A Staff Report from the Straight Dope Science Advisory Board

Why are red lights used in darkened control rooms and other low-light situations?

June 8, 2006

Dear Straight Dope:

I've heard for years that red light supposedly doesn't affect the ability of the human eye to see in the dark (or a "dark-adapted" eye, to be precise). In movies you see submarine control rooms bathed in red light, maybe because it won't mess up the periscope guy's night vision (or it looks cool), while BMW uses only red light to illuminate the gauges and dashboard displays of its cars. A couple of my friends are rabid red-light-believers but can't call on much data to back them up, other than "Well, if BMW says so . . ." Myself, I think it's a load of hooey that got started somehow and has careened on, urban-myth-like, into present day lore (and car marketing).

It's not a load of hooey, but it's not earth-shaking either. As you probably know, the retina contains two types of photoreceptor cells: rods and cones. Cones are predominantly located in the center of the retina and are responsible for color vision in relatively bright light. Rods, which are mostly found on the periphery of the retina, are much more sensitive to light than cones but lack the ability to discriminate between colors. Cone cells work poorly in low light, so the rod cells give us a black-and-white picture of our environment.  In near-absolute darkness, both cone and rod cells compensate by pumping out more light-sensitive chemicals. The more time spent in darkness, the more chemicals are produced. In about ten minutes, cone cells max out, producing as much as they are capable of holding. Rod cells max out in about thirty minutes. This process, called "dark adaptation," reverses in bright white light--the light-sensitive chemicals diminish, taking with them our ability to see in darkness. However, the light-adaptation period varies, depending among other things on the intensity of the light you're exposed to. Red light has minimal effect on night vision because its energy level is so low that the eye doesn't register it strongly enough to produce a compensatory reaction. (Red light, for those interested in the physics of these things, has the longest, slowest wavelength of any form of visible light and thus the least energy--as you shift toward the blue end of the spectrum, wavelengths shorten and energy levels rise.) Light of still lower energy is called infrared ("below red"), which is invisible to the naked eye and is more familiar to us as heat.

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