122 J{\1)I ATION 1!1()L(K;Y 



broadoiiinji; of the lines and the development of iippreciahle contirniou.s 

 l)a('kt>;roun<l ;is the ciincnt density and vapor |)ressure are inereased. 



PHYSICAL I'AHA.MKTKUS OK SOl'llCKS 



Two of the physical parameters which may influence tlu; ciioice of a 

 particular lif>;ht source were mentioned in the precedinj^ discussion of pos- 

 sible classification of sources, namely, the useful spectral range eovercjd 

 by the emitted radiation and the distribution of energy within tliat range 

 (i.e., continuous or discontinuous). A third factor of importance is the 

 amount of radiation emitted at a particular wave length or over a certain 

 band of wave lengths. The amount of radiation may be considered in 

 two ways: first, the total amount of radiation emitted from the whole 

 of the luminous body of the source. Second, the amount of radiation 

 may be considered to be that quantity emitted by a unit area or volume 

 of the source into a unit solid angle. The significance of these two modes 

 of expressing the intensity parameter of a light source will be discussed. 

 Following is a list of the terms which are used to describe the intensity 

 parameter: 



1. Radiant Flux: Radiant flux is the rate of flow of radiant energy with 

 respect to time. The quantity is also called "radiance" (P = dU/di) 

 where V is radiant energy and / is time. 



2. Radiant Intensity: Radiant intensity is measured hy the energy 

 falling in unit time upon an area subtended by unit solid angle about an}' 

 direction considered and at any distance from the source. This value is 



also called "steradiance" I ./ = , , ). The solid angle is represented 



by w. 



3. Steradiancy: Steradiancy is the radiant flux per unit solid angle per 



square centimeter of source w ^ = . , . . ). .4 is the area of the source. 



It is to be noted that the.se expressions contain no reference to wave 

 length. For studies of the biological effects of radiation it is frequently 

 necessary to know the value of one or more of the preceding intensity 

 expressions w^th respect to wave length. The maimer in which such 

 energy measurements are made will be considered on pages 130 to 142. 



CHOICE OF .\ SOURCE 



The choice of a source for a study of the biological elTects of radiation 

 depends on a number of factors which are inherent in the exact nature of 

 the experiment to be conducted. In a general way, decisions must be 

 made as to (1) the size of the area to be irradiated, (2) the range of the 

 ultraviolet spectrum to be covered, (3) the size of the band of energy wdth 

 respect to wave length, (4) the time in which the largest amount of energy 

 is to be delivered to the irradiated area. These factors are to some extent 



