LUMINESCENCE AND INCANDESCENCE 



23 



A body which emits light because of its (high) tem- 

 perature is said to be incandescent and we speak of tem- 

 perature radiation. We know, however, of many cases 

 where substances give off light at temperatures much 

 below 525° C. They do not follow the Stefan-Boltzmann 

 law. The light emission is stimulated by some other 

 means than heat. Such bodies we speak of as luminescent, 

 and in this category belong all luminous animals. The 

 distinction between light and luminescence was first 



FiQ. 2. — Distribution of energy throughout the spectrum of the sun, electric arc, and 

 gas light (after Nichols and Franklin). Ordinates show the relative intensities of different 

 wave-lengths emitted. The notches in the curve represent absorption bands and the dotted 

 line represents what the radiation from the sun would be if no selective absorption occurred. 

 V =violet and R ^red end of visible spectrum. (Courtesy Macmillan Co.) 



pointed out by Wiedemann (1888). It is usual to classify 

 luminescences, according to the means of exciting the 

 light, into the following groups : 



Thermoluminescence 

 Phosphorescence and Fluorescence 



Photoluminescence 



Cathodoluminescence 



Anodoluminescence 



Eadioluminescence 

 Triboluminescence and Piezoluminescence 

 Crystalloluminescence 

 Chemiluminescence 



