XIII. ACTION SPECTRA AND ABSORPTION SPECTRA 435 



incident radiation. In practice it is usual to ari-ange the apparatus 

 so that the measuring element can be interchanged with the surface 

 of the biological material under stud3^ The measuring element of 

 choice is a thermocouple or a bolometer, since these are nonselective 

 with respect to wavelength, but other, more sensitive instruments, 

 e.g., photoelectric cells, may be used if their spectral sensitivity is 

 known and can be taken into account. One usually measures the 

 intensity of the incident beam, and then allows the beam to impinge 

 upon the biological material for a certain length of time; the incident 

 energy being determined in energy units per unit time (/'), the dose 

 received by the biological system can thus be calculated in energy 

 units. It is not strictly necessary to measure the intensity in abso- 

 lute units, for in man\' cases only relative values are needed to inter- 

 pret the results. However, important information can sometimes be 

 gained by estimating the number of quanta absorbed, and for this 

 purpose energy values are required in absolute units. Moreover, the 

 reproduction of measurements l)\- other investigators may be facili- 

 tated if the intensities are accurately known; so these values should, 

 be presented in absohite units when feasible. 



2. Photochemical and Photobiological Aspects 



Any photobiological process is, generally speaking, a photochemi- 

 cal reaction; it may be an involved one, in which some of the com- 

 ponents are complex biological responses, based themselves on chemi- 

 cal reactions but about which relatively little is understood. In most 

 cases the intermediate steps between the primary photochemical act, 

 represented in equation (1), and the final measured response are vir- 

 tually terra incognita, which one attempts to explore by various ex- 

 perimental means. The interest in action spectra focuses on the 

 primary act — the absorption of quanta by the light absorber — and 

 although, as a rule, this can be studied only in terms of the final re- 

 sponse, one has the advantage that the intermediate steps can be 

 disregarded as long as the relationship between these initial and ter- 

 minal events is known. 



Reciprocity Law. The simplest relationship is a direct propor- 

 tionahty between the number of quanta absorbed in the primary act 

 and the final step in the photobiological reaction. In this case, a 

 given number of quanta produce a given measurable end response, 



