Light intensity, sometimes referred to as irradiance, is simply a measurement of how much light energy your device is delivering to your body. An underpowered device would mean extremely long treatment times, or no benefits at all.
Light intensity is measured in milliwatts per centimetre squared (mW/cm2) and you want to look for a light intensity around 100 mW/cm2 at treatment distance*
*While the "100 mW/cm2" is important, it is essential that the company tells you the distance that they were away from the device when they took this measurement.
Many underpowered devices will give a measurement as "100 mW/cm2" without telling you the distance. This usually means they took the measurement right up against the device, but when you’re a few inches away you’ll have no idea if you’re actually getting enough light for any benefits.
For full transparency the company will tell you the light intensity measurement at various distances.