Certified as a Medical Device
Light therapy products should certified to the Medical Devices Directive (93/42/EEC) and conform to European manufacturing standards and carry the correct mark (e.g. e.g. CE1639).
Guidelines for treatment time and distance
Light intensity drops dramatically over distance. A short treatment time may have been measured impractically close. Look for clear guidelines about distance so that you know how powerful the light really is and whether or not it will suit your way of using it.
Filtering of UV light
Most people are now aware of the dangers of harmful UV (ultraviolet) light. Properly designed light therapy products will incorporate special screens and filters to ensure that UV levels are negligible.
Shown to work in clinical trials
Light therapy makes a real difference but if you are not sure it can seem like an expensive risk. Reputable companies will offer a trial so that you can try the product out for yourself.
Extra considerations to look for in SAD lights:
Lux measurements linked to distance
A product description promising 10,000 lux is meaningless if it's not linked to a practical distance. A lightbox that produces 10,000 lux when measured 1cm away may emit only 500 lux at a more useful 50cm. Look for manufacturer's recommendations to follow when positioning your SAD lightbox for treatment.
Blue light devices
Use of blue light alone is still being investigated. Whilst wavelengths in the blue part of the spectrum are most effective at influencing the circadian clock and are beneficial in treating SAD and circadian disorders, there is currently nothing to suggest that blue light on its own is better than full spectrum light therapy. Furthermore, there are no studies into the long-term effects of using blue light only.