A reader asks:

 You suggest on your site that a light source should “provide 10,000 lux at a comfortable distance.” Yet one of your recommended light box models (the DayLightXL) provides only 7,000 lux. How do you reconcile this contradiction?


10,000 lux at a distance of 12 inches from the light box screen has become a widely used measurement standard in the field of light therapy. A light box rated at 10,000 lux at a distance of 12 inches mimics the brightness of a window view on a sunlit scene. This is the white-light level that provides the most efficient treatment available for a session 30 minutes long. The combination of 10,000 lux / 30 minutes works for the large majority of light therapy users, although some will do best with lower lux, different durations, or both. (Higher lux has not been tested and should be avoided.) If a fixture rated 10,000 lux at 12 inches is placed at a comfortable arm’s length distance about 24 inches from the eyes (twice the 12 inch standard), the eyes will receive only one quarter the brightness, or about 2500 lux. To compensate, the session should be longer than 30 minutes.

As examples: if the eye receives 7000 lux, a user may need 45 minutes; or at 2500 lux, 1-2 hours. Carefully understand that we have stated “may” need extra time. These average times do not necessarily apply to any given individual. For example, some particularly light-sensitive people may find 30 minutes (or even less) at 5000 lux fully adequate. Each person is different and must determine aspects of their personal bright light regimen, including adequate length of light exposure, the best placement of the fixture at a comfortable distance and the best time of day to use the lights. (For initial time-of-day recommendations, complete the Automated Morningness-Eveningness Questionnaire at www.cet.org.) The combination of all these factors must be taken into account for a positive response to the lighting regimen – and you can expect to find the right solution in about 10 days of use.