Hue Depends on Intensity

What color is 600 nm light? Most people who write of such things write as though there is an unambiguous answer to such questions, but it’s been known at least since Helmholtz in 1867 that that isn’t true. In this case I’m talking about the perceptions of an individual observer. It’s even less true if you try to compare perceptions across individuals even if your individuals are all humans. But let’s get back to what Helmholtz discovered. The figure below simulates what you would see if somebody showed you a spectrum of light (where each region of space has light with just one wavelength) as the intensity moves up and down. The simulation isn’t very good because computer monitors can’t do a good job of mimicking spectral (a.k.a. monochromatic) lights. However, it should be good enough...

If the figure isn’t animated, please see this page.

Look carefully at the different hue boundaries. In particular, look for the region where red blends into orange. As the intensity goes up and down, does that border stay in the same place? What about the border between blue and purple? If these and other borders don’t shift right and left as the intensity waxes and wanes, you probably need to get a new monitor. ☺

Whether or not this demonstration works for you, it certainly works under controlled settings. The hue (what most people think of when they say "color") of a light depends on intensity as well as wavelength distribution. So you shouldn’t say that a wavelength has a color!

This page was last edited on August 25, 2004.

© 2004, Mickey P. Rowe