About 8 percent of males are born with some degree of deficiency, but a small number of women, about 0.5 percent, can also have it. Although it is often referred to as “color-blind,” that isn’t really an accurate term. We can see the colors; we are just challenged to determine what color we’re really seeing.
Color vision is possible because of photoreceptors in the retina of the eye called cones. The cones have light-sensitive pigments that help us to recognize color. Each cone is sensitive to either red, green, or blue light and recognizes light based on the wavelength. Red-green deficiency is the most common form, and the intensity of the deficit is often dependent on the lightness or darkness of the colors.
Part of the FAA medical examination for any class of medical includes a test of color vision. Most AMEs have several color vision testing devices allowed by the FAA. The most commonly used is the Ishihara 14 plate pseudoisochromatic test. It is the most sensitive test in detecting even subtle degrees of color vision deficiency. I can identify two or three of the numbers presented in the 14-plate test. That’s a fail for an unrestricted medical that doesn’t include the “night flying prohibition” restriction. I am able to pass the Dvorine 2nd edition test that is less sensitive than Ishihara. For pilots who don’t meet the standard, the AME will issue a medical certificate but with “Not valid for night flying or color signal control.”
A signal light test administered by the control tower involves getting an approval from the FAA and scheduling the test with a local ATC tower facility. That test also has some pitfalls because of the technology of the signal light guns in use at tower facilities. Not all ATC towers have the newer technology LED bulbs in their light guns and use outdated incandescent bulbs that don’t produce the intensity of light like the LEDs. Try to locate a tower that has the new LED light gun, and you will probably have a better chance of passing.