VISIBLE AND NEAR-VISIBLE RADIATION 157 



DETERMINATION OF ABSORPTION CHARACTERISTICS OF TRANSPARENT OR 

 PARTIALLY TRANSPARENT MATERIALS 



This second general type of problem presents far less difficulty. The 

 material the characteristics of which are to be determined may be intro- 

 duced either before or after the monochromator, depending upon the 

 characteristics of the sample. If the material presents optical parallel sur- 

 faces, and produces no appreciable scattering of the radiation, it may be 

 introduced either in a parallel beam or in a somewhat convergent beam, 

 provided compensating focal adjustment is made. The former method 

 is preferred. In this case, one may use a compound condensing lens, 

 such as Ci and C2 in Fig. 6, made up of two parts separated by a sufficient 

 distance that the material may be placed between the two components, 

 being illuminated in this way by parallel light and so producing no 

 appreciable change in the focal length of the condensing lens. (A slight 

 change is actually produced because of the effective change in separation 

 of the components. However, in general this may be neglected.) If 

 the material whose transmission characteristics are to be determined 

 cannot be secured in sufficient size as not to restrict the aperture of the 

 condensing lens, it may be necessary to place the sample close to the slit, 

 making compensating focal adjustment for focal condition of the con- 

 denser system. In any method which places the sample before the slit, 

 care must be taken that the image of the source upon the sht is not 

 displaced, as such displacement may place upon the slit an element of 

 area of different radiance. If one is dealing with material which not only 

 absorbs but scatters, the figure obtained for absorption will represent 

 loss both by absorption and by scattering, as the spectrograph system 

 will effectively exclude most of this scattered radiation. In case of 

 scattering, if one wishes to obtain a figure for absorption alone, it is 

 necessary to use a detector of very much larger receiver area than the 

 beam incident upon the sample. Ideally, such a detector should be 

 either a hollow inclosure with a sufficiently large opening, or a portion 

 of a spherical surface of sensitive material whose radius of curvature lies 

 approximately at the center of the sample.' The thickness of the sample 

 should be small compared with the diameter of the beam. The portion of 

 the spherical surface must be large enough to include practically all 

 scattered radiation. Even under these favorable conditions, the appar- 

 ent transmission will be lowered owing to the greater path traveled by 

 the deviated radiation. The rigorous analysis of such cases is beyond 

 the scope of this discussion. A comprehensive treatment of such cases 

 has been prepared by Mestre (53). Calculations of characteristics for 

 nonscattering solid or fiquid material are made according to Tables 5 

 and 6, respectively. 



