64 KADIATION HIOLOGY 



ot incri'ury xapor prtictii-uUy in roiitacl with the luinp tuhe i.s aiiolTcctix-e 

 source of 2537 A energy. Thus, when several such tubes are placed in 

 contact, side by side to form a grid, only about one-third their total 

 ultraviolet output reaches a parallel irradiated surface of an area about 

 one-third the total surface areas of the tubes (Fig. 2-1 Oaj. It is of 

 interest to note that, with such a rectangular or sciuare grid source 

 formed by such an as.sembly, the intensity on the irradiated surface 

 remains constant for distances out to about half the width of the assem- 

 bly. As the distances are increased to the length of the rectangle, there is 

 a transition to a variation inversely with the square of the distance. 

 Single tubes are, in effect, rectangles of so small a width as to have the 

 characteristics of theoretical line sources (Fig. 2-9). 



(a) 



(6) 

 Fig. 2-10. Effect of bactericidal-tube spacing on utilization of total ultraviolet output. 

 (a) Close spacing, (b) Open spacing under reflectors. 



The high intensities possible with germicidal tubes in contact in a grid 

 pattern can be produced more economically with about one-third as 

 many tubes fitted with reflectors. In a tube-and-reflector system the 

 tubes should be spaced on centers three or four times their diameter 

 (Fig. 2-10^). 



All reflectors for practical uses with germicidal lamps should be of 

 speciallj^ processed, polished aluminum (60-70 per cent reflectance) or 

 polished chromium plate (40-50 per cent reflectance). Luckiesh and 

 Taylor (1946) have shown that no other reflecting materials are of 

 practical value. Special aluminum paint may be used in some places, 

 such as in air ducts, if the service or maintenance is such as to make 

 occasional repainting practical. Such a paint is made of pure aluminum 

 flakes in a vehicle of plastic lacquer of high ultraviolet transmission. 



Specular aluminum reflectors, designed to intercept about two-thirds 

 of the tube energy, redirect about (55 per cent of the energy to the irradi- 

 ated surface. The efficiency is therefore 65 per cent of two-thirds or 43 

 per cent plus the 33 per cent directly from the tube or a theoretical total 

 of ^^75 per cent. In practice, commercial etiuipment is only capable of 

 doubling the effective radiation from an equivalent grid of bare tubes. 



