A reader asks:

 I am ready to purchase a 10,000 lux light box, but am still confused about wavelength vs. degrees Kelvin (K) and full- vs. broad-spectrum light. If around 2800 degrees K is yellow-red and 11,000K is very blue, what is ideal? One unit I am considering specifies 3000K — would that be too yellow? Are the terms broad- and full-spectrum equivalent? Is it not true that both include all the colors — or, if colors are determined by wavelength, which ones are harmful? I know these details seem academic, but my eyes and skin are extremely sensitive and burn very easily. I would not be able to use a light box that failed to filter out all the harmful “rays.”


Color Temp

It would take a long essay to answer your important questions in detail. So let us try to give the bottom line. The higher the color temperature, the greater the representation of short wavelengths of light in the violet-to-blue range. Think of the “daylight” standard as about 6500K. Perceptually, that is a crisp, cool white. 3000K is a softer white with a slight pinkish tinge. Depending on the particular fluorescent bulb, the coloration is determined by a set of phosphors that fluoresce in relatively narrow wavelength bands. One needs to examine a graph of the wavelength pattern to determine how a given bulb mixes particular phosphors to create the overall coloration. While a 3000K bulb appears soft and easy on the eyes, it still contains a phosphor that emits photons in the short-wavelength blue range. So, do not listen to manufacturers who claim that higher color temperatures are “better” because they contain more blue. There is no clinical evidence for such claims. Light of lower color temperature has been thoroughly tested for clinical response, comfort and safety.

Full vs. Broad Spectrum

“Full spectrum” is a hyped term used in misleading advertising. It has a relatively high color temperature — white with a bluish tinge — and was originally developed actually to boost UV output, which can cause skin puffiness (erythema) and even burning under 10,000 lux conditions. There is absolutely no evidence that full spectrum bulbs have a therapeutic advantage. “Broad spectrum” merely means that a wide range of phosphors are activated to create a white, or whitish, light. Any light box, regardless of color temperature, should include a smooth diffusing screen that filters out 99% or more of ultraviolet (UV) radiation, the source of tanning. Long-term exposure even to low levels of UV constitutes a potential hazard to both eyes and skin, and must be avoided. We say: if you are choosing a light box, challenge the manufacturer to show you a graph of spectral output including the UV range between 200-400 nm, and make sure there is negligible UV transmission. So, we did write you an essay! . . .