The world in the eyes of dogs is not only black and white but also colorful

There are two kinds of color photoreceptors, or cone cells, on the dog's retina, which can distinguish between short wavelength and medium and long wavelength light, that is, blue ( short wavelength) and red-yellow (long wavelength) hues. Because dogs only have two types of cone cells, dogs can distinguish the same colors as humans who are red-green blind.
But dogs do see certain colors. Dogs can distinguish different shades of blue, indigo and violet, but they have no special sensitivity to high-chroma colors such as red and green in the spectrum. One method is to shine a colored light source into a puppy's eyes and analyze the spectrum, or pattern, of the reflected light, and then compare the results with the analysis of the same light source into a human's eyes.
Red is a dark color to dogs, and green is white to dogs, so a green lawn looks like white grass to a dog.

Dogs are very sensitive to changes in grayscale

Dogs have a strong ability to distinguish between shades of gray and rely on this ability to distinguish The changes in light and dark on the object produce a three-dimensional visual image. Dogs have poor vision. The lenses in their eyes are twice as thick as humans, so dogs are myopic. The adjustment ability of its eyes is only 1/5-1/3 of that of humans. It can be seen clearly within 50 meters, but cannot be seen clearly beyond this distance. But the moving target can feel the distance of 825 meters.

How do guide dogs detect changes in traffic lights?

Blind dogs do not distinguish between traffic lights by distinguishing red and green lights, but by the brightness of the two lights. , although red light and green light are both gray in their eyes, different grays can be distinguished. Dogs have a strong ability to distinguish subtle changes in gray shades. Relying on this ability, relying on this ability, It can distinguish the changes in light and dark of objects and produce three-dimensional visual images.
There is a very special kind of cell in the dog’s eyes. Because of this kind of cell, they can still distinguish the absorbed colors according to different spectra. This sounds a bit convoluted, but to put it simply, in the eyes of dogs, different colors are not indistinguishable. The difference is not in color, but in brightness. These colors will be converted into shades of brightness and fed back to the dog's brain. In other words, in the eyes of dogs, the brightness of red lights and green lights are different! Through the difference in brightness between the two colors, dogs can easily distinguish traffic lights.