■ APPLICATIONS AND SOURCES OF ULTRAVIOLET 51 



KILLING EXPOSURES 

 RECIPROCITY OF TIME AND INTENSITY 

 Like the photographic effects of Hght, the germicidal effect of ultra- 

 violet results from an exposure (intensity times time). The basic factors 

 in an exposure are the incident power, the time, and the irradiated area. 

 The erg, often used in biophysical work, is a unit of energy only and must 

 be referred to time in order to define power. Ergs per second become 

 units of power, and ergs per second per square centimeter become units of 

 uitensity. Ergs per second per square centimeter-second, or, more 

 usually, ergs per square centimeter, become units of dose or exposure. 

 In practical work there are advantages in basing the intensity unit 

 directly on the watt since it is also used to define the power output of 

 ultraviolet sources. The microwatt, equal to 10 ergs/sec, becomes a con- 

 venient unit of power, and the microwatt per square centimeter becomes 

 a convenient unit of intensity. For practical purposes the microwatts 

 per square centimeter equal the milliwatts per square foot, the multiply- 

 ing factor being 0.9290. The microwatt per square centimeter-minute, 

 often written as microwatt-minute per square centimeter, becomes a prac- 

 tical unit of exposure equal to 600 ergs/cml The microwatts per square 

 centimeter-minute emphasizes the reciprocity of exposure intensities and 

 times which may be adjusted over a very wide range to obtain a specified 

 exposure under various conditions. 



Theoretically, an e.xposure of 25 MW-min/cm-' or 1500 ergs/cm^, for 

 example, may be obtained either in a long time (1 day) with a low inten- 

 sity (0.018 ultraviolet /xw/cm^) or in a short time (0.001 min) with a 

 high intensity (25,000 ultraviolet /xw/cm^). In practice, the exposure 

 time IS usually determined by the nature of the job to be done and ranges 

 from a fraction of a second for the disinfection of rapidly moving air or 

 products to 1-10 min for air disinfection in relatively quiet places. The 

 exposure intensity must then be adjusted to obtain an adequate exposure. ' 

 Such intensities may range from a few ultraviolet microwatts per square 

 centimeter for bacterial air disinfection to several ultraviolet milliwatts 

 per square centimeter for product disinfection from molds, as suggested 

 by Fig. 2-7a,b and in greater detail for the short exposures in air ducts 

 by Fig. 2-8. 



The reciprocity of time and intensity is also illustrated by the upper and 

 lower legends on the otherwise identical scales of exposure of Fig. 2-2. 



ERYTHEMAL ACTION OF WAVE LENGTHS 2537 AND 2967 A 



As shown graphically in Figs. 2-1 and 5, the germicidal ultraviolet is 

 also erythemal in action, the effect at 2537 A being about half as great as 

 at the optimum wave length of 2967 A. As indicated in Figs. 2-1 and 7a 



