A reader asks:

 The CET Store offers two different light boxes, one of which uses a color temperature of 6500 Kelvin and the other, 4000 Kelvin. Also, one produces 7500 lux light intensity, and the other, 10,000 lux. Do these two units have different effectiveness, results, and purposes? How do I decide which would be better for me?


Before we answer, please note that last year’s (2003) model of the 10,000 lux device used lamps with 3000 Kelvin color temperature, which is perceptually a bit pinkish relative to the current 4000 Kelvin. Higher Kelvin does not necessarily translate into higher light dose, however. Both of our recommended units have been demonstrated to have antidepressant efficacy, although formal clinical trials for SAD have yet to be performed for the 7500 lux model.

The 7500 lux device was originally designed for enhanced work station lighting, to be flexibly used throughout the day at either low, medium or full intensity. This device has now also been positively evaluated at full intensity in patients with non-seasonal antepartum or bipolar depression, and there is no reason to think that it would not also work for SAD.

With lower maximum light output, some people will find that they need to increase exposure duration beyond the 30-minute average used with the 10,000 lux device. In antepartum depression studies, for example, patients used the 7500 lux device for 45-75 minutes; in a past SAD trial with another device providing about 7500 lux, everyone was treated for 90 minutes per day.

Because the manufacturers of these devices have chosen to use different color temperatures, however, it is impossible strictly to compare lux levels. All other things being equal, 6500 Kelvin appears whiter and brighter, while 4000 Kelvin appears softer. Both contain wavelengths that are active in the therapeutic range. We have named the 10,000 lux device a “clinical” unit, mainly because it has been extensively formally tested in clinical trials of SAD and chronic depression.