The Penetration of Living Tissue by Ordinary Radiant Energy

Abstract

Since water in a layer 10 millimeters or more thick is well known to be inpenetrable by ordinary radiant energy (that is, in the ultraviolet, visible, infra-red, and heat spectrum) of wave lengths longer than 1.4 micron, it has been suggested by Coblenz 1 that living tissue will, in general, be permeable only to wave lengths shorter than these. Sonne 2 found penetration of living tissue in his so-called “visible range”, but his filters (water and 5% ferro-ammonium sulphate in glass chambers) apparently transmitted waves to a length of at least 1.3 micron. 3 On the other hand, Bachem and Reed 4 have recently shown that wave lengths shorter than 0.6 micron practically do not penetrate living skin. Evidence exists to show then, that the wave length range of ordinary radiant energy capable of penetrating living tissue lies between 0.6 and 1.4 microns, i. e., it includes those infra-red rays lying near the red rays of the visible spectrum. Independent experiments initiated by me furnish further evidence in this regard, and indicate the extent to which these rays penetrate living tissue. The required measurements were made by G. H. Cartwright at the California Institute of Technology under the direction of R. C. Burt.

For convenience, the experiments on transmission were performed on one subject's cheek which averaged 5 mm. in thickness. Initial experiments indicated that this tissue transmitted about 14% of infra-red rays of a wave length of 0.86 micron. These experiments were made by means of a spectrograph and photographic plates. Opal glass 2 mm. thick was placed in front of the slit of the spectrograph to diffuse the light, the source of which was a 15 Watt electric light bulb.