214 RADIATION BIOLOGY 



intensity by a factor of 5 and still obtain stable operation. However, 

 it must be kept in mind that, if the cathodes of the hot-cathode lamps 

 do not stay sufficiently hot for adequate emission, the resulting high field 

 intensity is apt to damage the cathodes. 



The spectral energy distribution of the phosphors of the fluorescent 

 lamp does not change appreciably with change in current or power input. 

 Varying the power by means of a series resistor is a convenient method 

 of controlling the intensity of these lamps over a considerable range. 



NEUTRAL FILTERS 



A wide variety of materials may be employed as spectrally neutral 

 filters for reducing the irradiance by a precisely known factor. These 

 include perforated diaphragms, woven wire cloth or screen wire, evapo- 

 rated metal films (Banning, 1947), blackened photographic emulsions, 

 dyed films (Eastman Kodak Company, 1951; Withrow and Price, 1953), 

 and neutral gray glasses (Corning Glass Works). Where large areas and 

 distributed sources are involved, perforated metal diaphragms and woven 

 wire cloth are convenient neutral filters. By drilling a large number of 

 small holes in a uniform pattern in thin sheet metal and beveling the 

 edges of the holes to a thin edge, diaphragms can be obtained which have 

 precise transmission factors and are spectrally neutral throughout the 

 ultraviolet, visible, and infrared. If the holes are arranged on a square 

 pattern and are of sufficient number to obtain uniform sampling of the 

 radiant flux from a diffuse source, the following formula may be used in 

 calculating the diameter d of the holes and the center-to-center spacing w 

 required for a given transmittance T: 



cP = Aw^'T/tt. (3-27) 



Woven wire cloth may be obtained commercially with specified factors 

 of "open" spaces to total screen area. These factors are approximately 

 equal to the transmittance of a blackened wire cloth. Screens of most 

 metals except aluminum may be blackened with electrodeposited plati- 

 num black (Britton, 1951). Copper and high-copper-content alloys, such 

 as brass and bronze, are readily blackened with alkaline oxidizing baths 

 (Raymond) or with proprietary solutions such as Ebanol C. 



For very low values of transmittance, screens can be placed in tandem, 

 provided that sufficient distance is interposed between the screens and 

 provided that the wires of the screens are not parallel to each other. A 

 well-collimated beam of parallel flux passing through two screens in tan- 

 dem can be made to vary in intensity from zero to the maximum allowed 

 by the transmittance of one screen by small variations in position. For 

 this reason, multiple screens should be used with considerable caution. 



Neutral filters made of evaporated metal films, photographic emulsions 

 that have been blackened to controlled degrees of density, and gelatin 



