The physiology of seeing colors

More colors today, partly in honor of the fact that Alan’s color idioms project just crossed the 700 idiom mark. It’s idiotastic! Since it got picked up by a few blogs (like this and this), it’s even surpassed the Elvish page to be the single busiest page on my site.

This particular post, however, isn’t about how we talk about color but rather how we see color. Here is a very basic question: What is color?

You might reasonably expect a solid physics-based answer to this question. But it’s not so simple. To start the discussion, here’s a diagram that used to puzzle me extremely.

It’s our friend the electromagnetic spectrum. It spans an absolutely vast frequency range of the various kinds of wiggling that photons can do. But look at that rainbow band in the middle that defines visible light. It’s a teeny weeny little thing. You’re being bombarded by electromagnetic radiation all the time and from every direction, and all you can make out is this tiny little sliver of it. Why is that? Why can’t you see ultraviolet and infrared? What color are microwaves? Your iPhone is brilliant twinkling flashlight that distant antennas can see. Why can’t you?

It was a long time before I realized that the problem is framed partly by the chemistry of the eye. Your retina is a delicate lawn of special proteins that sense light when they’re jostled by arriving photons. But if you jostle them too little or too much, no light gets sensed. By way of analogy, think about the annoying wind chimes on your neighbor’s porch. If the wind is a soft lazy breeze, no chime. If it’s a hurricane, the whole apparatus blows away. No chime. When infrared comes knocking on the old eye-door, it may warm your retina, but there’s no chime.

In short, color is defined by physiology. Color is the thing that looks like this. The human is in the loop. I can write the most quantitative of equations about the frequency, wavelength and energy of photons, but as soon as I mention color (or sound for that matter) I have to put an unreliable blob of protoplasm in the driver’s seat. And the way humans sense color is very complicated. And that is what leads us to this enjoyable essay by Jason Cohen of the Smart Bear blog: Color Wheels. For even more detail, Jason points us to this monograph by the enigmatic Bruce MacEvoy.

One thought on “The physiology of seeing colors”

  1. Very nice. I think the wind chime analogy is spot on, insofar as all of our color photoreceptors (cones) are cellular and biochemical decendents of our main photoreceptors (rods), such that the tweaking of the chemistry that gives us our color vision can only cover a narrow range without significantl changes to different molecules. The caveat being, as mentioned in the Bruce MacEvoy post, that all of our photoreceptors are centered around sun and daylight illumination; in other words, we can only perceive a narrow band of EM, but it is the most useful narrow band here on Earth for getting around without bumping into things.

    I, too, have always been fascinated by the fact that the EM spectrum is linear, but our color perception is circular. The telecommunication industry recognized this long ago when they created the RGB CRT that maps satisfactorally to the excitatory peaks of our RGB retinas, completely ignoring the linearity of the EM spectrum. This gives them the one thing you can’t get from physics: purple. Our eyes and our monitors are all about relative intensities of signals and not wavelengths, so we can see purples and magentas in the space between blue and red, as long as there isn’t any green. In the EM spectrum between blue and red there are no magenta wavelengths, only yellows and greens. There is no wavelength for purple, but it is as real as any other color. It really is all physiology.

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