198 RADIATION BIOLOGY 



than the fabric dyes, but they are not available in so great a variety of 

 spectral characteristics and are more expensive. The water-soluble dyes 

 may be incorporated in water-dispersible or water-permeable plastic films 

 of gelatin, polyvinyl alcohol, cellophane, or nylon. 



Since gelatin is relatively transparent to the visible and ultraviolet to 

 beyond 300 m/n and is water-dispersible, it forms a convenient dispersing 

 medium for the water-soluble organic dyes. Dyed gelatin films, such as 

 Wratten filters (Eastman Kodak Company, 1951), have been used exten- 

 sively in photography and photochemical investigations. Large dyed- 

 gelatin filters can be made in the laboratory by the direct casting of 

 dye-gelatin solutions on glass (Withrow and Price, 1953). The alcohol- 

 soluble dyes can be incorporated directly into organic solutions of the 

 colorless plastics. A series of visible-absorbing, infrared-transmitting 

 filters have been developed in this manner (Blout et al., 1946; Shenk et al., 

 1946). Infrared-transmitting filters with high thermal stability can be 

 prepared by the thermal polymerization of plastic films in which the 

 plastic itself becomes the filter (Blout et al, 1950). 



Cast sheets of the acrylic resins, such as Plexiglas or Lucite, strongly 

 absorb the middle and far infrared. A sheet of clear plastic 5 mm thick 

 absorbs the infrared beyond 2500 m/z; a 2-cm sheet absorbs beyond 

 1700 m^; ultraviolet absorption begins at about 290 m/x. There is 

 no appreciable absorption in these plastics between 320 and 900 m/i. 

 Because they are thermoplastic, they cannot be used where high-power 

 dissipation is required, but they are very useful for protecting thermal 

 detectors from long-wave-length infrared emitted by warm objects. 



3. Glass filters. In the infrared and ultraviolet, certain of the glass 

 filters (Coblentz and Stair, 1929; Corning Glass Works; Jena Glass 

 Works; Stair, 1948; Stair et al, 1949) have spectral characteristics that 

 cannot be duplicated with dyes. This is especially true of the infrared- 

 absorbing Aklo type and the ultraviolet-transmitting filters. The glass 

 filters are particularly useful where permanence and heat resistance are 

 prime considerations. 



Optical Filters. The optical filters have greater spectral range and reso- 

 lution than the selective-absorption filters, but they all require some 

 degree of beam collimation ; thus only high-intensity concentrated sources 

 can be used. The optical systems may be quite simple, since an appreci- 

 able beam divergence can be tolerated without seriously widening the 

 transmission band, and a projection-lamp filament at the focus of a simple 

 plano-convex lens is usually adequate. 



1. Christiansen filter. The Christiansen filter consists of a transparent 

 couvette containing small particles of a transparent solid in a liquid. 

 The solid and liquid are so chosen that a wave-length plot of their refrac- 

 tive indexes yields two curves that cross at one point where the refractive 

 indexes are the same (McAUster, 1935; Minkoft" and Gaydon, 1946; 



