348 History of Luminescence 



traces of heavy metals control phosphorescent properties. His re- 

 searches were well planned. He analyzed commercial phosphors 

 prepared from mussel shells and foimd that, in addition to CaS, 

 traces of Si, Mg, Bi, SO4, and PO4 were present. A pure Ca sul- 

 phide did not phosphoresce but gave a brilliant phosphorescence if 

 impurities were added. Bismuth was responsible for a violet color. 

 He found that both the color and intensity of sulphide phosphors 

 could be controlled by traces of impurities and came to the conclu- 

 sion that " all substances which are capable of vitrifying the surface 

 of the calcium sulphide without coloring it render the product very 

 phosphorescent. 



This conclusion is not the whole truth, as two additions must be 

 made to a sulphide to obtain a good phosphor. The trace of metal 

 is actually an " activator," responsible by means of electron shifts 

 for the light emission. There must also be a flux. The alkaline 

 earth phosphors, prepared by V. Klatt and Philipp Lenard ^* in 

 1889 were made up of sulphide plus a flux, usually a sodium sul- 

 phate or borate, to which Cu, Bi, or Mn were added in traces as 

 activators. The metals were found to be responsible for certain 

 definite bands in the phosphorescence spectrum, which determined 

 the color of the luminescence. Each band has its characteristic 

 period of decay. Further details were published in three extensive 

 later papers by Lenard and Klatt in 1903 and 1904.^^ 



Thus the control of " impurity phosphors " was not fully under- 

 stood until near the end of the nineteenth century. Even the great 

 Edmund Becquerel originally thought that the characteristics of 

 phosphor luminescence were determined by physical characteristics, 

 but his last papers (1886, 1888) dealt with the importance of traces 

 of heavy metals. Following the studies of Crookes and of Lecoq de 

 Boisbaudran during the eighteen-eighties on cathodoluminescence, 

 which indicated the importance of impurities, a number of new 

 workers continued to add metal traces to prepare new phosphors. 

 Today a phosphorescence of any color and any duration can be 

 obtained by the proper mixture of pure materials and the metal 

 activator. 



The subject of new phosphors should not be left without mention 

 of another type of phosphor, prepared by Wiedemann in 1887 and 

 studied more extensively by Wiedemann and Schmidt (1895) and 

 Schmidt (1896) . It may be described as a solid solution. For 

 example, analine dyes, which in water exhibit true fluorescence, 



^* These " Lenard " or " impurity " phosphors are often designated thus: Ca Mn S, 

 with the activating impurity Mn between the alkaline earth and sulphide, without 

 mention of the flux. Omission of the activator abolishes the ability to phosphoresce. 



