VISIBLE AND NEAR-VISIBLE RADIATION 153 



the diffraction characteristics arising from the sUt. In absohite intensity 

 measurements, it is necessary to take into consideration the transmission 

 of the condenser lens as well as that of the monochromator. A third 

 method of sUt illumination will be taken up more specifically in connection 

 with absolute measurements of intensity of source, the above providing 

 methods of procedure more widely applicable to various problems. 



ABSOLUTE MEASUREMENTS OF THE INTENSITY DISTRIBUTION OF RADIATION 

 FROM SOURCES AS A FUNCTION OF WAVE-LENGTH 



Our problem is to determine the amount of energy of a given wave- 

 length range supplied by a source of defined extent through a specified 

 solid angle. We are thus able to arrive at the radiance of the source 

 for a given wave-length, this corresponding in radiometric measurements 

 to the quantity termed brightness in photometric measurements. A 

 knowledge of this quantity will enable us to determine the irradiation 

 provided by the source under various conditions. 



One method is to use a source whose area can be determined, at a 

 sufficient distance from the first slit that the solid angle of illumination 

 thus provided less than fills the lens and prism system. The dimensions 

 of the source thus determine the aperture utilized. The slit area deter- 

 mines the solid angle of radiation measured from any point on the source. 

 One obtains, therefore, an average value of the radiance of the exposed 

 source. If a black-body detector, such as a thermocouple, is used, the 

 energy escaping from the second slit may be completely absorbed by 

 the thermocouple. The thermocouple-galvanometer sensitivity may 

 then be determined by comparison with the total radiation from a stand- 

 ard lamp. This comparison provides the information — energy per 

 second for a given galvanometer deflection. Multiplying the deflection 

 observed by this factor, one obtains the radiant power from the second 

 slit of the instrument under the conditions specified. Dividing this by 

 the transmission of the instrument, one obtains a value for the radiant 

 power Ps passing through the first slit, provided the second slit has been 

 so adjusted that it does not obstruct the energy of the given wave-length. 

 Dividing this value by the solid angle subtended by the slit, one obtains 

 the radiant intensity (watts per steradian). Or dividing by the area As 

 of the slit, one obtains the irradiation (watts per square centimeter) 

 provided by the source at that distance. If one wishes to determine 

 the radiance R, one must further divide the radiant intensity (watts per 

 steradian) by the projected area A of the source. In the case where the 

 source is a selective emitter, the instrument has been adjusted for a 

 given wave-length, and all other wave-lengths excluded by the second 

 slit. One now has all the necessary data on the source for the wave- 

 length indicated. 



