how powerful is this LED?
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4870908/
seems to suggest you need several watts..
"snouf and colleagues reported that only 34% of an 850 nm continuous light source at 100 mW could penetrate 0.784 mm of human skin. Approximately 89% of 820 nm light will penetrate 0.4 mm of epidermis (Kolari, 1985) and about 13.5% traverses 2 mm of skin (Henderson and Morries, 2015a). Our laboratory research shows considerably less penetration by low level NIR. We found energy from a 50 mW 810 nm LED did not penetrate 2 mm of human or sheep skin (Henderson and Morries, 2015a). Similarly, no energy could be detected penetrating either human skin or sheep skin from a 200 mW LED (650 + 880 nm). Using higher energy devices, we found 9% of the energy from the 10 W combined 810/980 nm continuous wave infrared laser passed through 2 mm of skin (human or sheep)."
Interesting article. The dosage is controlled by J/cm^2 in most of the studies I'm looking at - so time and distance from the emitter are the other factors.
eg, This study used an array of 500mW LEDs and a 10 minute exposure to achieve 13 J/cm^2.
Alas, I didn't measure it with a meter (since I don't have any), but according to the spec it draws 14W with a panel size of ~70 square centimeters. There's diffraction and everything, but since I put it directly on the skin, it's fair to assume that the theoretical absorption is 200mW/centimeter. BUT, power loss is huge (cheap infrared LEDs are below 50% efficiency), I don't think that, all in all, more than 30% of the input energy are preserved. So that would be 60mW/centimeter.
