158 BIOLOGICAL EFFECTS OF RADIATION 



PROVISION FOR MONOCHROMATIC IRRADIATION OF MATERIALS 



In this case, the combmation, source phis monochromator, becomes the 

 source of monochromatic radiation. The second sUt of the mono- 

 chromator may be treated as a new source. Where selective emitters 

 are used and the resokition of the instrument is such that only a single 

 line passes through the second slit, the wave-length range of major 

 intensity is limited by the inherent line width, which for all practical 

 photochemical or biological work may be neglected, the only impurity 

 being the scattered radiation discussed above. If, however, a continuous 

 source is used, the wave-length range will depend upon the slit adjust- 

 ment and dispersion of the instrument. With a single monochromator 

 of 10 cm. focal length, a quartz prism of 60-deg. angle, and 0.1-mm. 



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slit width, the wave-length band will range from roughly 5 A at 2250 A 



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to 200 A at 6500 A. With the double monochromator, wave-lengths 

 will be restricted to one-half this range. This is offered only to give a 

 rough idea of the practical values available. For twice the slit width, 

 there would be four times the wave-length range and correspondingly 

 greater intensity. 



The outstanding difficulties presenised by such a source are: (a) The 

 size of the source is limited to the size of the slit, namely, from 0.1 to 

 2.0 mm. in width, and from 1.0 to 5.0 cm. in length, depending, of course, 

 on the size of the instrument and the purity required. (6) The solid 

 angle of available radiation from such a source is again limited by the 

 numerical aperture of the instrument, (c) Practical considerations of 

 expense. 



In using such a source, a secondary black-body detector, such as a 

 thermocouple, may be used to evaluate the power output by direct 

 comparison with the total radiation from a standard lamp. In practice, 

 however, it is desirable to use a secondary control either upon a portion 

 of the beam from or through the spectrograph, or directly upon the 

 radiation from the source. Any type of dependable detector may be 

 used for this purpose. A photocell is often placed near the slit or con- 

 densing lens and read at intervals during the exposure in order to evaluate 

 the changes and fluctuations in the source. This enables one to make 

 correction on intensities determined before and after exposure. In such 

 irradiation experiments, it may be desirable to know not only the power 

 output which, of course, enables one to compute radiance and irradiation 

 at the sample, but also the amount of absorbed energy. 



Two general methods for evaluating the amount of absorbed energy 

 present themselves: (a) To evaluate by computation from a knowledge 

 of the absorption characteristics of the sample and the irradiation pro- 

 vided. (6) To make black-body determinations of radiation incident 



