The effective range for red light therapy appears to be from 670 nm to 720 nm, allowing photons to penetrate deep into the dermis.

“So the range that seems to be helpful, and, I confess, I use a red light panel for other things, is 670 nanometer out to about 720 nanometer, so it looks like red and very dim lights, dim red and bright red. And the idea is that red light can travel, the photon and energy is such that it can travel down through the deep layers of the dermis of the skin.”