358 History of Luminescence 



TEMPERATURE EFFECTS 



The effect of temperature on phosphorescence has been noticed 

 more or less casually since the studies of John Canton (1768) , 

 already mentioned. N. Hulme (1801) also demonstrated that Can- 

 ton's phosphor would emit a brighter light on slight warming and 

 observed no light in a freezing mixture (salt and snow) , although 

 luminescence reappeared on thawing.^^ Grotthus (1815) was the 

 next to try low temperatures (—25° C) . His results led him to 

 declare that cold favored the taking up of light and heat the emis- 

 sion of luminescence. Osann (1825) found no luminescence of phos- 

 phors at high temperatures. 



These early workers did not realize how complicated the behavior 

 of phosphors might be at different temperatures. There is not only 

 an effect on absorption of light and on intensity of emission, but 

 also on total light emitted, on rate of decay and on color. In addi- 

 tion, the whole problem is greatly complicated by delayed emission, 

 i. e., thermoluminescence. Different phosphors behave in different 

 ways. Some of the observations on these effects are recorded below. 



Becquerel's (1839) first studies on temperature had to do with 

 the effect of illuminating Canton's phosphor at two different tem- 

 peratures, minus 20° C and plus 20° C, after which samples were 

 kept at these two temperatures. For as long as fifteen minutes they 

 luminesced with equal intensity, but when the light disappeared, 

 only the sample at low temperature emitted more light on warming. 

 Becquerel came to the conclusion that a phosphor was more excit- 

 able the lower the temperature. 



This conclusion is not universally true. His later (1859) work 

 indicated that other phosphors behaved differently. Some showed 

 an optimum temperature for greatest intensity at low temperatures 

 and others at high temperatures. Becquerel's final conclusion was 

 that at various temperatures phosphors absorb " a certain amount 

 of light which is always the same for a given temperature and once 

 placed in darkness a quantity of light is emitted corresponding to 

 the total absorbed, more slowly under ordinary conditions and more 

 rapidly the higher the temperature or under the influence of infra- 

 red rays." What this statement '^" amounts to is that in general after 

 irradiation the luminescent light intensity increases and the dura- 



'^ Hulme (1801) came to the conclusion that Canton's phosphorus behaved toward 

 temperature in the same manner as the luminescence of fish, referred to as " spon- 

 taneous light," his principal object of study. 



'^ Becquerel's statement, quoted in the previous paragraph, has been quite generally 

 confirmed, that the total light emitted by a phosphor after a definite exposure to light 

 is a constant quantity whether it be emitted rapidly or slowly. 



