Visual perception starts with a complex image analysis done by neurons in the eye. That data is sent to the brain, which interprets information from the eyes by using information gained from prior experience (and some fancy processing tricks). Unlike a photograph, where what you see is what you get, real life visual perception hinges largely on things that are inferred.
Consequently, you can be fooled fairly easily when you place yourself in environments and circumstances that violate the visual rules that you learned through prior experience. Sometimes the fooling around can be fun, as with textbook optical illusions. Other times however, vision can fool you into a tragic mistake.
Although understanding the basis of an illusion rarely diminishes its effect, knowledge of the illusion might prevent an inappropriate response.
Picture yourself stopped at a red light in your car. Most people will push harder on the brake pedal if a large vehicle next to them begins to move forward. Even though your body tells you youre stationary, the eyes dominate and render a compelling feeling that youre moving backward. In this example, your brain believes your eyes, which say youre moving, rather than your vestibular (balance) system, which you are stationary. The relative dominance of the visual system is actually fortunate for pilots because often it is our vestibular system that is the infidel, for example telling us we are climbing when in fact we may be in a tight descending spiral.
Adaptation
An important characteristic of our sensory systems that contributes to many illusions and errors of judgment is adaptation. Adaptation causes sensory systems to be most sensitive to rapid change. When external stimuli do not change, the system becomes less sensitive to those stimuli.
Adaptation is the reason why we do not constantly feel the pressure of our clothes on our body. Also, if objects in the visual world are moving at a constant rate and direction then the receptors become less sensitive to that motion.
As a result, if this motion suddenly ceases, motion in the opposite direction will be perceived even when the objects are not moving. This illusion has been called the motion after-effect and the waterfall illusion. Many of the illusions discussed in aviation textbooks can be traced to adaptation (for example the leans experienced after a prolonged turn).
A common illusion experienced by pilots is the narrow runway illusion which makes pilots believe that they are higher than they really are. This illusion is a good example of how our prior experience influences what we perceive. Since these common illusions are covered well in many aviation textbooks, lets look at other important characteristics of our vision.
Acuity
The ability to see and avoid as well as being able to read and interpret instruments and charts depends on acuity. Acuity is usually limited by how well the light rays are focused on the back of your eye.
Light rays entering the eye are mostly focused by the cornea, the clear front surface of the eye. Fine focus is accomplished by the lens inside the eye, which thickens to focus light reflected from near objects and flattens when we focus on distant objects.
Just how fine that adjustment must be also depends upon the size of the pupil. In bright light, when pupils are small, focus of the lens becomes less critical. If you had a pinhole-sized pupil you would not need to focus at all because objects at all distances would be clear. However, the amount of light getting to the eye would be very small and therefore you wouldnt see very well in any but the brightest conditions.
When the pupils are large, in dim light, focus is much more critical. This is the reason why many people who dont need glasses during the day require them at night – proper optical correction becomes more important.
Unfortunately, the lens of the eye gradually becomes stiffer with age (at least something does) and a condition called presbyopia sets in. Beginning around 45 year of age, the ability to focus on close objects begins to decline. When we discover that our arms are too short for reading, reading glasses or bifocals are the usual solution.
For pilots, it is critical to be able to rapidly switch focus between near and far, so full reading lenses are generally a poor choice. Bifocals, variable power lenses or half lenses for those who do not require additional correction are more practical solutions.
Scanning
The best method for scanning depends on the circumstances. How you scan the instrument panel in solid IMC will be different from how you scan outside the cockpit to detect traffic. However, there are certain principles of vision that should serve as a general guide to efficient scanning.
For panel scanning in IMC, pick a pattern that radiates from a central instrument, most likely the attitude indicator. Scan out to other instruments and return to the central instrument regularly. Scan to the engine instruments often and scan to the other flight instruments more often. After you develop a pattern that works for you, try to stick with it.
A practiced scan pattern will gradually create less fatigue and become more natural. Focus on each instrument only long enough to positively gather the information. A good rule of thumb is that you should focus on an instrument just long enough to quickly say what the instrument has told you (e.g. altitude 5,500).
The AIM and many textbooks wisely suggest that one should scan for traffic by continuously focusing for brief periods and shifting your fixation after several seconds. When scanning outside the cockpit, be aware that the visual system does not gather information efficiently when the eyes are moving.
The brain actually ignores most signals sent from the eyes when shifting rapidly from one fixation point to another. It is essential that you stop your eye movement for brief periods at different spots. Ive seen students madly darting their eyes around in an attempt to spot traffic, but without actually focusing on one spot those attempts are futile indeed.
In fact, an interesting illusion can occur under these circumstances. If a light is flashed just as you shift your visual fixation, the apparent location of the flash will be displaced in the direction that your eyes move, sometime as much as 30 degrees. The glint of sun off the window of an approaching airplane may lead you to think the airplane is actually someplace else.
When the eyes are fixed, however, the visual system is extremely adept at detecting moving objects. The exception to that rule is if a target is on a collision course with you. In that case, there will be no apparent movement of the object, it will just get bigger. So if ATC calls out traffic at 12:00 and your altitude and you cannot locate the target immediately, change course.
The change in the apparent motion of the target caused by your own course change will often rapidly reveal the traffic (of course, if the traffic was close, changing course would probably be smart anyway).
When scanning at night it is important to know that your highest sensitivity to dim lights is actually not at the center of your gaze but is offset by about 10 degrees. Take a tip from stargazers and look a little to the side when trying to discern a dim target.
Color Vision
Color vision is probably one of the most misunderstood of our visual capabilities. The details of color vision fills textbooks, but there are some important factors for pilots to consider. While complete color blindness is quite rare, partial losses of color vision are quite common, up to 8 percent of Caucasian males.
Most of the people we describe as color blind actually do have color vision, although a reduced form. The most common color vision losses are genetic and affect the red-green dimension of color experience. In addition to the common deficiencies there are a host of other types of color vision changes caused by normal aging, disease and even medications such as Viagra.
Color blindness is generally not debilitating, but it has been argued that for pilots a failure of color discrimination could be fatal. The debate about how disadvantaged a pilot actually is has raged for years.
For now at least in the United States, color-blind individuals can become licensed pilots under most circumstances. Even most of the U.S. armed forces will accept pilots with mild forms of color blindness. Many other countries however, will not license color-blind pilots at all.
One common form of red-green color blindness poses particular problems. A person with this condition is termed protan and has reduced sensitivity to long-wavelength lights (those at the red end of the spectrum). For a protan pilot, not only is color vision affected but long wavelength lights will appear dim or not at all. For example, blood would appear almost black and a red flashing beacon may not be detectable at all. Obviously, a pilot or prospective pilot with this condition would be hard-pressed to function in many flight environments.
Cockpit Lighting and Sensitivity
The primary concern many pilots have when it comes to vision is what kind of cockpit lights are optimal for night flying.
This depends on what you need to see. Assuming that you would like to be able to see and avoid, as well as locate approach lights that may be partially obscured, its obvious that you will need to detect dim lights as well as you can. This task requires high sensitivity.
We would also like to be able to see inside the cockpit and read our instruments and our charts without error. This task requires high acuity and possibly color vision to interpret color-coded charts.
The visual system can impressively handle both requirements. It can detect extremely dim light as well as discriminate wavelengths of light (colors) that differ by a tiny fraction. However, the visual system uses two different and somewhat complementary sets of receptors, the rods and cones to perform these two tasks.
Under bright light, the cones provide for high acuity and color discrimination at the expense of sensitivity to dim lights. Under low light, the rod receptors take over. They are extremely sensitive but sacrifice high acuity and color vision. High light levels are required for the cones to function, while for the rods the light levels must be low enough to avoid becoming desensitized.
In planning cockpit lighting, a pilot should have a light that will be bright enough to provide high acuity but not reduce sensitivity – two goals that may seem mutually exclusive. Fortunately, rods and cones differ in another important way. Rods have their maximum sensitivity in the shorter wavelength region of the spectrum (blue-green) whereas most of the cones are sensitive in the middle to long wavelength part of the spectrum (toward the yellows and reds).
Researchers determined long ago that a moderately bright long-wavelength light (red) was an ideal compromise. That way, you can provide the cones with enough light for high acuity and, because the rods are not very sensitive to the long-wavelength light, retain high sensitivity.
This plan works quite well with one caveat. Color discrimination suffers because you actually need light from all parts of the spectrum (white) to allow for accurate color discrimination. Nonetheless, it was suggested that a red light would be best for use in the cockpit and red lighting has rightfully become the standard.
Recently, blue-green lights have become popular in general aviation. Blue-green lighting was originally introduced for military use because these lights do not interfere with night vision systems, which detect and amplify light in the infrared part of the spectrum. Obviously red cockpit lights interfere with the night vision systems.
Blue-green lights have subsequently caught on with manufacturers of otherwise excellent cockpit lights and are touted as being the latest in military technology. They are said to maintain sensitivity while still providing high acuity and good color vision.
However, unless you are using a military night vision system, it would seem that blue-green light should be the worst possible choice. If the blue-green light is bright enough for the high acuity cone system to use, then the rods will be desensitized because they are extremely sensitive to this light.
Measurements recently made in our laboratory have shown that this is exactly what happens. Color discrimination with blue-green light is slightly better than with red light for some parts of the spectrum but still falls far short of that measured under equally bright white light.
In addition, discerning the colors on VFR charts appears to be slightly easier with blue-green light than with red, but most people tested report that colors on the IFR charts are easier to discern with red light than with blue-green.
How bright is too bright when it comes to blue-green light? An episode from the lab provides poignant illustration. A colleague was running an experiment that required measurement of a persons sensitivity after adapting to complete darkness for 45 minutes.
One subjects results were inconsistent, and his sensitivity after 45 minutes in the dark was abnormally low. It was later discovered that he was wearing an indiglo watch that glowed blue-green and was periodically checking the time while waiting in the dark. They ran the subject again without the watch and he showed normal sensitivity.
To maintain high sensitivity, pilots must avoid exposure to bright lights, particularly if they are not red. If you inadvertently expose your eyes to even a brief bright light you will have lost sensitivity for an extended period of time.
It takes about 45 minutes to gain maximum sensitivity after exposure to the bright light typically found outdoors in the summer. If you dont think thats likely to happen at the airport, try looking into an airliners landing lights.
If you cannot help exposing yourself to high light levels you might take a trick from WWI soldiers and cover one eye, so at least the covered eye maintains high sensitivity.
So what is a good pilot to do? Keep the lights as low as comfortable in the cockpit and use red lighting. Gradually turn down the lights as your eyes become more sensitive over time. Avoid blue-green lighting.
If you are having trouble discerning colors on a chart, briefly use a dim white light and make the discrimination with one eye. Avoid bright lights, especially before you attempt to let down to minimums and search for the runway lights while scanning for scud runners.
Also With This Article
Click here to view “Red light outshines blue/green for night cockpit lighting.”
Click here to view “Ratio of active cones/rods under different kinds of light.”
-by Michael Crognale
Michael Crognale is a CFII and a researcher in visual neuroscience on the faculty in the Departments of Psychology and Biomedical Engineering at the University of Nevada at Reno.
