432 



HAROLD F. BLUM 



involving problems that are essentially biological rather than physi- 

 cal. 



1. Sources and Measurement of Radiation 



The exact determination of action spectra requires an appropriate 

 source of monochromatic radiation. However, as may be judged 

 from inspection of the action spectra shown in Figure 3, the purity 

 (monochromaticity) of the monochromatic radiation required may 

 depend upon the detail of the particular action spectrum and the 



2- 



> 



E ° 



CD 



Z 



(rt -I 



•" -2 



-3 - 



-4 - 



-5 



0? 



UJ 



a: 



o 



- 4 



-5 



350 400 450 500 550 600 650 700 750 

 WAVELENGTH, m^ 



Fig. 4. Spectral sensitivities, measured as reciprocal of threshold ( 1/7 'o), for 

 foveal and peripheral cones (photopic vision) and peripheral rods (scotopic vis- 

 ion) (38). All sensitivities are expressed relative to the maximum sensitivity of 

 the fovea. 



use to which the information is to be put. It ma}^ frequently be 

 necessary to sacrifice purity in order to obtain enough intensity to 

 elicit the photobiological process under study, for in some instances 

 quite high intensities are needed. These are matters that cannot be 

 determined without some preliminary information about the material 

 to be studied. In some cases approximate measurements using 

 polychromatic radiation may be as interpretable as those that can be 

 obtained with monochromatic radiation. 



