430 HAROLD F. BLUM 



separated epidermis in which the intensity was measured by an ele- 

 ment that could, by use of stops, be caused to accept the light from 

 various solid angles, extrapolation then being made to 180° solid 

 angle. These methods have been used chieflv for measurement of 

 transmission (///o) by a scattering layer, e.g., the human epidermis, 

 rather than for determining absorption by the layer. For the latter 

 purpose one needs to take into account the light scattered throughout 

 a 360° solid angle, a more complicated measurement. 



Attenuation Coefficient. In a homogeneous scattering and ab- 

 sorbing medium, the integrated intensity decreases with the dis- 

 tance traveled, in the same manner as in a nonscattering medium. 

 Thus, one may write, in form comparable to the Bougeur-Lambert 

 law (equation 5) : 



I = lo e-"' (10) 



where a is a constant, known as the attenuation coefficient, which 

 recognizes both absorption and scattering. In systems containing 

 no particles larger than molecules, a relationship corresponding to 

 Beer's law may be expected to hold, at least within limited ranges of 

 concentration. Equation (10) should hold for systems in which 

 scattering is due to particles larger than molecular size, so long as 

 their size and distribution are homogeneous, but, unfortunately, 

 living tissues are seldom homogeneous throughout any great thick- 

 ness, and hence attenuation coefficients cannot be accurately cal- 

 culated. The concept of the attenuation coefficient is, however, a 

 useful one in the idealization of such systems for theoretical considera- 

 tion (e.g., 14). 



4. Fluorescence 



Many biological substances fluoresce, i.e., the molecule after ab- 

 sorbing a quantum emits within a very short interval of time an- 

 other quantum, usually smaller than that absorbed. Hence a certain 

 portion of the absorbed radiation is re-emitted as radiation of longer 

 wavelength. The direction of this fluoresced radiation, like scattered 

 radiation, bears no relationship to the direction of the incident beam. 

 Thus fluorescence introduces error of the same kind as scattering, 

 but in most cases the magnitude of loss due to this factor is not 

 great. 



