58 RADIATION BIOLOGY 



mercury arcs lias long taiitali/od exporimoutcrs. From the well-known 

 absorption of the 25.37 A resonance line by mercury vapor and from an 

 erroneous association of efficient 2r)37 A production solely with low 

 mercury-vapor pressure, the experimenters have inferred that water or air 

 cooling of a lamp should permit a great increase in the electric power 

 input and the 2537 A emission. In the search for the optimum con- 

 ditions for 2537 A production, rather definite optima of vapor pressure 

 and power input, corresponding to lamp-tube temperatures of 40°-60°C, 

 have been found. Radical decreases in mercury pressure by cooling or 

 increases in power input, either separately or concurrently, produce rela- 

 tively small changes in 2537 A output but produce radical changes in 

 efficiency. The output ratings of commercial sources are the maxima 

 consistent with good efficiency and life. Users of 2537 A sources who 

 may be willing to sacrifice both life and efficiency for higher output power 

 density must now be reconciled to a maximum emission of the order of 

 30-50 ultraviolet mw/cm- of source surface provided by about 0.1-0.15 

 watt of electrical power input per square centimeter of tube surface. 

 This is about twice the output of commercial sources. The difference 

 between power input and emission (3 to 1) results from the inefficiency 

 of the conversion of electrical power to radiant power in the lamps and a 

 subsequent absorption of about 20 per cent in the glass tube. Tenfold 

 increases in power input, which are possible by water or air cooling, pro- 

 vide but slight increases in the 2537 A output per unit of source area. 

 These generalizations have little or no bearing on the radiation character- 

 istics of higher pressure mercury arcs discussed later. 



SOURCES OF 2537 A ULTRAVIOLET 



Table 2-3, based partially on the lES Lighting Handbook, 2d ed. 

 (1952), presents the physical, electrical, and radiation characteristics 

 of most of the commercially available sources of 2537 A energy. In each 

 ease the amount of electrical input (in watts) to the arc, the length of 

 the radiating source, and the total radiated 2537 A energy in watts, here- 

 after called "ultraviolet watts," and the ultraviolet watts per square 

 centimeter at a distance of 1 meter are closely associated. This permits 

 calculation of the efficiency of the sources, of their input and output per 

 unit of source length and area, and of the ultraviolet intensity provided 

 by them at various distances. Division of the intensity in ultraviolet 

 microwatts per sc^uare centimeter at 1 meter by 10,000 provides a useful 

 practical rating in ultraviolet watts per square foot at 10 ft. Multipli- 

 cation of ultraviolet microwatts per square centimeter at 0.9290 converts 

 to ultraviolet milliwatts per square foot, but for practical purposes they 

 are equivalent. Multiplication of the 10-ft rating by 100 and division 

 by the distance squared provides intensity in the same units for other 

 distances greater than the length of the radiating source. 



