

APPLICATION OF FLUORESCENT SCREENS 51 



screens are used in visual fluoroscopic examinations of body structure. 

 If the mounted compounds emit radiations predominantly in the blue, 

 photographically sensitive wavelength region, then these mounts are 

 designated as intensifying screens and are used for roentgenographic 

 work. 



In the construction of a screen the compound is carefully sifted for 

 uniformity of fragments. Marcotte (British patent 184,485; 1921) 

 claims that the fluorescence varies with crystal size, first rising to a 

 maximum and then decreasing. He also maintains that the color of the 

 fluorescent light from the tungstates of calcium and cadmium undergoes 

 a parallel variation with crystal size, changing from yellow-green, blue- 

 green, to blue-white, and that ungraded crystals in consequence give a 

 luminosity of mixed color. 



The proper grade of crystals is mixed with a binder of cellulose, ace- 

 tone, or amyl or methyl acetate (20 per cent); the resulting mass is 

 heated to 45° C and then poured into molds and rolled to a thickness 

 of 0.02 in. These screens are claimed to be inert to soap and alkali. 

 In the trade they are referred to as " washable screens." 



For radioscopic work the screens must satisfy the following require- 

 ments : 



1. Brilliancy. A yellow-green with maximum intensity at 5560 A is 

 recommended. 



2. Clearness of definition. Coarse crystals produce great brilliancy 

 but poor definition. Small crystals give less brilliancy and better defini- 

 tion. 



3. Contrast. This depends on the use of a fluorescent material of 

 heavy atomic weight, such as BaPtCN 4 , in which the fluorescent atom 

 is the platinum atom and not the barium atom, since absorption takes 

 place in the region of the K and L platinum spectral lines. With the 

 presence of this heavy atom, variations of absorption and fluorescence 

 can be obtained as the hardness of the exciting radiation varies. Addi- 

 tional contrast may be obtained if greenish fluorescent emission screens 

 are used and if the general illumination in the x-ray room is strong red 

 light such as supplied by a bright photographic ruby light bulb. 



4. Absence of after-glow. Successive exposures to the exciting 

 x-radiation must be spaced for sufficient time to elapse so that the accom- 

 panying phosphorescence, if present, has had time to decay below the 

 threshold of absolute visibility of the eye. The after-glow phosphores- 

 cence can be controlled by dilution of the sensitive material with insen- 

 sitive crystals, but unfortunately at the expense of its original brilliancy. 



5. Protection. The fluoroscopic screen is mounted in a frame cov- 

 ered with a lead glass plate -j^- in. or more in thickness, to protect the 



