Nonisotropic propagation of light in the human forearm

Abstract

Light propagation in the skin of the human forearm was investigated in vivo and in situ. In this study, a sensitive photomultiplier tube with a spectral range of 300 similar to 650 nm was used to detect the extremely weak light that is propagated in skin from 17 mm up to 33 mm. We devised a control system that enabled us to guide the optical fibers so that the input and the detection could be exactly located on the skin in complete darkness. We obtained an effective attenuation coefficient whose values were 0.21 +/- 0.03 cm(-1) and 0.28 +/- 0.06 cm(-1) in the longitudinal and the transversal directions, respectively, over a long distance (> 17 mm). Those value were much smaller than the previously known value of 2.7 similar to 3.3 cm(-1) for a short distance (<= 2 mm). In addition, we found that light propagated better longitudinally than transverse, which contrasted with the isotropic propagation at short distances. These results might have significance for phototherapy.