Color Rendering: Asking the Question

Two white lights may have the same chromaticity, and yet when used to light an array of objects may differ in their ability to reveal colors. For example, any white light can be matched by a mixture of two narrow-band lights, a yellow plus a blue. In this extreme case, reds and greens become black or brown and the red-green dimension is lost. At the other extreme, a light with three narrow bands, at the proper wavelengths, can brighten reds and greens and increase redgreen contrast, relative to a broad-band light such as daylight. Many commercial lights tend to dull reds and greens, relative to broad-band sources, a central reason that color rendering is a practical concern. A telling example is neodymium glass, a yellow-absorbing filter that is sometimes used to improve color rendering. This article seeks to bring these ideas to life through detailed graphical examples.