Fly Well: Green eggs, ham, and Isaac Newton

In addition to defining the laws of gravity that pilots joyously flout, Sir Isaac Newton maintained that objects reflect a certain wavelength of light, absorbing all others; it is the reflected beams we experience as color. So Newton’s apple is not inherently red, it just reflects that particular part of the spectrum. Objects like a runway can appear black because all wavelengths are absorbed, and runway lights are white because all wavelengths are reflected. Red lights tell us to hold, a green light from the tower invites us to land our nonradio aircraft, and blue marks a taxiway; combining these three can produce all the colors of the spectrum, and knowing which is which is important to pilots.

Light strikes the cornea, a membrane covering the eye; this and an underlying lens focus the image, upside down, projecting it onto the retina. Here, millions of specialized cells, rods and cones, convert light into nerve signals that are transmitted to the brain via the optic nerve, and with a nod to aerobatics, the brain flips images right side up. About 120 million rods are gathered around the periphery of each eye and are best suited to interpreting dim light, which also seems less colorful to us.

Although one can adapt to driving a car if color blind, for instance by knowing what order traffic lights change, pilots have other issues to contend with.Centrally, 5 million to 6 million cones are mission critical for color and sharp vision; they are divided into three types, some handling long (red), some medium (green), and some short (blue) wavelengths. As roughly 60 percent of cones are configured to respond to warm tones (yellow to red), we can interpret more nuanced colors at that end of the spectrum. The combined signals allow our brain to dictate that a chosen tie definitely clashes with that shirt.

Color vision impairment is much more common in males. Roughly one in 12 men, usually of northern European stock, and one in 200 women are so affected: inability to recognize differences between red and green, the most common defect; or, less often, problems discerning blue from yellow. Gray days are common flying in the northeast winter, but zero color differentiation—achromatopsia, only seeing shades of gray—is rare and accompanies poor general vision; lazy eye; preference for dim light; and nystagmus, eyes that flicker incessantly.

As one might conclude, missing or malfunctioning cones might lead to some degree or severity of color “blindness” in either all lighting conditions or dim light. The condition usually afflicts both eyes, is congenital (present at birth), is of genetic origin, and does not change much with age. Other causes are glaucoma or macular degeneration, head or eye trauma, various diseases (e.g., diabetes, Parkinson’s, leukemia, Alzheimer’s, sickle cell anemia), medications (hydroxychloroquine to treat rheumatoid arthritis), and alcoholism.

Color blindness may not be diagnosed unless a color test is conducted, coming to light (pun intended) when a parent endeavors to teach a child color names. Developed more than 100 years ago by Japanese ophthalmologist Ishihara Shinobu, the eponymous color charts display embedded numbers or shapes seen—or not—in colorful circles. Congenital color vision deficiency cannot be treated, but if there is an underlying cause as above, addressing that may improve the situation.

Although one can adapt to driving a car if color blind, for instance by knowing what order traffic lights change, pilots have other issues to contend with, such as not landing on taxiways. You can read about requirements for flight and color vision in the FAA’s Guide for Aviation Medical Examiners (faa.gov).

As pilots, we see many great sights, but the privilege to fly depends partially on color vision. If you already have your certificate, you must have passed the color vision test, so any change would suggest a causative disease or condition as above—another good reason to see an eye doctor annually.

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