Why can't we see colors well in the dark?
If you've ever gotten dressed in the dark and later realized that the shirt you were wearing was not the color you thought it was, you're not alone. Identifying colors can be challenging in the dark, and even in low light, different colors can look remarkably similar.
But why is it harder to discern colors in the dark than it is in bright light?
Humans' ability to perceive color varies due to how we see under different lighting conditions. Human eyes contain two types of photoreceptors, or nerve cells that detect light: rods and cones. Each photoreceptor contains light-absorbing molecules, called photopigments, that undergo a chemical change when struck by light. This triggers a chain of events in the photoreceptor, prompting it to send signals to the brain.
Rods are responsible for enabling vision in the dark, known as scotopic vision. They're made of layers and layers of photopigments, said Sara Patterson, a neuroscientist at the University of Rochester in New York.
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Rods are particularly good at picking up light even when it's dark because "every single one of those stacks is a chance for photons to get absorbed," she said. Photons are particles of electromagnetic radiation — in this case, visible light — and rods can be activated by exposure to relatively few photons.
Cones, on the other hand, are responsible for vision in bright light, or photopic vision. Most people have three types of cone cells, each of which is sensitive to a different range of wavelengths of visible light, which correspond to different colors. Small changes in the light-absorbing molecules in different cones make them specialized in detecting red, green or blue light.
But importantly, individual cone cells can't distinguish between colors, said A. P. Sampath, a neuroscientist at UCLA. When a molecule inside the cone cell absorbs a photon, it only activates the cone; at that point, no information about the light's color or intensity has been processed. Color vision arises when the brain combines the responses from all three types of cones in the eyes — tiny biological circuits transform those responses into the colors we see.
Cones dominate vision in bright light because rods quickly become saturated, or overwhelmed with photons, and the brain essentially tunes out the rods' activity. That's why we can see colors easily in bright light. But as it gets darker, as the sun sets or you switch off the lights in a room, rods start to take over because they're more sensitive to light than cones are.
The rods dominate night vision, while cones are only weakly activated. Unlike cones, though, rods come in only one type. Color vision comes from comparing the responses of the three types of cone cells, which isn't possible in rod-dominated vision. So, in the dark, we can't distinguish colors well.
However, rods might still influence color perception under certain conditions. In dim light, our eyes operate in an intermediate range known as mesopic vision, in which both rods and cones contribute to vision but neither dominates.
"In this mesopic range, there's reason to believe that rods may contribute to color processing as well, by providing a distinct spectral sensitivity to compare against the cones," Sampath said. Rods are most sensitive to green light, and in this intermediate range, they provide extra information to the brain to compare against that from the cone cells.
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This crossover between rod vision and cone vision also produces the Purkinje effect, in which red hues look dark or bluish under dim light and purple, blue and green suddenly pop, Patterson said. The Purkinje effect is particularly noticeable at twilight or during a total solar eclipse.
Even though we can't see color well at night, our visual system lets us take in information over an enormous range of light intensities, from a moonless night to blindingly bright ski slopes, Sampath said.
"One of the things that's amazing about the visual system is that we have this enormous range of intensities and it's shifting continuously," he said. "And yet we can accommodate 12 orders of magnitude of light intensity. There's no synthetic detectors that can manage this type of performance."
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