A reader asks:
What is the correct way to measure the distance from the box, which determines lux level? If it is at eye level, then obviously it is just the horizontal distance in front of one. When it is above you, do you still just measure horizontal distance, or do you measure the diagonal from your eye up to the box?
The configuration we recommend — see the illustration under Bright Light Therapy at the CET Store — uses a tilted box, and we measure the distance horizontally on a plane between the eyes and the lower 1/3 of the screen, which is directly ahead. It sounds like you are raising an untilted box above your head, in which case the light would be projecting forward, not toward your eyes. That sounds non-optimal, and the distance rule would not apply. Rather, you would need to place a light meter that reads in lux at the position and orientation of your eyes as you take treatment.