128 BIOLOGICAL EFFECTS OF RADIATION 



being derived from these, simply as a matter of convenience in expressing 

 the spatial distribution of energy transfer and in dealing with different 

 orders of magnitude. Such a system is entirely independent, therefore, 

 of the characteristics of vision or any other biological response. Other 

 systems are, however, in common use, so that it will be desirable to discuss 

 them briefly. Since our concepts are largely based on visual impressions, 

 and since for many purposes observers have been chiefly concerned with 

 those visual impressions, a complete system of measurement has sprung 

 up, which is dependent upon the characteristics of the visual mechanism. 

 In an effort to gain a universal basis for such measurement, a standard 

 observer has been defined. Unfortunately, however, a large proportion 

 of the published measurements depend upon the characteristics of some 

 particular observer, and even the so-called standard observer has been 

 redefined a number of times. 



SYSTEMS OF MEASUREMENT 



Three parallel types of measurement have sprung up : (a) radiometry, 

 (6) spectrophotometry (illumination), (c) colorimetry. Each has devel- 

 oped its system of terminology, its units, and its methods. In descrip- 

 tion of apparatus and specification of characteristics, we shall find 

 sometimes one system and sometimes another. Often the specifications 

 are inadequate and indeterminate. It is important, therefore, to recognize 

 the place and scope of each system, and as far as possible, the interrela- 

 tion between them, wherever a definite relation exists. 



Radiometry is the system based entirely upon the objective physical 

 methods and specified in fundamental c.g.s. units. It will be the basis 

 of our discussion throughout. 



Spectrophotometry is an extension of photometry. In photometry, 

 the quantitative aspect of radiation is evaluated in terms of its effective- 

 ness as the stimulus of vision. A standard source is made the basis of 

 comparison. The starting point is thus wholly arbitrary. The magni- 

 tude of effectiveness of other sources is determined by visual comparison. 

 Obviously, the relative effectiveness of different sources will depend upon 

 the relative wave-length distribution. If sources differ too greatly as 

 to color, difficulties are encountered in evaluating the comparative 

 effectiveness. Spectrophotometry, by a combination of visual and 

 physical means, compares two radiations at the same wave-length, the 

 eye being used for the most part as a null instrument. 



In the definition of a standard observer, the relative effectiveness of 

 different wave-lengths is arbitrarily specified. From the standpoint of 

 practicability, it is desirable that this standard observer approximate 

 the average characteristics of a large number of observers, but from the 

 standpoint of definiteness and reproducibility, this is of no consequence. 



