When you close your eyes in a perfectly dark place, be it a lightproof photo development room, a deep cavern, or just beneath a snug fitting sleep mask, you won’t, counter-intuitively, see inky pitch black darkness. Instead, you’ll perceive a very faint gray color known as “eigengrau”. Why do you see any color at all and why the odd name?
The reason we don’t see perfect darkness even when there are absolutely no photons entering our eyes is because of a particular protein in our retinas called rhodopsin. This protein, a component of the retina’s rods, is extremely light sensitive and helps us see even in very low light conditions.
When we experience a dim gray color in the absence of any actual light, what we’re actually experiencing is the thermal isomerization of rhodopsin (a process where heat triggers a chemical reaction that causes a molecule to reform into a new and different molecular structure, but still have the same atoms).
This process triggers false positives in the rods and creates the illusion of faint light hitting the retina. When you see this faint gray behind your eyelids as you’re drifting off to sleep in a pitch black room, what you’re really seeing is the side effect of biochemistry playing out across your retina.
As for the name, “eigengrau”, it’s derived from German and literally translates as “own gray”—a fitting term for a faint gray color that is specific to the biochemical processes going on in our own eyes.