A reader asks:

I am confused. A recent Ask the Experts answer said that color temperature and spectral output matter. I understood your previous answers to suggest they did not — that any fluorescent bulb spectrum is fine as long as the UV is filtered out. Is this new info, or do I misunderstand? I am looking into buying a light box and am now unsure if one of my considerations should be the claims about spectral output?


As a general rule, any white or whitish fluorescent lamp will contain adequate spectral output for a positive treatment effect because it will project short, medium and long wavelengths.

There is recent evidence that short wavelengths — in the blue or bluish range — mainly affect one (but not all) of the retinal receptors that input the circadian clock. That does not mean you should be seeking blue light, however. Seek a broad, balanced spectral output, which one obtains from tri-phosphor, multi-phosphor, daylight and softer fluorescent light. If a manufacturer is boasting about optimum spectral output of a particular apparatus, it is a sign of deceptive hyping.

We recommend light boxes of 4,000 Kelvin, which are visually comfortable. Higher color temperatures, which project more blue, are no more therapeutic than lower color temperatures despite occasional hype to the contrary.
For more information on light boxes, please see our page on light boxes, and light therapy.