A reader asks:
Most of the equipment referred to by your questioners, as well as that sold for depression treatment, uses fluorescent tubes. Have incandescent sources been used in trials? Would it be effective, since color temperature does not seem to be a factor?
There were a couple of very early clinical trials of bright light therapy for SAD, which used incandescent bulbs, but that avenue of R&D quickly died out. Fluorescent tubes are used in preference to incandescent bulbs for several reasons. Importantly, for the high therapeutic intensities involved, incandescent lamps would radiate too much heat to be practical. Secondly, most incandescent lamps provide imbalanced spectral output biased toward longer, reddish wavelengths of light, while most fluorescent sources provide peaks of energy throughout the wavelength spectrum.
Color Temp Matters
In contrast to what you say, color temperature may indeed be a factor. For example, we are learning that the circadian rhythm system and the melatonin system are both impacted more strongly by shorter (bluish) than longer (reddish) wavelengths. We are not yet certain that the antidepressant system responds in a similar way, but a good guess is that it does. Our current recommendation is to use a “soft white” fluorescent lamp that contains both short and longer wavelengths. We warn against the use of intense blue light.