Re: What is the best color of light to use at night to preserve night vision??

Dear Mike:  
Thanks for your question about what color to use at night to best preserve 
night vision.  There are a couple of ways to think about your question.  
The first way is to think about the wavelength of light that can be 
detected best at night; the second way is to consider the wavelength of 
light that is detected least by the photoreceptors responsible for night 
vision.

The human contains four different types of photoreceptors.  One type is 
the rods which function in low light, such as at night.  These rods have a 
maximum sensitivity to light with a wave length of about 590.  On the 
spectrum, light with a wavelength of 590 is indicated as a yellow-green, 
although when only our rods are activated in low light  we do not detect 
color at all.  You can check this out yourself.  

The three other types of photoreceptors in your eye are the cones.  These 
photoreceptors function when the intensity of the light is higher, they 
are responsible for color vision, and because they are packed densely in 
the center part of your eye called the fovea, they are responsible for 
high resolution vision.  The cones come in three flavors, as I indicated, 
S,L and M cones. S-cones are most sensitive to short wavelengths of light 
(420 or violet), L-cones are most sensitive to long wavelengths of light, 
and the M-cones are most sensitive to medium wavelengths of light. 
Actually, L and M cones have quite similar peak sensitivities to light 
with wavelengths of about 500-510 or red light.  I should point out that 
the sensitivity cures of the cones are rather broad and overlapping, and 
the color you “see” depends on the combination of cones that are 
activated.   

So - to your question.  Let’s consider first the light you can detect best 
when the intensity is low.  You would select light that activates the 
rods.  This is light in the yellow-green part of the spectrum.  Infrared 
converters used in night-vision goggles convert light to this wavelength, 
and when viewed through these goggles all the world turns a strange yellow-
green.  

If you wish to see with your  cones without activating the rods, you would 
select a wavelength of light to which your rods are relatively 
insensitive. Furthermore, it would be logical to select a color close to 
the maximum sensitivity of the L, M or S cones.  There are many more L and 
M cones in the human eye than S cones, so the color that activates the L 
and M cones, a red color,  would probably be your best bet.  Be aware that 
in order for you to see, the red light you use to activate the L and M 
cones has to be brighter than the yellow-green light you would use to 
activate the rods.  

I hope this has been helpful.