APPLICATIONS AND SOURCES OF ULTRAVIOLET Gl 



of all commercial sources are such that the output ratings should be used 

 or approxmiations only. A description of a lamp type and of its elec- 

 trical characteristics is an essential guide to the mechanics of a laboratory 

 research but is. of little value as a means of specifying the radiation 

 nitensities provided. The effective intensities obtained fn any „h 

 should be measured at the irradiated surface or throughout the irradiated 

 volume and should be specified in general terms entirely independent of 

 the source, the usual laboratory unit being the microwatt per square 

 centimeter, and the corresponding engineering unit, the milliwatt per 

 square foot. ^ 



INTENSITY VARIATIONS WITH DISTANCE 



For distances greater than the length of the radiating source the 



intensity varies inversely as the square of the distance. For distances 



ess than about one-third the radiating length of these linear type sources 



he intensity varies inversely as the distance. The variation at the 



transition distances can be measured directly or estimated from the 



ac-tual mtensities produced by such typical sources as are shown in 



HIGH INTENSITIES AT CLOSE RANGE 



The maximum hitensity prcn-ided by a single tubular source is at its 

 own surfac-e. This is a useful point at which to start a study of the 

 variation of intensity at short distances from the tube. Distances are 

 measured from the center of the tube although it radiates ultraviolet 



r' r^ 1 T r^ ""^ ^^' *"^' '''^'^ *^^ «^^^^^- The effective emitting 



ng h of the l-in.G30T8 tube of Fig. 2-9 is 32 in., and its circumferenc^ 



•s 314 m. From this surface area of about 100 sq in., 7 ultraviolet watts 



^00 ' "">' T""'T '""^'''"'^^ "^ ^-^^ ultraviolet watt/sq in. or 



il^800 Mw/cm-. A surface or a material in contact with the tube would 

 therefore be irradiated at that intensity. 



Similarly, a cylindrical surface 2 in. in diameter would have double the 

 tube area^ J* .would intercept practically all the radiated 7 ultraviolet 

 watts with half the intensity. Similarly, the intensity on a 3-in cylin- 

 drical surface would be one-third as great, and on a 4-in. cvlinder one- 

 ourth as o^reat On cylinders of these diameters, small compared with 

 heir eng h, the ultraviolet intensity is uniformly distributed, except 

 tor a length at each end equal to about the radius of the tube Th,> 

 intensity on the surfaces of such irradiated cylinders varies with their 

 diameters or inversely as the distance from the tube axis to the irradiated 

 suitace. This relation is true out to distances of the order of one-third 

 the effective length of the source, as shown in Fig. 2-9. 



