PHOTOGRAPHIC SENSITOMETRY 



169 



If we measure all the light transmitted through the negative, i.e., if we determine 

 all the light in the envelope (by means of an integrating sphere, for example), as shown 

 in Fig. 8, we obtain what is known as diffuse density. 



The light transmitted through a negative may be measured by means of an inte- 

 grating sphere and radiant-energy detector. The integrating sphere consists of a 

 hollow sphere whose inner surface is covered with some coat of diffusing and reflecting 

 material. The film is placed in contact with a small aperture in the integrating sphere, 

 the aperture being small compared with the dimensions of the sphere. Thus the film 

 and aperture are sensibly a portion of the spherical surface of the integrating sphere. 



Fig. 7.- — Polar diagram illustrating the scattering of light in passing through a photo- 

 graphic film. The relative intensity of the emerging beam in any direction is proportional 

 to the length of the arrows in that direction. 



The detector may be a phototube and associated electrical circuit, a thermocouple, 

 bolometer, or similar device. Preferably a nonselective indicator should be 

 used. Direct rays from the aperture are prevented from reaching the indicating device 

 by means of a shield, and only that energy coming through the aperture which has 

 been reflected by the spherical walls of the integrating sphere affects the indicating 

 device or detector. 



If now, the film is placed very close to the aperture, as in Fig. 8, so that it forms a 

 portion of the wall of the integrating sphere, the transmitted rays normal to the film, 



Fig. 8. — By measuring all the light Fig. 9. — By measuring only that por- 



passing through the negative, the diffuse tion of the light passing straight through 

 density is determined. the negative, specular density may be 



determined. 



as well as the diffused rays coming through the negative, will be collected by the 

 integrating sphere and will determine the reading of the indicating device. 



Diffuse and Specular Density. — The value of the density computed from the inci- 

 dent and emergent light in this case is known as "diffuse density." It is based upon 

 measurement of the total transmission of the negative (both normal or undeflected, as 

 well as diffused light) and will give the smallest possible value of density. The meas- 

 urements made on the basis of diffuse density are appropriate for contact prints, since 

 contact prints use all the transmitted light. 



If now the film is removed a considerable distance from the aperture, as in Fig. 9, so 

 that the only rays passing through the aperture are those which are transmitted by 



