Uusally you want one With a 10,000-lux intensity (a measure of the strength of the light illumination. light therapy typically involves daily sessions of about 20 to 30 minutes. But a lower-intensity light box, such as 2,500 lux, may require longer sessions. You should be able to find on the box or product specifications the lux. Word of warning, going online you may find some bill themselves as for light therapy, but if they don't have 10,000 lux it may not truly be therapeutic. Also watch if you have bipolar, light therapy can trigger mania or hypomania