126 RADIATION BIOLOGY 



1. FUNDAMENTAL CONCEPTS 



Radiant energy is that form of energy which is propagated through 

 space as electromagnetic waves. On interaction with matter, such energy 

 behaves not as a continuum, but as a series of indivisible discrete packets 

 known as "quanta" or "photons." Our present discussion concerns the 

 properties and experimental technics pertaining to that region of the spec- 

 trum in which the photons have sufficient energy to alter the outer elec- 

 tronic energy levels of atoms but not sufficient energy for complete ioni- 

 zation, thus encompassing only the near ultraviolet, the visible, and the 

 very near infrared (Brackett, 1936; Daniels, 1948; Forsythe, 1937; Habell 

 and Cox, 1948: Illuminating Engineering Society, 1952; Richtmyer and 



Kennard, 1947). 



NOMENCLATURE 



Before entering into a full discussion of the fundamental concepts of 

 radiant energy, it is desirable to present an outline of the nomenclature 

 to be subsequently employed, particularly since the complexity of the 

 terminology has led to much confusion and the inconsistent use of terms 

 (Withrow, 1943). The Illuminating Engineering Society of America 

 (1942, 1952); the Optical Society of America, Committee on Colorimetry 

 (1944a,b,c, 1953); and the National Bureau of Standards, through Brode 

 (1949), Gibson (1949), and Judd (1950), have been especially active in 

 the United States in developing a standardized nomenclature. 



Within the visible spectrum, radiant energy may be evaluated either 

 as a physical entity by the use of physical detectors or as a psychophysical 

 entity by visual means. Thus two systems of nomenclature have been 

 evolved, one dealing with the evaluation of radiant energy as a purely 

 physical entity (radiometric terms) and the other with psychophysical 

 evaluation (photometric terms). This dual nomenclature has led to the 

 frequent use of photometric quantities as substitutes for radiometric 

 terms in technical writing. Such misuse undoubtedly results from the 

 tendency of the investigator to extend terms applicable to his own visual 

 experiences to the experimental system under consideration, but this 

 practice frequently leads to ambiguity in scientific writing. 



The physical and psychophysical radiation terms are compared and 

 briefly defined in Table 3-1. At the bottom of the table are given the 

 connotations of the various prefixes and suffixes used; these are consistent 

 with those employed generally in scientific nomenclature. In Table 3-2 

 defining equations and symbols are given for the more commonly used 

 quantitative radiometric and photometric terms. 



Certain terms are misused more than others and merit special mention. 

 Although the nomenclature committees make a distinction between light 

 and luminous energy, for practical purposes these may be considered as 

 synonymous with radiant energy evaluated over the luminosity curve of 



