4 History of Luminescence 



and its duplication has long been the goal of the illuminating 

 engineer. 



When Wiedemann (1888) recognized luminescence as the anti- 

 thesis to incandescence, he also classified luminescences into six kinds 

 according to the method of excitation. No better basis of classifica- 

 tion is available today. He recognized photoluminescence, thermo- 

 luminescence, electroluminescence, crystalloluminescence, tribolumi- 

 nescence, and chemiluminescence. The designations are obvious, 

 characterized by the prefix. Photoluminescence of solids is excited 

 by light itself and is subdivided into fluorescence and phosphores- 

 cence. Thermoluminescence is light from gentle heating. Electro- 

 luminescence appears from gases in electrical fields; crystallolumi- 

 nescence and triboluminescence occur when solutions crystallize or 

 when crystals are crushed or broken, and chemiluminescence may 

 appear during chemical reaction. All bioluminescences are examples 

 of chemiluminescence. 



Although present ideas of the mechanism by which light is emitted 

 have changed and many more types of luminescence have been recog- 

 nized, the new varieties all belong in the six categories of Wiede- 

 mann. In order to clarify the historical treatment and to orient the 

 reader in modern terminology, a list of the present recognized types 

 of luminescence, with definitions, follows: 



Fluorescence, the emission of light from substances only during 

 the time they are exposed to radiation of various kinds— visible light, 

 ultraviolet light, X-rays, gamma rays, cathode rays (electrons) , and 

 other types of particles. The terms photoluminescence^ cathodo- 

 luminescence^ anodoluminescence^ radioluminescence ^ are some- 

 times used to indicate the type of exciting radiation, whether light, 

 cathode rays (electrons) ,* anode rays. X-rays, or y rays. The modern 

 fluorescent lamp makes use of a special powder excited by ultra- 

 violet light in the glass tube. 



If the light emission persists ^ after the exciting radiation is cut 



^ The word " radioluminescence " has often been used as a general term for any 

 luminescence resulting from bombardment by various particles other than light, such 

 as electrons, alpha particles (ions) , etc., including X-rays and 7 rays. Various radio- 

 active disintegrations may cause luminescence by virtue of the particles formed. Chap- 

 ter XII deals with the history of radioluminescence used in this sense, and includes 

 cathodoluminescence (or electroluminescence) , anodoluminescence (or ionolumines- 

 cence) , and radioluminescence proper, sometimes called roentgenoluminescence. 



* When electrons strike molecules, the resulting luminescence may be called an 

 electroluminescence. 



^ Persistence can only be detected by the eye, if the time is around O.I second. The 

 duration of the excited state of non-metastable atoms or ions is around 10"* second, 

 and some workers designate as fluorescence, luminescence emissions with a persistence 

 not longer than this. 



