PHOTOGRAPHIC SENSITOMETRY 



173 



In the deflecting type of densitometer, the difference in intensity of the measuring 

 and comparison light beams is measured directly by some deflection instrument, and 

 no effort is made to create equality between the intensities of the two beams. 



Automatic-recording densitometers are elaborations of the null or deflecting type 

 of instruments. In place of the deflecting type of indicator, a recording pen is pro- 

 vided which makes a plot of the density as the position of the photosensitive material 

 under measurement is changed. 



A wide variety of densitometers and microdensitometers has been described in the 

 literature for general photographic research and for such specialized branches of 

 photography as radiography, astronomical photography, spectroscopy, etc. A few 

 representative densitometers for general photographic work will be described, but for 



Fig. 11. — Diagrammatic illustration showing the essential design of the Capstaff-Green 

 densitometer which makes use of the inverse-square law of illumination. 



additional detailed information or for information on densitometers for specialized 

 applications, the technical literature should be consulted. 



Capstaff-Green Densitometer.- — ^The Capstaff-Green densitometer is of the subjec- 

 tive type and depends upon the inverse-square law for its calibration and for its method 

 of balancing the equality of the comparison and measuring light beams. A diagram 

 showing the essential elements of construction is shown in Fig. 11. This densitometer 

 consists of a light source which is split into two beams. One beam traverses the path 

 from lamp / to the reflecting mirrors M, M', M" and finally falls upon a partially 

 reflecting mirror M in the eyepiece H. The other beam travels directly from the lamp 

 / through a diffusing screen S, upon which the negative to be measured is placed, and 

 thence to the partially reflecting mirror M. The lamp may be moved up and down on 

 a suitable carriage. It is driven by a steel tape passing over pulleys, the tape being 

 graduated to read density directly. The lamp is adjusted to its zero or reference 

 position with the negative removed until both fields of the mirror in the eyepiece are 

 equally illuminated. The negative is then inserted on the screen S and the lamp 



