The Reason Why Astronomers Need To Use Red Flashlights At Night

There are three types of cones, each detecting a different color (red, green, and blue), while the more numerous rods can’t provide any color details. Both can get overexposed if they’re blasted with too much light and basically become blinded by the light. Like when you turn on a bright flashlight at night.

Night vision, also known as dark adaption, is governed by two things. First, the pupils must open up to let in more light, which doesn’t typically take more than a handful of seconds. The second part of the equation is the chemical change that must also occur. Once the rods and cones are in the dark, they start to create a pigment called Rhodopsin (also known as visual purple), which takes anywhere from 20 to 40 minutes to kick in before “night vision” becomes effective. Once achieved, any bright light will disrupt it, and the process has to start all over again (via NPS).

Which absolutely no one wants when standing around in the dead of night looking up to the heavens. Thankfully, there is a workaround. The rods in the human eye have a peak sensitivity of around 530 nm, but red light has a wavelength between 635 nm anf 700 nm and is therefore undetectable by the rods and won’t ruin the night vision effect. Thus making a flashlight that shines red light the ideal tool to take with you during your sky-watching session.