VISIBLE AND NEAR-VISIBLE RADIATION 155 



If, however, one is working with a continuous source, the value 

 obtained is representative of a wave-length band, the extent being deter- 

 mined by the sUt width and dispersion of the instrument. Further 

 experiments are necessary in order to evaluate the effective wave-length 

 range. This may be done either analytically or by the use of an auxiliary 

 spectrograph or monochromator. 



An example of absolute measurement of the spectral characteristics of 

 a source is furnished by the measurements on the quartz mercury arc 

 by McAlister, and illustrated in Fig. 8 (50). These data are given in 

 terms of the irradiation at 25 cm., furnished by a 20-mm. midsection of a 

 250-volt uviarc, operated on 143.5 volts, 4.5 amp., the temperature 

 having been maintained to provide these characteristics. 



If a condensing lens is used, the limiting aperture must be determined. 

 If the prism of the monochromator provides the limiting aperture, its 

 projected area at the condenser can be computed. This area, together 

 with the distance of the condensing lens from the source, determine the 

 solid angle. The region of the source imaged upon the slit determines 

 the projected area of the source contributing. If this image is magnified 

 or reduced, the magnification factor must be taken into account in com- 

 puting the effective area of the source, i.e., the slit area must be divided 

 by the magnification due to the condensing lens. 



In determining the radiance R one proceeds as follows: The radiant 

 power Ps, passing through the first slit is divided by the soUd angle 

 (determined by the projected area of the limiting aperture on the con- 

 densing lens and the distance of the condensing lens from the source). 

 One thus obtains the radiant intensity. This in turn is divided by the 

 projected area of the effective source (slit area vl, /magnification Mc of 

 the condenser). Thus 



J, PsMc 



The first method outlined may be preferable provided sufficient 

 energy is available for measurement. The second method must, how- 

 ever, be resorted to for weak sources. In practice, often a combination 

 of the two is used, the absolute values for strong lines being obtained 

 by the first method and the values for the weaker lines interpolated 

 by the second method. This procedure was followed by Dr. McAlister 

 in the work illustrated. 



The type of undertaking described above is one of the most difficult 

 in optical practice. Such determinations cannot be made unless exten- 

 sive preparation and a great deal of time are devoted to them. This 

 accounts for the fact that so few absolute-energy data are available 

 concerning even the most frequently used sources. 



