A reader asks:

 If I buy a small light box with less lux output, is it simple math to adjust my sessions — for example, half the light output equates to twice as long in front of it — or is it not that simple?


Although there is a general trade-off between light intensity and exposure duration, no one knows the exact formula. (Manufacturers who provide a “linear solution,” like in your illustration, are just guessing, which is irresponsible.) All the relevant research has varied light intensity while the size of the box is kept constant. Box size introduces a major complication. With small boxes — which naturally tend to be less expensive — even slight head movements can remove you from the therapeutic range of light intensity. That is why we strongly recommend large-size screens. This is a case of “smaller is not better”!