XIII. ACTION SPECTRA AND ABSORPTION SPECTRA 



431 



C. ACTION SPECTRA 



The action spectrum for a photobiological process is an expression 

 of the variation of the effectiveness of the exciting radiation with the 

 wavelength; examples appear in Figures 3-6, 8, and 9. There is no 

 generally accepted way of expressing action spectra that corresponds 

 to the use of the absorption coefficient in absorption spectra. The 

 reciprocal of the incident energy (1//') required to produce a given 

 photobiological response is probably the expression most commonly 

 employed (as in Figs. 4 and 5), but has numerous disadvantages and 

 its use is by no means universal. In fact, with regard to the whole 



\F 



300 



400 



700 



800 



500 600 

 < WAVELENGTH, m/i 



Fig. 3. A number of action spectra (^). The ordinates are I/I'o corrected to 

 unity at the point of maximum action. (A) Hemolysis of erythrocytes. (B) 

 Killing of a bacillus, E. coli. (C) killing of a yeast, Saccharomyces cerevisiae. (D) 

 Erythema of human skin. (E) Vesiculation of a protozoan, Paramecium multi- 

 nucronucleata. (F) Vision of an insect, Drosophila. (G) Tropic bending of the 

 oat coleoptile. (H) Photosynthesis of the wheat plant. (/) Scotopic vision 

 (rods) of the human eye. (/) Photopic vision. (K) Photosynthesis by purple 

 sulfur bacteria, Spirillum ruhrum. The curve reaches a maximum about 900 

 m/n. The original references are cited in Blum {2). Accuracy of some of these 

 action spectra may be questioned; but wavelength limits are probably nearly cor- 

 rect, and this alone serves to illustrate spectral range of such effects. 



subject of action spectra, the systems studied and the problems of 

 measurement are so diverse that very few general rules can be laid 

 down. The purpose for which the action spectrum is obtained may 

 dictate the mode of expression. For example, in illuminating engi- 

 neering, and in many other problems involving human vision, one is 

 concerned with the amount of incident energy required to stimulate 

 the visual response (Fig. 4), and this is usually measured in ergs or 

 some other energy unit. On the other hand, if one wishes to deter- 

 mine the nature of the light absorber, i.e., the photosensitive retinal 

 pigment in this case, a mode of expression is desirable that permits 

 one to deal in terms of the number of quanta absorbed. To estimate 

 the absorption, indirect measurements are often required, sometimes 



