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Name: Kathleen
Status: other
Grade: 12+
Location: Outside U.S.
Country: USA
Date: Spring 2014

What color of light does not affect rods and helps us see in the dark

As an amateur astronomer I got interested in this question years ago. Traditionally, the color red was used in places like night time military applications (e.g., planes or submarines) on the basis that the eye is least sensitive to red wavelengths and least likely to trigger the eye's daytime response (i.e., activation of daytime cone cells, which see poorly at night). As a result, many people have used red lighting in flashlights and other night sources.

But other studies point out that the human eye is most sensitive to wavelengths at the opposite (blue/green) end of the spectrum, regardless of illumination level. In other words, if you can avoid activating the cone cells, blue/green is usually the best wavelength to see things in low light. This probably explains why emergency vehicles in most countries use blue or green wavelengths instead of (or in addition to) red. And explains why modern automotive and avionic dashboards are not dominated by red colors at night. Likewise it probably explains why some backpacking flashlights often favor green wavelengths for night time purposes.

Here's another way to look at it. At night time astronomy outings, which color of laser is used to point at stars in the sky, red or green? It's green. As long as it's not bright enough to activate the cone cells and disrupt night vision, it is more visible than red.

In other words, like many scientific subjects... it's complicated!

Paul B.

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