134 RADIATION mOLOGY 



(for a general description of the properties of photographic materials, 

 see James and Higgins, 1948). Furthermore, since the sensitivity and 

 contrast vary with wave length, the plate must be calibrated at all wave 

 lengths for which measurement is desired. (For data on the ultraviolet 

 characteristics of spectroscopic plates, see Harrison, 1925a; Jones and 

 Sandvik, 1920; Johnson and Hancock, 1933; Amstein, 1944. Fraser, 

 1950, has obtained data on the ultraviolet characteristics of several types 

 of motion picture film.) 



For homochromatic photometry, in which the only desire is to compare 

 radiation intensities at a given wave length, the plate or film may be cali- 

 brated by means of any device which produces a scale of plate blackening 

 versus source intensity on which the desired unknown intensities may be 



(a) (b) 



Fig. 4-2. Rotating sectors: (a) stepped, (b) continuously variable. (Reproduction 

 from Practical Spectroscopy, by G. Harrison, R. Lord, and J. R. Loofbourow, Prentice- 

 Hall, Inc., 1948.) 



read (Harrison, 1934b). Calibrated step wedges or rotating sectors are 

 commonly used for this purpose. Thin films of platinum (Merton, 1924; 

 O'Brien and Russel, 1934; Uber, 1939) or Chromel A evaporated onto 

 quartz (Banning, 1947a) are frequently used for the former, since they 

 are nearly constant in optical density over a wide range of wave lengths, 

 but for accurate work they must be calibrated. The rotating sector may 

 either be stepped or it may vary continuously in exposure time (Fig. 

 4-2). It is truly a "neutral-density" device. 



For heterochromatic photometry, in which it is desired to compare 

 radiation intensities at different wave lengths, the relative sensitivity of 

 the plate as a function of wave length must be determined. This is most 

 easily done with a source of previously determined spectral energy distri- 

 bution, preferably one with a spectral continuum, such as the hydrogen 

 discharge tube. 



Ordinary photographic plates are sensitive to ultraviolet radiations to 

 wave lengths as short as 2300 A. The faster of these plates reciuire a net 

 exposure to 0.1 1.0 ergs/cm- to produce a plate density of 0.5-1.0. 

 Below 2300 A, the absorption of the gelatin matrix for the silver halide 

 grains prevents the radiation from penetrating beyond the upp«n- layer of 



