What is young Helmholtz theory?

The Young Helmholtz theory (based on the work of Thomas Young and Hermann von Helmholtz in the 19th century) is a theory of trichromatic color vision the manner in which the photoreceptor cells in the eyes of humans and other primates work to enable color vision. In 1802, Young postulated the existence of three types of photoreceptors (now known as cone cells) in the eye, each of which was sensitive to a particular range of visible light.

He proposed that there are sets of color perceiving elements on the retina. These colors are red, green and blue. The trichromatic theory states that these three colors are perceived from the sensations of the three elements, if any one element is absent, it will lead to a disability in which this color cannot be seen by the person. There are sensitive cones on the retina responsible for the color sensation.

The cones are like broad band receptors that perceive colors of different wavelengths. Color perception happens due to the overall stimulation pattern of the three receptors. For instance, yellow light uses different proportions of red and green, but little blue, so any hue depends on a mix of all three cones, for example, a strong blue, medium green, and low red.

Moreover, the intensity of colors can be changed without changing their hues, since intensity depends on the frequency of discharge to the brain, as a blue-green can be brightened but retain the same hue. The system is not perfect, as it does not distinguish yellow from a red-green mixture, but can powerfully detect subtle environmental changes.

So there is some truth to the three-color theory. However, other aspects of color vision cannot be accounted for by the trichromatic theory. For example, there is the phenomenon of color after images. If you stare at a red dot, then move your gaze to a white wall, you will see a green dot as an afterimage. If you stare at a green dot, you will see a red after image. The same thing happens with yellow and blue.