130 



BIOLOGICAL EFFECTS OF RADIATION 



and the subjective response to light, termed color. While a wide range 

 of terminology has been developed to describe color, three essential 

 attributes are commonly recognized: brilliance, hue, and saturation. 

 The following table shows the relation between the terms describing 

 subjective response (color), objective stimulus (light), and physical 

 phenomenon (radiation). 



Table 1 

 (56, cf. page 178) 



Table 2 indicates the hue which we commonly associate with the 

 different representative wave-lengths of the pure spectrum. 



Table 2 



Other hues are combination effects of different wave-lengths, 

 instance, purple is due to the combination red and blue. 



For 



UNITS AND DEFINITIONS 



The complete physical measurement of radiation requires a determina- 

 tion of the energy for each wave-length and its relation to space and time. 

 Table 3 defines the units commonly used. 



In many problems it is assumed that one might have a finite amount 

 of energy transferred by a parallel beam, i.e., a wave motion in which 

 maxima and minima lie in parallel planes. For such an idealized case, the 

 flux density in a direction normal to the wave plane is often spoken of as 

 the "intensity" or "beam intensity" and measured in watts/cm. 2. 



Practically, such a condition cannot be reahzed. It may be ap- 

 proached where the radiation is restricted to a small solid angle. In 

 such cases, the term intensity or beam intensity is commonly used. 



Quantities expressed in units of the absolute system cannot be con- 

 verted into quantities expressed in terms of the illumination system, 



