CHAPTER XI 



FLUORESCENCE 



Introduction 



BY DEFINITION the word " fluorescence " applies to an emission of 

 light during the irradiation of a substance by any type of radia- 

 tion, provided the emission ceases immediately when the exciting 

 radiation is cut off. If the luminescence persists, the word " phos- 

 phorescence " is used to describe it. Obviously the words " imme- 

 diately " and " persists " are relative terms, depending on the sensi- 

 tivity of the instrimient used to measure them. Beccari's (1744) 

 box and device for exposing a material to sunlight and rapidly 

 examining it in the dark revealed substances whose phosphorescence 

 lasted seconds or tenths of a second, while Becquerel's phosphoro- 

 scope (1858) decreased the time between irradiation and examina- 

 tion to 10"* second. Since then the ability of phosphoroscopes to 

 detect a rapid decay of phosphorescence has increased. Time inter- 

 vals of the order of 10"^ second can now be measured. Thus the 

 distinction between fluorescence and phosphorescence seems rather 

 arbitrary and the two are usually treated together. 



The term " true fluorescence " has sometimes been applied for 

 light emissions lasting of the order of 10"^ seconds. Such effects are 

 observed only with gases,^ liquids, or materials in solution, and the 

 very short decay of light in fluids contrasts with the longer decay in 

 solids. However, an organic compound (eosin or fluorescein) in 

 solution in water or alcohol at room temperature, may exhibit " true 

 fluorescence," but at very low temperatures or when dissolved in a 

 boric acid glass (see Tiede and Wulff, 1922) or a water poor gelatin 

 gel (Wiedemann, 1888) or when absorbed on surfaces at room 

 temperature, the material will phosphoresce for minutes after 

 irradiation. 



It is significant that E. Becquerel never recognized a distinction 

 between phosphorescence and fluorescence and did not use the latter 

 word, although he did discuss the work of Stokes. Immediately after 

 Stokes' introduction of the term " fluorescence " in 1853, attempts 

 were made to find criteria other than duration of light emission, 

 which would distinguish between fluorescence and phosphorescence. 



^ Fluorescence has been applied to the luminescence of gases in vacuum tubes excited 

 by electrical charges or emitted rays, a phenomenon also spoken of as electrolumi- 

 nescence. 



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