because there are no rods here, directly looking at an object will cause an object to disappear, therefore you must offset where you are looking
Parallax Error:
Parallax Error is the apparent displacement of an object caused by a change in the position from which it is viewed
This error occurs to some extend when looking at an analog instrument from any direction other than head-on
This would be most apparent when looking from one side of the dashboard to the other
This error is removed when reading digital displays such as the G1000 series instrumentation
Despite digital instrments, there are still analog instruments which may be even farther away (i.e., more parallax error) on the dashboard
Parallax error also effects the way a pilot views the outside world in flight, most pronounced during turns in aircraft with side-by-side seating (the pilot is seated on either side of the logitudinal axis)
Assuming the pilot is sitting in the left seat enters a left turn, the nose appears to rise, while if in that same left seat a turn is performed to the right, the nose appears to fall
This error results in either an instinctual push down on the stick with a turn to the left (altitude loss) or a pull-back on a turn to the right (altitude gain)
To overcome this error the pilot must first be aware of the illusion to avoid improper inputs while maintaining an instrument scan to backup inputs
Visual Limitations & Impairments:
Smoking, alcohol, oxygen deprivation (hypoxia), diet, medication, and age affect vision, especially during night operations
Vision at night is impaired based on physical limitations
Knowing these limitations (rods vs. cones) is important to improving vision
Dust
Fatigue
Emotion
Germs
Fallen eyelashes
Age
Optical illusions
Alcohol
Atmospheric conditions
Windscreen distortion
Oxygen
Acceleration
Glare
Heat
Lighting
Aircraft design
The "mind" (day dreaming)
Sunglasses:
The FAA recommends pilots utilize sunglasses with Ultra Violet (UV) protection to safeguard their eyes from the sun and to assist in locating other traffic, while minimizing color distortion
Pilots should avoid polarized sun glasses as they tend to interact with instrumentation, making it difficult to see
Empty Field Myopia, sometimes called Empty Space Myopia, is a condition in which the eyes, having nothing specific within the available visual field upon which to focus, focus automatically at a range of the order of a few feet ahead
This condition usually occurs when flying above the clouds or in a haze layer that provides nothing specific to focus on outside the aircraft
This causes the eyes to relax and seek a comfortable focal distance which may range from 10 to 30 feet
Usually occurs flying above clouds or in a haze layer that provides nothing to specifically focus on
The eye will relax and seek a comfortable focal distance about 10-30' in front of the aircraft
This will make focusing on distant objects very difficult
To combat this focus the eye to infinity (something well outside the cockpit)
You must refocus the eye each time you look in and then back out
Conclusion:
Your eyes are not perfect and can play tricks on you, especially at night and in instrument conditions
