The spectral energy of a light transmitted through the cheek was tested photographically in the region of λ0.86µ, and also by a spectrometer throughout the whole spectrum. A very sensitive vacuum thermocouple was used. The photographs of the spectrum showed 14% transmission of λ0.86µ through the cheek. The cheek transmitted a maximum of 20% at λ1.15µ and the water in the body cut off the longer wave lengths. The spectral energy transmitted by bacon fat was found to be similar to that of the cheek; however, it transmitted nearly five times as much energy. The ordinary incandescent electric light with filters to cut out the visible and infrared beyond λ1.4µ is ideal for deep penetration of light into the body.
C. HAWLEY CARTWRIGHT, "INFRA-RED TRANSMISSION OF THE FLESH," J. Opt. Soc. Am. 20, 81-84 (1930)