# Young-Helmholtz's Trichromatic Theory of Color Vision

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

How do we see color? It's a complex topic and this lesson will look at the foundational theory of Young and Helmholtz and see how this theory was developed.

## Seeing in Color

Have you ever watched old movies in black and white? Yeah they're a lot of fun, but when you switch over to a color program in HD you remember: color is awesome. We love color. Colors can affect our moods, alter our understanding, and bring us together. It's no wonder that so many artists across history have focused the bulk of their efforts on understanding color. However, a real understanding of color comes from knowing more than just how colors match on a color wheel. You need to understand how we actually see and perceive color at a biological level. Luckily, you don't have to drop out of art school and get a PhD in biology first. There are a few theories to help us understand. One of the most important is called the Young-Helmholtz trichromatic theory.

## The Young-Helmholtz Theory

At its most basic, the Young-Helmholtz theory states that within your eye are tiny cells that can receive waves of light and translate them into one of three colors: blue, green, and red. These three colors can then be combined to create the entire visible spectrum of light as we see it. That's the 2-second version of the theory, but let's take a deeper look at where this comes from.

## Thomas Young

In the early 19th century, English physicist Thomas Young was conducting experiments with light. Around 1801, he demonstrated that light was composed of waves and soon after began realizing that different colors of light had different wavelengths. Following on a theory of Isaac Newton's, Young calculated the approximate wavelengths of the seven colors: red, orange, yellow, green, blue, indigo, and violet (the colors of the rainbow). From there, this early scholar of color perception claimed that the eye must contain certain cells called photoreceptors and argued that each photoreceptor could only translate light waves of a single color. Therefore, the eye had to have multiple kinds of photoreceptors to see all the kinds of light. As an interesting side note, Young ended up taking a break from this research around 1814 to pursue another interest: the deciphering of the newly-discovered Rosetta Stone.

## Hermann von Helmholtz

Fast-forward a few decades to 1860 where a German scientist named Hermann von Helmholtz had continued Young's research. Young had been the first to propose that the human eye could have a few as three kinds of photoreceptors and still see the entire color spectrum, but it was Helmholtz who would really prove this to be true. Helmholtz proposed that the human eye contained specific three kinds of photoreceptors. Some could only receive light of a short wavelength. Some cones could only receive light of a medium wavelength. And some cones could only receive light of a long wavelength. While scientists of the time disagreed on which three colors these wavelengths represented, Helmholtz was able to demonstrate that all the colors we see could be created by a combination of three basic colors.

The basic idea was that if all three receptors were stimulated simultaneously and at equal intensity, the eye would perceive the color white, which he demonstrated by combining various lights of various wavelengths into white light. This experiment was essentially the opposite of running white light through a prism to separate it out. However, if the intensity of one wave was diminished, the end color would change. Furthermore, Helmholtz demonstrated that all three colors had to be active for this to work. If only two wavelengths were used, they could not be combined.

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