The Trichromatic Theory of Color Vision

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According to the trichromatic theory of color vision, also known as the Young-Helmholtz theory, there are three receptors in the retina that are responsible for the perception of color.

How Color Vision Works

The retina contains millions of photoreceptors called rods and cones. When light enters the pupil of our eye, it travels to the retina in the back of the eye. When the rods and cones detect light, they send a signal to the brain for interpretation.

The rods are sensitive to light and help us to see in dim lighting, whereas the cones allow us to detect color and detail in normal lighting. Of the three types of color receptors, one is most sensitive to the color green, another to the color blue, and a third to the color red. The combinations of these three colors produce all of the colors that we are capable of perceiving. Researchers suggest that people with normal color vision are able to distinguish between as many as one million different colors.

The ability to perceive any color requires interaction between at least two types of photoreceptors (dichromatic vision). However, the availability of all three color receptors (trichromatic vision) allows combinations that form all of the visible colors in the spectrum. 

Background of Trichromatic Theory

Color is a noteworthy part of our visual experience. It can affect how we interpret things about the world, influence our appetite and mood, and even carry symbolic meaning for some people. 

But what exactly explains our experience of color? A number of theories have emerged to explain this phenomenon, and one of the earliest and best-known was the trichromatic theory of color vision.

Renowned researchers Thomas Young and Hermann von Helmholtz contributed to the trichromatic theory. The theory began when Young sided with the unpopular theory that light traveled in the form of a wave, and he calculated the wavelength of each color in the visible light spectrum. In 1802, based on the understanding that combining three colors in different proportions could create all of the colors in the visible spectrum, he suggested the eye only required receptors that were sensitive to three of the color wavelengths (red, yellow, and blue), as opposed to needing a receptor for each color.

It was later in the mid-1800s that researcher Helmholtz expanded upon Young's original theory and suggested that the photoreceptors of the eye were either sensitive to short-wavelength (blue), medium-wavelength (green), or long-wavelength (red). He also proposed that it was the strength of the signals detected by each of the receptor cells simultaneously that determined how the brain interpreted color in the environment.

Through a series of experiments, Helmholtz demonstrated that people with normal color vision needed to have three unique photoreceptors that were preferentially sensitive to one of the three proposed wavelengths of light in order to perceive all of the colors in the visible light spectrum.

Young Helmholtz Theory

  • Helmholtz used color-matching experiments where participants would alter the amounts of three different wavelengths of light to match a test color.
  • Participants could not match all the colors if they used only two wavelengths but could match any color in the spectrum if they used three.
  • The theory became known as the Young-Helmholtz theory (also known as the trichromatic theory of color vision).

Color Receptors

The identification of the three cone receptors responsible for color vision did not occur until more than 70 years after the proposal of the trichromatic theory of color vision. Researchers discovered that pigments in the cones (opsins) have different levels of absorption of light waves.

3 Different Cone Receptors

  • Short-wavelength cone receptors (S-cones)
  • Medium-wavelength cone receptors (M-cones)
  • Long-wavelength cone receptors (L-cones)

Humans can see wavelengths that span from 400 nanometers (violet) to 700 nanometers (red). The full spectrum of visible light includes seven main colors: red, orange, yellow, green, blue, indigo, and violet (remembered with the mnemonic ROY G BIV).

Each of the three cone receptors is most sensitive to the specific wavelengths of either blue, green, or red and there are a million variations of color when these three are combined. Photoreceptors also tend to have different sensitivity levels. Blue receptors are the most sensitive and red the least.

The perception of color by the brain requires input from at least two different types of cones. The brain must interpret information about both the wavelength and the intensity of the incoming stimulation. By comparing the degree of input from each cone that has been stimulated, the brain can interpret the color of the source of that stimulation.

Most people with normal vision are trichromats, but in rare cases, some women have a genetic mutation giving them four types of cones that allow them to see exponentially more colors than most people. This is known as tetrachromatic vision.

Color Blindness

We cannot talk about color vision without mentioning color blindness. The underlying cause of color blindness is a genetic alteration in one or more of the cone pigments.

Typically, people with color blindness lose the ability to differentiate between reds and greens, which indicates a loss or mutation of M- or L-cones causing a decrease in sensitivity to either green or red wavelengths.  Less common is blue-yellow color blindness caused by a loss of S-cones and a decrease in sensitivity to blue light. People with this type will have difficulty discerning between blue and green, or between red and yellow. In rare instances, all three cone pigments are absent, and the world is seen in shades of gray.

Types of Color Vision

The four types of color vision include:

  • Monochromatic: 500 shades of gray
  • Dichromatic: 10,000 colors
  • Trichromatic: 1 million colors
  • Tetrachromatic: 100 million colors

Trichromatic Theory and Opponent Process Theory

In the past, the trichromatic theory was often presented as competing with the opponent-process theory for dominance in explaining color vision. Today, it is believed that both theories can be used to explain how the color vision system operates and that each theory applies to a different level of the visual process.

The highlights:

  • Opponent process theory: Color vision at the neural level
  • The trichromatic theory: Color vision at the receptor level

Takeaway

Color vision and perception is a complex process that involves the eyes and brain. The trichromatic theory explains one part of this process, focusing on the photoreceptors in the eye that then send signals to the brain. Learning more about this aspect of color vision is an important part of understanding how we perceive things about the world that make up our visual experience.

9 Sources
Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Kendra Cherry, MSEd
Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."