Optical irradiance is the power density delivered to your skin, measured in milliwatts per square cm. See the photo of the irradiance levels by distance from the light.
This panel gives 174 mw/cm2 irradiance when standing 12” away, which is substantial power. Different goals require different levels of irradiance.
(While we cannot give medical advice, we suggest customers do a little research online to understand what distance you need to mount your panel to get the optimal irradiance for the therapy type you need. Some needs may require the light to be closer or farther from the body.)