High Quality Content by WIKIPEDIA articles! The Optical window (also known as therapeutic window) defines the range of wavelengths where light has its maximum depth of penetration in tissue. Within the NIR window, scattering is the most dominant light-tissue interaction, and therefore the propagating light becomes diffuse rapidly. Since scattering increases the distance travelled by photons within tissue, the probability of photon absorption also increases. Because scattering has weak dependence on wavelength, the optical window is primarily limited by absorption, due to either blood at short wavelengths or water at long wavelengths.