146 RADIATION HIOLOGY 



is specified on the assumption that the filter will be used with radiation at 

 normal incidence (Buc and Stearns, 1950). 



More complex "multilayer" interference filters (Banning, 1947b; 

 Bolster, 1949, 1952), which rely on the cancellation of rays multiply 

 reflected between sandwiches of dielectric layers of appropriate thickness 

 aiul refractive index (replacinji; the silvered surfaces), can provide higher 

 transmission (70 80 per cent) and narrower band widths (50-00 A) at 

 half-maximum transmission) . 



The simple interference filters cannot be satisfactorily made for wave 

 lengths less than 3600 A because of the decline in the reflectivity of silver. 

 Aluminum reflectors have not proved satisfactory. It would seem possi- 

 ble to extend the range of the multiple layer dielectric filters farther into 

 the ultraviolet if dielectrics combining proper refractive indices and ultra- 

 violet transparency can be found. 



Christiansen Filters. If rough chips of transparent dielectric are sus- 

 pended in a cell containing a transparent liquid, the resultant mass will be 

 highly scattering and hence of low transmission, except at or near the 

 wave length at which the refractive index of the liquid matches that of 

 the dielectric. When employed with an appropriate optical system, such 

 a cell constitutes a Christiansen filter (Christiansen, 1884). 



Such filters can be made with large cross section. Their spectral selec- 

 tivity depends inversely on the angular divergence of the radiation pass- 

 ing through them, directly on the difference in the slopes of the refractive 

 index versus wave-length curves of the liquid and solid at their point of 

 intersection (Raman, 1949) (the curve for the liquid always has the 

 greater slope), directly on the thickness of the cell, and also on the size of 

 the dielectric chips, for which there appears to be an optimum (Denmark 

 and Cady, 1935). As the refractive indices of liquid and dielectric gen- 

 erally vary at different rates with temperature, the wave length of peak 

 transmission of Christiansen filters is strongly temperature dependent. 



Appropriate dielectric and liquid mixtures have been described for the 

 visible region by MacAlister (1935), for the 3100 4000 A region by Kohn 

 and von Fragstein (1932), and for the 2300-3100 A region by Sinsheimer 

 and Loofbourow (1947). A filter for the mercury 2537 A line has been 

 described by Minkoff and Gaydon (194G); von Fragstein (1938) mentions 

 filters centered at 2610 and 2450 A. It should be emphasized that the 

 transmission and spectral selectivity of these filters depend strongly on 

 the optical system in which they are employed (Weigert and Staude, 

 1927; von Fragstein, 1938). The transmission of Christiansen filters 

 does not decline to zero outside the transmission band but to a minimum 

 d(;pendent on the opti(!al system employed. 



Focal Isolation Fillers. The focal length of a simple uncorrected lens 

 depends on its refractive index and hence on the wave length of the radia- 

 tion. At the focal plane of any gixcii wave length, radiation of othei' 



