@masterpopo The basis for stimulating testosterone production by shooting red light and near-infra red light (yep, especially on your testicles) lies on the mechanism how red (or infrared) wavelengths work inside the cell. The key is that they stimulate ATP production in Leydig cells, thus increasing the energy available for the cells. This means more testosterone production.
Avoid heating the testicles, since the heat will destroy sperm cells and have a negative effect on the Leydig cells.
Avoid blue light and UV light exposure on testicles (blue light inhibits ATP production in mitochondria).