270 HANDBOOK OF PHOTOGRAPHY 



dissimilarity of the methods employed to express the sensitivity characteristics of 

 photographic materials as well as the dissimilar numerical units employed, it is 

 difficult to make any quantitive statements concerning the relation between the 

 density of the silver deposit and the sensitivity or speed of the photographic material. 

 In several common methods of expressing film characteristics the sensitivity or speed 

 is a measure of the minimum exposure which would just produce a deposit of silver 

 if the D-logio E characteristic (as shown in Fig. 8) were a straight line. All the methods 

 of determining the speed or sensitivity of photographic plates are alike in that the 

 higher numerics of any given system of speed notation represent the more sensitive 

 or the faster emulsions. Consequently, it may be said that, for a given light source, 

 the density of the deposit will tend to increase as the speed number in any given 

 system of the photographic materials is increased. But it is not possible to determine, 

 from speed ratings alone, what the ratio of the densities will be for two films exposed 

 in a given manner but whose speeds are given in different methods of rating. Thus 

 it may be assumed that, for a given light source exposed to a plate for a given time, a 

 denser deposit of silver will be obtained for a photographic material having a speed of 

 500 H and D than for one having a speed of 100 H and D. Conversely, a denser 

 deposit for identical exposure and processing conditions will be obtained for material 

 having a rating of 26° Scheiner than for one having a rating of 18° Scheiner. We 

 cannot, however, make any statements as to the density due to a given exposure 

 between films of 500 H and D and 26° Scheiner or of 100 H and D and 18° Scheiner 

 without making measurements on the processed films. 



Effect of Spectral Characteristics of Film and Light Source.- — In color photography 

 and in monochrome photography where colored objects must be photographed so that 

 the final print shows the image in black, white, and shades of gray which are propor- 

 tional to the luminosities of the various portions of the original subject, the spectral 

 characteristics of the light source and of the photographic material are of considerable 

 importance. 



The spectral-sensitivity characteristics of the photographic material must be 

 studied together with the spectral-energy distribution of the light source, for both 

 characteristics enter into the correct tone rendition of the colored image. It is 

 possible to study the spectral characteristics of either the film or the light sovirce alone, 

 if we maintain constant the characteristic of the other factor. But in any event it is 

 essential that the spectral characteristics of both of these two factors be known. 



Let the spectral distribution of energy intensity in a Ught source be represented 

 by the symbol J\, and let the spectral sensitivity of the photographic material be desig- 

 nated by the symbol S\, where both J\ and S\ are continuous functions between their 

 upper and lower wavelength limits, which theoreticall.y are zero and infinity. Then 

 the photographic intensity of the light soiirce, whose characteristics are specified 

 by J\, when used with a photographic material whose spectral characteristics are 

 specified by S\, is given by 



'^'^JxSxdX (3) 



( 



=x 



The equation for the photographic intensity may be represented by the area under a 

 curve whose boundary is determined by the boundaries of the curves representing the 

 spectral characteristics of the film and the light source. 



For those who may not be mathematically minded, the following illustrative 

 examples may be more enlightening than the above equation. Let Fig. 9 represent 

 the relative spectral sensitivity of several photographic materials, as indicated, and 

 let Fig. 10 represent the relative spectral-intensity distribution of three common light 

 sources employed in photography. Since we shall be concerned only with the relative 



