XIII. ACTION SPECTRA AND ABSORPTION SPECTRA 437 



tumor appearing only after the lapse of several months during which 

 many doses are delivered (12). In all these effects reciprocity holds 

 at least over wide ranges of intensity, but the experiments must be 

 properly designed to demonstrate this point. 



In order to obtain an interpretable action spectrum, it is necessary 

 in most cases to know that reciprocity is obeyed, or to know the in- 

 tensity dependence so that appropriate correction can be made. The 

 tlependence uj)on intensity may also give important clues regarding 

 other aspects of a photobiological mechanism, and hence the reciproc- 

 ity relationship is one of the first that should be explored in the in- 

 vestigation of a photobiological process. It is possible in some in- 

 stances, however, to compare minimal (threshold) stimulating inten- 

 sities, as in investigations on the human eye, and so to obtain action 

 spectra without reference to reciprocity. 



Quantum Efficiency and Quantum Efifectiveness. Measure- 

 ments of intensity or dosage are usually made in terms of energy 

 units, whereas photochemical, and hence photobiological, efficiency 

 must be reckoned in terms of the number of quanta absorbed. The 

 efficiency of a photochemical reaction is measiired by the quantum 

 yield, which is expressed as follows : 



number of molecules changed chemically 



quantum yield = 7 — 



number 01 quanta absorbed 



The quantum yield for the primary act, as described by (equation 

 1) would be unity; but for a reaction involving several steps it may 

 range from a small fraction to a value much larger than unity in the 

 case of chain reactions. In photobiological processes the quantum 

 yield can seldom be calculated, photosynthesis being an exception. 

 Thus the term effecliveness is more appropriately applied to most 

 photobiological processes than ifficiency, the more specifically defined 

 term. 



In comparing the effectiveness of different wavelengths it is neces- 

 sary to take into account the relationship between incident energy and 

 incident number of quanta. From equations (1) and (2) we see that 

 the niim ler of quanta, N , is related to the energy, Q, as follows: 



^ .V - Q/hv = Q\/{hc) 02) 



and since h and c are constants, we may write; 



Af oz Q\ (13) 



