Some people are more sensitive to changes in light than others. Doctors believe light has a biological effect on brain hormones and function. Treatment is based on the interaction of light with the eyes, not the skin. Typically the patient is directed to sit directly in front of the light once or twice a day, from 30 minutes for the brighter light boxes to a couple hours for the lights that are not quite as intense. The time required varies between different people.
Opinions differ as to what kind of light and what intensity.
The standard for professional light boxes usually are fluorescent tubes. These give even disbursement of light and cool operating temperatures. Some individuals are sensitive to flourescent light, so there is no one answer, and a doctor should always be consulted. To combat seasonal affective disorder, the best context is a healthy mental and physical lifestyle. Some ideas you may not have thought of.