Radiometry, part 2: Spectra and photometry

The radiometric quantities introduced in part 1 of this series are completely color-blind. That is an obvious drawback for rendering. Thus far, our notion is that radiance is a single number, not a color. A fairly common practice in rendering is to have separate radiance values for red, green and blue. If your scene happens to be lit exclusively by light sources with only three wavelengths and does not exhibit fluorescence, that is a physically accurate approach. Unfortunately, real scenes are not that simple. In this post, we describe the physical reality more completely. We consider spectral versions of all radiometric quantities. Then we take a look at human perception of color and brightness, which leads to photometric quantities. Next, we figure out how to handle that efficiently in a renderer. And finally I rant a bit about all the confusion that surrounds this subject.