A reader asks:

What is the difference between color temperature and the color rendering index (CRI)? Which is more important for light therapy?


Color temperature reflects the distribution of wavelengths of light produced by a bulb. The white or whitish light used therapeutically is created by a particular set of phosphors that fluoresce in the tube, and these vary widely. Successful treatment has been obtained across a wide range of color temperature, from about 3000 degrees Kelvin to 6500 degrees. (Important note: higher does not mean “better”!) Outside that range, we see distinctly colored light, red or blue, which we do not recommended for light therapy at high intensity. The CRI measures how accurately you will perceive colored objects — say, clothing fabric or skin tone — in comparison with how you would see them in outdoor light. This, in itself, is not important for light therapy — unless, of course, you are working on an oil painting while taking the treatment!