Red light penetrates through the skin several inches. I make a circle using the velcro on the belt there is a section of about 4 inches that is not lights where the belt overlaps, and is velcro. But almost all of the circle is lights on the inside. I slip the belt over my head, and set it for 20 minutes, level 4, both uv and red light. While the belt is on my head, I keep my mouth open so the light gets through the skin, but is also able to get to the inside where bone would stop it from penetrating. When the 20 minutes is over, the light automatically shuts off.