XIII. ACTION SPECTRA AND ABSORPTION SPECTRA 425 



made with a thermocouple or bolometer, or are referred to these in- 

 struments, which measure radiant energy flux per unit area in terms of 

 energy units. Hence it is general practice to express intensity in 

 terms of energy units per unit area per unit time; and herein /' 

 will be used to designate the intensity when expressed in these units. 

 Since the absorption coefficient is determined from the ratio of intensi- 

 ties, the numerical value is the same whether / or /' is used, i.e., 

 numerically Iq/I = Iq /I' . However, when one wishes to compare 

 the effectiveness of radiation of different wavelengths, as in dealing 

 with action spectra, the definition of these terms becomes important 

 (see Sect. C2). 



Measurements. In biochemistry, absorption spectroscopy is 

 applied extensively in the identification and estimation of compo- 

 nents of biological systems; in most cases the material is removed 

 from the living system, and its absorption spectrum measured in 

 some solvent. The absorption spectrum of a substance may differ 

 considerably according to the solvent in which it is dissolved, and 

 identification is therefore made by comparison of the absorption spec- 

 trum to those of known compounds in solution in the same solvent. 

 Since certain structures have characteristic absorption, such studies 

 may assist in determining the chemical structure of substances of un- 

 known constitution although advances in this direction are somewhat 

 limited {6,7,2Ji). The determination of absorption spectra under 

 conditions where one deals with homogeneous substances in known 

 solvents and where scattering of light is usually not an important 

 factor is quite straightforward, since appropriate equipment is avail- 

 able for this type of measurement (see 5,8,9a). Thus in biochemistry, 

 methods of extraction and problems of organic structure may out- 

 weigh those of absorption spectroscopy. The biophysicist and 

 physiologist, on the other hand, are more commonly presented with 

 the problem of measuring absorption spectra of substances inside liv- 

 ing systems, and here new factors enter, for example, light scattering. 

 Particular difficulties are encountered in attempting to determine the 

 light absorber in a photobiological process, where the action spectrum 

 is compared to the absorption spectrum of a substance that must, 

 as a rule, be removed from the living system to an entirely different 

 environment, i.e., to some solvent that does not resemble closely the 

 milieu of the living organism. Before considering some of these dif- 

 ficulties, methods used for measurement of absorpt ion spectra will be 

 discussed briefly. 



