Eigengrau
Close your eyes.
What color do you see?
Disregard any of the artifacts that linger from light sources that impacted your eyes before you closed them. Those kaleidoscopic blobs are fascinating, but they’re not what we’re after here. They should fade relatively quickly. Some people experience these light shows all the time, thanks to brain or eye injuries, but they will abate for most humans. Instead, look at the background color. If you can’t produce a calm vision, imagine your eyelids as a movie screen. Toss out any of the projections and just snag the background hue. What color is your cinematic canvas?
Most people will answer black. That’s a perfectly rational response. Even if one didn’t partake in the experiment, black might be the guess. As we learned recently, in the absence of photons, humans perceive blackness. Our eyelids do a wonderful job at keeping out light, so, unless you’re sitting in front of a massively luminant source, black is a good guess. If you want to take this experiment to the maximum, go into a dark room or closet. Let your eyes adjust for a while, then close them. Do you see black?
If your answer is still jet, ebony, or raven, attempt to compare the color you see to the blackest blacks we recently investigated, such as Vantablack.
Is your eyes-wide-shut screen as dark as the hue above?
For most people, the answer is no. Instead of complete blackness, most people see a lighter color, a dark shade of grey known as eigengrau.
This name is German for “intrinsic grey.” In the past, some have also called it eigenlicht, which means “intrinsic light.” Other terms include “dark light” or “brain grey.” Scientists tend to eschew these names, opting instead for terms such as “visual noise” or “background adaptation.”
The difference between black and eigengrau is slight, but it does exist:
Why does this phenomenon occur and how did we ever notice it?
In the 19th century, physicist Gustav Fechner (great name) wanted to test the human eye’s ability to perceive changes. He developed an experiment called The Method of Limits. In this investigation, Fechner would display a color to a subject. He would present a slightly lighter version of the color, over and over, until the person could no longer distinguish the color from the background. Fechner discovered that lighter colors vanished for subjects faster than darker colors. From white to black, the points at which pigments disappeared for a subject followed a pattern.
Except for one: grey.
This exception fascinated Fechner. Why did this one color buck the trend? Fechner theorized that the human eye, for some reason, does not see black in total darkness, after all. Instead, he believed the brain must perceive a dark shade of grey.
Now that someone had uncovered eigengrau, scientists could attempt to figure out why this non-blackness transpires.
Eye researchers realized in the 1860s that experimental data related to intensity and sensitivity illustrated the retina likely yielded inherent noise. When one closes one’s eyes, anything one sees other than black is “noise.” Our eyes, it seems, make noise on their own.
Our peepers feature two types of cells: cones and rods. Cones handle color; rods register light intensity and contrast. When one enters a dark room, the cones take a hike and rods take over. Rods contain proteins called rhodopsin, which starts what scientists call the transduction train. This process is how the body translates inputs to the images our brains perceive. When photons hit rhodopsin, the sight procedure begins. However, rhodopsin also seems to fire randomly, without sensory input. Researchers believe these intrinsic trips of the protein cause eigengrau.
To our eyes, a photon and a random activation produce the same thing: the perception of light. So, when we close our eyes, even though light from the natural world isn’t hitting rods, we still sense a little light when the proteins trigger.
The result is a color called eigengrau.
Further Reading and Exploration
The Method of Limits – Oxford Reference
Impact of Noise on Retinal Coding of Visual Signals – Journal of Neurophysiology
Eigengrau: The Shade You See When You Shut Your Eyes Isn’t Perfect Black – IFLScience
Did You Know The Color You See When You Close Your Eyes Isn’t Black? It’s A Different Color. – Medium
Two components of electrical dark noise in toad retinal rod outer segments. – Journal of Physiology
