VISIBLE AND NEAR-VISIBLE RADIATION 133 



necessary to establish the value of the maximum absolute visibility, 

 T^o.555^,. The most recent determination of this value is Fo.555m = 624. 

 The reciprocal of this value, known as the least mechanical equivalent 

 of light, is 0.001602 watt per lumen (67, 40). (This is, of course, the 

 ratio of the unit lumen to the unit watt for wave-length 0.555 yu.) 



Thus, if one knows for a given source, the power Px emitted at each 

 wave-length, one obtains the luminous flux by multiplying the radiant 

 power by the absolute visibility. 



Lx= ViPxi= FxX Fj.555,Px) 



Integrating over the wave-length range, one obtains the total luminous 

 power of the source. 



Often it is most convenient to perform the integration graphically, 

 plotting Lx as a function of X, and measuring the area under the curve 

 by means of a planimeter. 



GENERAL CONCEPTS 



Having so agreed upon our methods of specification of radiation and 

 light, we may now" attack the simple problem of the passage of a parallel 

 beam of radiation through a portion of matter bounded by two parallel 

 flat surfaces. At the first surface, a portion of the radiation is turned 

 back, a portion continues through. Of this, the portion which is turned 

 back is spoken of as reflected; the portion continuing, as transmitted. 

 As the radiation passes through the body of the material, part of the 

 energy is lost by absorption. At the second surface, a portion is again 

 reflected and a residue transmitted. In the case where the incident 

 beam falls vertically upon the surface, the terms in Table 5 may be used 

 to specify the phenomenon. 



Since t = -\/T = \^E-i/Ei = e~\ then E2 = Eie~''', a common expres- 

 sion for Lambert's law. 



For a wide range of solutions, the absorption of radiant energy is 

 proportional to the number of molecules of dissolved material in the 

 path. For such cases, the terminology in Table 6 has been established. 



Here it will be seen that t = \/T may be written T = e~'^*^, the expo- 

 nent being proportional to the concentration and the length of path. 

 This form is perhaps most frequently given to Beer's law. The specific 

 transmissive exponent i depends upon the type of dissolved material. 

 While the specific transmissive index k is commonly spoken of as the 

 absorption coefficient, the specific transmissive exponent i is often referred 

 to by the same term, so that the logarithmic base must be indicated 

 whenever the term is used. 



