The Biological Layers of Color Perception

Sixth in a series of blog entries about color theory with live help from the ColorTheory (Step 6) .  First_post,  Prev_post,  Next_post

Colors are fun to investigate, because there are so many ways to look at them, all different. all seemingly contradicting each other, and yet all correct. There are a wide variety of different color wheels, each of which illustrates something different. And here we start to look at them.

As discussed in a previous post , this arrangement originated with Newton and his investigation of the spectrum. But it’s not quite what the spectrum really shows.

Color Wheel according to spectrum

The spectrum color wheel distributes the colors equally across the spectrusm. Note that blue and green make up much more of the spectrum than the traditional wheel allows.

We now see the wavelengths (in nanometers) of light equally across the wheel. But we also see two additional labels for the ends of the spectrum, ‘>660′ for the Infrared, and ‘<420′ for the Ultraviolet. This represents the ends of the rainbow. The magenta colors between them are “nonspectral”- they will not appear in a rainbow or a prism, and they can’t be represented by a single frequency of light. Yet we see them.

Because our eyes can see all the wavelengths, all at once.

The sweep of the wavelengths as it enters our eyes is call the reflectance . A rough analogy may be through musical notes. The reddish hues are the low notes, the blues are the high notes, and green is in between. Our eyes can “see” the chords of the colors. The spectrum only shows a single note at a time. Those “non-spectral” colors between the red and the blue are the optical equivalent of chords of music with only high and low notes, missing the middles notes.

As a result, except for scientific studies, the spectrum really results in a somewhat unsatisfactory way to look at color. But how can we hope to model the hundreds or thousands of possible chords of light?

Cones responses to Color

Luckily I lied just three paragraphs ago when I said our eyes can see all the wavelengths. We do, but when we do, we lump what we see into only three categories, because our eyes use three types of optical cones. The figure below shows the current best estimate of the eye cone responses (the “Stiles-Bursch” data).

What this means is our response to colors always start with three values, our three cone responses. And so we can start the characterization of our perception of color with those same three colors. And that leads us to an RGB color model. Only unlike the discussion about sRGB or the “Yurmby” wheel in our second and fourth posting , in this case we are interested in how our eyes see colors, rather than how to mix lights to generate colors.

sRGB Color Wheel

But the three lights chosen to generate colors also, for good efficiency reasons, closely match the actual responses of the three cone cells. So we will use it as a close surrogate to a wheel based exactly on the cone response.

sRGB Cone Response to Spectrum

So now to the right we can see that the cone responses of the eye are more sensitive to yellow/greens, and less sensitive to the green/blue colors.

But to my eye, and to others as shown by scientific study (although color perception does vary individually), the green sectors in the sRGB wheel blend together into a single green from 540 to close to 570, while the reds and oranges seem to vary significantly from 570 to 600. So it seems the cone responses aren’t enough to explain why and by how much colors differ.

And this is born out by scientific studies– the optical cone cells are only the first layer of neurological processing that the optical system uses to perceive color. We’ll discuss the additional layers in our next post.

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