. The American journal of roentgenology, radium therapy and nuclear medicine . defined, fluorescence is a phe-nomenon in which radiant energ^^ of oneparticular wave length is absorbed by amaterial and emitted again as radiantenerg>^ of a longer wave length. Fluores-cence takes place only while the excitingradiation is allowed to impinge on thematerial. It is thus difi:erent from the phe-nomenon of phosphorescence in whichthe time-lag between excitation and emis-sion may be considerable. Of the materials which fluoresce toroentgen rays in the range of frequenciesfrom the ultra violet to the
. The American journal of roentgenology, radium therapy and nuclear medicine . defined, fluorescence is a phe-nomenon in which radiant energ^^ of oneparticular wave length is absorbed by amaterial and emitted again as radiantenerg>^ of a longer wave length. Fluores-cence takes place only while the excitingradiation is allowed to impinge on thematerial. It is thus difi:erent from the phe-nomenon of phosphorescence in whichthe time-lag between excitation and emis-sion may be considerable. Of the materials which fluoresce toroentgen rays in the range of frequenciesfrom the ultra violet to the red, there areonly a few which can be used efficientlyfor photographic intensification. All of * Real bafore th3 niiitjenti a-nial msj^inj of ths A-nirican Ro^nt^en Ray So^isty, Chattanooga, Tenn., September, 1918. 596 Physical Characteristics of Intensifying Screens ^tf n fei. 1 ■ 1 k ...iiiniiiiiiiDliH •Ill ^B B ■ :;--^^- ^ ^^ ■ Ik. .^ k . 1 mnllnilpnnfflflHIl iHrr m Ml Iik^^ k. ^d ^. .ifflilffllffllMMI a ^ .*^ - r Hfflllinl>. Fig. I. 1. Roentgen Ra} Plate. 2. Ordinary Plate. 3. Ortho-chromatic Plate. 4. Pan-chromatic Plate. these substances must be in the crystalHnestate. Barium-platino-cyanide, barium sal-icylate, calcium tungstate, molybdenumtungstate, magnesium tungstate and somedouble tungstates of these metals fluoresceto a greater or less extent to roentgen rays,and the radiation is more or less activephotographically. Of these materials crys-talline calcium tungstate is by far the bestwith present photographic rcentgen-raymaterials. The salt is usually powdered and coatedwith suitable binder on a support of somematerial of slight roentgen-ray absorption, siirh a^ rarrlhoprrl or nl^^]^^^n,\A TViio cr^^r^-r-, is then placed in contact with the photo-graphic surface and exposure made througheither the screen or the photographic plateor film. The efflciency of any radiator as asource of photographic stimiilation dependsprimarily on the comparative spectraldi
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