The wavelengths of light. Blue light and UV light, for example, get almost completely blocked by the skin and do not penetrate much more deeply than the very surface layers of skin. Whereas red and near-infrared penetrate much more deeply. And within that category, near-infrared has significantly greater penetration depth than red light, and there are even some small differences between specific wavelengths of red and near-infrared light in terms of penetration depth.
