I have been enjoying the book The Future of the Mind, by physicist Michio Kaku. It is interesting and well researched, but he does make one statement that I have to take issue with. He says that humans “see” only three colors. This is not the first time I have heard this claim from someone who ought to know better. (I suspect that vision experts he consulted for the book told him this, and he didn’t question it.)
The notion that we see only three colors is based on the fact that there are three primary colors and we have three types of color photoreceptors, or cones, in our retinas. The idea is wrong, however, on two counts.
For starters, it is important to understand that our eyes detect all frequencies of light in the visible spectrum. We are not limited to the three primary colors. In fact, each type of cone has a response function that spans most of the visible light spectrum. The response peaks at a different spectral frequency for each cone type, and it tapers off as the frequency gets further from the optimum for that type. The peaks do not even correspond with red, green, or blue colors.
Almost any frequency of light in the visible spectrum will cause a response in all three types of cones. Our visual systems combine the three response levels to discern the frequency. For example, light with a 500 nanometer wavelength, which has a blue-green shade, results in a strong response from the “green” cones, about 7/10 as strong in the “red” cones, and only about 2/10 as strong in the “blue” cones. That combination of responses allows us to distinguish between various light frequencies. (There is a lot more to it than that, of course. Color can be influenced by context, for example, and we have a remarkable ability to compensate for different types and levels of lighting.)
We need at least two types of cones in order to distinguish frequencies this way, and three are better. With only one type of cone, we could not tell a dim stimulus at the optimal frequency from a brighter stimulus at a less responsive frequency.
With three types of cones, our visual systems can distinguish a rich variety of many colors, more than two million for most people. I don’t know about you, but my perception of those colors is that they are all different, not just one of three.
It is true that color televisions use only the three primary colors, but that is not because those are the only colors we see. Rather, it is because three colors are sufficient to produce an experience of seeing most of the visible spectrum. With the right combinations of those primary colors, we can stimulate the cones at levels nearly identical to the levels produced by other colors of light, fooling the brain into thinking it is seeing the other colors.
But there is another, more mind-blowing problem with the claim that we see only three colors. That statement might lead one to believe that we are missing out on so many other colors in the visible spectrum that we cannot truly see—and that is usually the intent of the person making the claim. It is wrong, though, because there is no color in reality. Light waves have physical properties including wavelength, frequency, photon energy level (all interrelated), and intensity, but not color.
Color is an interpretation imposed by our brains, as a way of making meaningful sense of the complex mix of light frequencies and intensities striking our retinas. We rarely see pure spectral colors, outside of rainbows and reflections from surfaces that diffract light (such as CDs and DVDs). Most everyday objects reflect many different frequencies of light. The color we perceive is a result of the combined sensations caused by that assortment of frequencies. Whatever the total cone response is, our brains assign a corresponding color to it.
Color perception is one of the most profound examples of the way our brains construct representations of the world around us from sensory inputs. As real and as important as color seems to us, it nonetheless originates within our minds. For that reason, there can be no colors other than the ones you see. But for most of us that range of colors is rich and wondrous, and certainly more than three.