142 RADIATION BIOLOGY 



The use of modulated radiation, giving rise to an oscillatory current, 

 permits the use of alternating-current amplifiers and thus simplifies the 

 associated electronic circuitry. Alternatively, a steady photocurrent 

 may be converted to an oscillatory current, either by magnetic modula- 

 tion (Kalmus and Striker, 1948) of the photocurrent, or by mechanical 

 interruption (Liston et al., 1946; Lash, 1949). 



Image Orthicon. The application of television techniques (Zworykin 

 and Ramberg, 1949, Chaps. 16, 17) to ultraviolet spectroscopy through 

 the medium of ultraviolet-sensitive image orthicon tubes has significant 

 potentialities. The orthicon is in effect a two-dimensional photoelectric 

 detector which permits the application of photoelectric techniciues to 

 problems that previously could be adecjuately approached only by photo- 

 graphic means. The use of the image orthicon by Pai-part and Flory for 

 the \isualization of ultraviolet microscope images (Purpart, 1950; Flory, 

 19")1). and the study of source spectral characteristics (Benn el al., 

 1949; Agiiew et al., 1949) may be cited as examples of the potential 

 applications. 



METHODS OF SPECTRAL ISOLATION 



One of the important data in any problem in radiation biology is the 

 variation of the subject under investigation (absorption, fluorescence, 

 photobiological or photochemical effect) with the wave length of the 

 radiation concerned. To obtain these data, spectrally defined beams of 

 radiation must be available. A wide variety of devices have been devel- 

 oped to provide such spectrally defined beams ; these devices differ in basic 

 principles and in range of application and, in general, may have specific 

 advantages or disadvantages for a particular application. For the iso- 

 lation of well-separated spectrum lines from a discontinuous source, much 

 simpler technicjues can be employed than are necessary to isolate narrow 

 spectral band widths from a source of spectral continuum. For some pur- 

 poses, high intensity or large total energy of radiation are more important 

 than purity of wave length. For others, flexibility and the possibility of 

 easy, rapid change of wave length are important. The optimum means of 

 spectral isolation can be chosen only after the research objectives are 

 clearly defined. 



The various means employed for spectral isolation may be somewhat 

 arbitrarily grouped into two classes: filters, which by one means or 

 another block or prevent transmission of all save the selected band of 

 wave lengths, and dispersing systems, which transmit all wave lengths, 

 but disperse them in space so that })arti('ulai' regions may be selected. 



FILTERS 



Absorption Filters. The simplest filters are absorption filters, liy 

 virtue of the al)sorption spectra of their components these filters absorb, 



