APPLICATIONS AND SOUKCES OF ULTRAVIOLET 63 



AVERAGE INTENSITY IX SPACE 



In the disinfection of fluids, the average ultraviolet intensity, or average 

 radiant-energy density, throughout the concentric cylindrical space sur- 

 rounding the tubular sources becomes a basic factor. It is especially 

 basic in the case of air where there is no absorption to modif}^ the linear 

 decrease in intensity inversely with the distance from the lamp tube. 

 It can be the basic intensity factor in the disinfecting exposure of air 

 where it is possible to provide enough turbulence of flow to expose the 

 air to the full range of intensities, and so to an average intensity, during 

 its travel through an irradiated zone. Since the intensity at less than 

 source-length distances from linear sources varies inversely as the distance 

 and the volumes of successive increments of annular space increase 

 directly as the distance, the products of annular volume increments and 

 their energy density become constant. Thus, within source-length dis- 

 tances, the average intensity occurs at the average distance of one-half 

 the radius of the irradiated zone and is twice the intensity at the outer 

 limits of the zone, as pointed out by Luckiesh and HoUaday (19-l:2a,b). 



At distances greater than source length, as in directly irradiated rooms 

 or very large plenum chambers of ventilating systems, the intensity 

 throughout the spherical space surrounding a central ultraviolet source 

 varies inversely as the stjuare of the distance, and the volumes of suc- 

 cessive increment shells of space increase directly as the square of the 

 distance, so that the products of successive volume increments and their 

 energy density become constant. Here again the average intensity 

 would occur at the average distance of l/\/3 or 0.577 the radius of the 

 irradiated spherical volume and would be three times the intensity at 

 the outer limits of the volume, as developed by Wells (1940), if it were 

 not for the toroidal rather than spherical spatial distribution of the 

 energy about a linear source. Also, since irradiated rooms are cubical 

 rather than spherical in form, the average intensity occurs at more 

 nearly half the average radial distances to the walls and is again about 

 half the intensity at the outer limits of the spherical or cubical zone. 



INCREASE OF INTENSITY AND UTILIZATION BY' REFLECTORS 



Efficient sources of the germicidal ultraviolet are inherently low in 

 intensity compared with high-pressure sources designed for photo- 

 chemical and therapeutic use. A maximum intensity of 10-20 ultra- 

 violet watts/sq ft (10,000-20,000 ultraviolet-/iw/cm'") can be available 

 at a tube surface for experimental work, but, at practical working dis- 

 tances, only about one-fourth this intensity can be obtained as irradiation 

 over an extended area (see Fig. 2-106). 



The ionized mercury vapor in germicidal tubes almost completely 

 absorbs any 2537 A energy which might otherwise pass through the glass 

 tube itself from an outside source. For this reason, only the thin layer 



