ULTRAVIOLET SPECTROSCOPIC TECHNIQUE 129 



(photographic) effects and the electrical effects make possible the meas- 

 urement of small (quantities of such radiations. 



FLUORESCENT SCREENS 



A great variety of substances fluoresce under ultraviolet irradiation 

 (DeMent, 19-15). The properties of certain of these, of especial interest 

 to the lamp industry, have been studied in considerable detail (Kniger, 

 1948; Fonda and Seitz, 1948). 



Zinc silicate (willemite) powder can be used to make very satisfactory 

 fluorescent screens over the wave-length region 1000-3000 A (Beese, 1939; 

 Lui, 1945). Maximum excitation is obtained with radiation near 2500 

 A, for which the quantum efficiency of fluorescence is nearly unity 

 (Fonda, 1939; Schulman, 1946). The emission spectrum of manganese- 

 activated zinc silicate peaks at 5250 A but the emission can be shifted 

 throughout the visible spectrum by addition of beryllium and of various 

 activators (Leverenz and Seitz, 1939). Throughout the region 2200- 

 3000 A nonabsorbent silicone resins, such as General Electric #9980, may 

 be used as a binder. Magnesium tungstate may also be used as a phos- 

 phor throughout the wave-length region 2200-3000 A with a quantum 

 eflficiency nearly unity (Fonda, 1944; Oszy, 1951). Data on other phos- 

 phors useful on this spectral region are summarized by Thayer and 

 Barnes (1939). 



For the 3000-4000 A region, sulfide phosphors are quite effective 

 (Klasens et al., 1948; Studer and Larson, 1948; Pringsheim, 1949, pp. 

 582^., pp. 594^".) with high quantum efficiency about 3650 A (Leverenz 

 and Seitz, 1939). A large number of varicolored pigments are known 

 which respond to radiation in this region (Barnett and Grady, 1949). 



Special phosphors have been developed to convert ultraviolet radiation 

 at 2537 A to ultraviolet radiation at other wave lengths for particular pur- 

 poses (Froelich, 1947). Thus ultraviolet-sensitive phosphors emitting 

 radiation in the erythemal region (2900-3200 A) (Clapp and Ginther, 

 1947; Nagy et al., 1950) and the "black-fight" region (ca. 3600 A) (Beegs, 

 1943; Clapp and Ginther, 1947) have been described. 



Fluorescent coatings may be employed to extend the usefulness of 

 phototubes to wave-length regions shorter than the transmission limits of 

 their envelopes. Dejardin and Schwegler (1934) used sodium salicylate 

 to extend the effective range of a potassium hydride surface phototube 

 from 3400 to 2200 A. A constant quantum efficiency of fluorescence was 

 obtained over this region. Coatings of salicylate and other materials 

 have been used to extend the sensitivity of photomultiplier tubes to 900 A 

 in the vacuum ultraviolet (Johnson et al., 1951). Again a constant quan- 

 tum efficiency was found with salicylate, independent of the wave length 

 of excitation. 



It would seem quite feasible to make use of the threshold wave lengths, 



