What Color Is Darkness?
What’s the darkest color you’ve ever seen? To most people, the obvious answer would be the black, more specifically the black of total darkness. After all, black by definition is the absence of light.
But total darkness isn’t as dark as you might think. Paradoxically, we need light to see the darkness.
Let's try to understand how we perceive darkness with a little experiment. Pick a black object in front of you and stare at it for about second …. Now close your eyes for a few seconds and allow them to adjust. You may need to cover your eyes if you're in a bright room or outside. Now open them quickly and look at the black object. It may take you a few tries to fully see it, but you’ll notice that the black object actually appears darker than the black of total darkness.
This phenomenon is caused by the way we see the world. When our eyes are open the light-sensitive layer of cells at the back of our eyeballs, called the retina, are bombarded by packets of light energy called photons. The photons that represent visual light trigger nerve impulses on the retina that pass via the optic nerve to the brain, where a visual image is formed.
But when we close our eyes or are in total darkness most people see a vague grey field usually composed of changing regions of tiny black and white dots. This color is called Eigengrau, a German word that means “intrinsic gray”. What we’re seeing is actually visual noise and it’s the “static” of our retinas.
The photoreceptors in the human retina come in two flavors. Rods and cones. Rods are responsible for vision at low light levels. This is called scotopic vision. They lack spatial accuracy or the ability to mediate color, and they exist primarily around the outer edges of the retina, forming a large part of our peripheral vision.
When photons hit rod cells, a photoreceptive pigment within the cell, called rhodopsin, changes its shape. This initiates the processing of triggering a nerve impulse. Rhodopsin can also change its shape spontaneously from ambient heat. This triggers a false nerve impulse. The rate of these spontaneous false triggers is temperature dependent, though in humans it averages about once every 100 seconds. With over 120 million rod cells in the human eye, the cumulative effect of these false triggers forms the visual noise we see in total darkness. Because of this intrinsic visual noise, most of us have never actually experienced true darkness.
The same amount of visual noise is present whether our eyes are open or closed. But we usually don’t see it when we look at the world. So how can we see an object as darker than the visual noise if we can’t see the noise itself?
When our brains process visual information, contrast is more important than absolute brightness. Darkness as we see it is relative to the brightest thing we're looking at. To see how much our brain prioritizes contrast, let's do another experiment. In a few moments, this video will show all black for a few seconds. If you’re on a smartphone or a tablet, pause the video during this and go full screen so that the entire screen is filled with black. First, look at the screen in a lit area, then look at the screen in a totally dark room.
In a lit area, the brighter our surroundings, the darker the black appears on the screen. But in a room with no light, because the device’s screen is backlight, that same black now becomes the brightest object in view and appears as a glowing greyish color.
The contrast of the visual noise of our retinas is relatively low. Combined with the low spatial accuracy of rod cells, it all blurs together into the vague grey field when we close our eyes. With our eyes open and taking in light, that contrast of that noise is dwarfed by the contrast of the world around us. The relative intensity of a dark object is much further away from a bright object when compared to the intensity of visual noise. An extreme example of this is a clear, moonless night sky. The darkness of space appearing simultaneously with the brightness of stars creates our representation of the night sky as a black canvas dotted with bright white dots. Our brains quite literally synthesize the darkness we can never truly experience.
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