Red light is most often used to enhance, or more properly, not harm
night vision. Red light provides great contrast and is not visible to
the human eye over long distances. It also works well with night vision
scopes and goggles. Photographers use red colored light when doing
night-time photography because it provides contrast while preserving the
natural shadows developed by darkness. Red light cuts through smoke
better than white light and, with the exception of red lines and marks
on maps and charts, it makes reading in low, or no, light possible.
Police and military units like the red LED for night operations because
it will not interfere with night vision technology and cannot easily be
picked up by other people.