GENERATION, CONTROL, AND MEASUREMENT 179 



253.7 mn to a low-energy quantum of visible at 507.4 mn entails a 50 per 

 cent energy loss. 



If the luminescent substance emits a quantum within 10~^ sec to 

 a few microseconds of the time of activation by an ultraviolet quan- 

 tum or a high-speed electron or ion, the process is called "fluorescence." 

 If, however, there is an appreciable delay in the reemission of quanta, 

 the process is known as phosphorescence. Many of the phosphors used 

 in fluorescent lamps are sufficiently phosphorescent so that there is a 

 marked reduction in "flicker" on 50- and 60- cps a-c circuits. 



Although a very large number of organic and inorganic substances are 

 luminescent when exposed to ultraviolet flux, only a few of the inorganic 

 compounds have the required high sensitivity at 253.7 m/z, high quantum 

 efficiency, desirable spectral-emission characteristics, and physical and 

 chemical stability for use in fluorescent lamps. The phosphors most 

 frequently used are either double oxides or salts of inorganic acids, i.e., 

 boric, silicic, phosphoric, and tungstic. The base metal is usually of 

 magnesium, beryllium, zinc, cadmium, or calcium. 



Most synthetic phosphors consi.st of a highly purified o.xide or salt to 

 which is added a trace of an activator, usually a heavy metal such as 

 manganese, nickel, or silver. In the pure state many crystalhne materials 

 exhibit little photoluminescence, but minute traces of activator materials, 

 to the extent of less than 1 part per million, wifl increase the luminescence 

 by a hundredfold. The activator also controls in part the region of maxi- 

 mum sensitivity and the spectral energy distribution. 



SPECTRUM 



The complete emission spectra of some typical fluorescent lamps are 

 given in Fig. 3-15. The "white " fluorescent lamps employ combinations 

 of two or more phosphors. By controlling the proportions of blue-, 

 green-, and red-emitting phosphors, various colors of "white" radiation 

 can be obtained. The complete emission spectrum of the fluorescent 

 lamp consists of the phosphor emission upon which is superposed a weak 

 fine spectrum of the mercury discharge. 



COLOR TEMPERATURE 



The color of "white" fluorescent lamps is characterized by the temper- 

 ature of a complete radiator having the closest color match. Since the 

 spectrum of mixtures of various phosphors never produces a spectral 

 energy distribution that even closely approximates the Planckian radi- 

 ation curve, color temperature is useful only as a means of approximate 

 color specification. The white lamps contain phosphor combinations that 

 emit radiant energy approximating color temperatures from 3500° to 

 6500°K. The 3500°K lamp approximates the color of incandescent-lamp 

 radiation, and the 6500°K lamp approximates sun and sky radiation. 



