PHOTOGRAPHIC SENSITOMETRY 171 



well. Table III, taken from Jones, ^ shows the manner in which Q depends upon the 

 density of the negative. 



Effective Density of Printing. — Since the conditions under which contact printing 

 is done make use of all the transmitted light, it is evident that the most useful density, 

 or the effective value of density for contact printing is diffuse density. For all prac- 

 tical purposes the diffuse density may be used as the effective density values for contact 

 printing. 



Unfortunately, the effective densitj^ of a developed film or plate cannot be so 

 readily specified for projection printing. The reason lies in the fact that projection 

 printing corresponds more nearly, in its optical system, to some intermediate value of 

 density which varies with the enlarging conditions. In some instances, however, the 

 conditions for projection printing may closely approach the conditions for specular 

 density. They are generally nearer to specular than to diffuse densitj^. The effective 

 density of a developed negative for projection printing depends upon the optical 

 system used in the projection equipment. 



Densitometers. — Having exposed the sensitometric strips in a known and predeter- 

 mined manner and having developed them under standardized or known conditions, it 

 is now necessary to determine quantitatively the effects produced by the exposure. 

 This is done by nieasuring the amount of silver deposit per unit area or the density 

 of the negative. Such a measurement is made most readily with a special form of 

 photometer designed to measure the magnitude of the light transmitted by the silver 

 deposit on the film. Such a form of specialized photometer is called a densitometer, 

 or, if the area being measured may be limited to that of a small hole or slit, a micro- 

 densitometer or microphotometer. 



There are various types of densitometers, operating on various principles. All 

 of them, however, make use of determinations of the radiation incident on a detector, 

 both without and with the negative — whose density is to be determined — in the path 

 of the light beam. 



All densitometers contain as fundamental features a source of light to provide 

 the measuring beam passing through the negative, a means of limiting this beam to the 

 desired area of the negative, and a means of comparing the brightness of this beam with 

 that of another (or a part of the same beam) which has not been made to pass through 

 any portion of the negative. The densitometer must, therefore, provide some means 

 for measuring the intensity of the Kght without the negative in one of its beams 7o 

 and the intensity of the same or an equal light beam with a portion of the negative in 

 its path / and must provide some means for comparing these two intensities. Thus, 

 essentially, all density (or transmission) measurements may be regarded as determi- 

 ning the difference between two different conditions. 



Some detecting or indicating device is required to compare the relationship of 

 these two conditions. The comparison between the intensities of these two light 

 beams, which should haA^e the same spectral distribution, can be made by visual obser- 

 vation. In this case, accurate and reproducible results can be obtained only when the 

 two light beams being compared can be placed side by side in an optical system and 

 when provision has been made for diminishing the intensity of the stronger beam con- 

 tinuously, bj^ known amounts and without changing its spectral distribution, until it 

 matches the intensity of the weaker beam. In densitometers which make use of visual 

 comparisons, a good balance can be obtained only if the two light beams are of the 

 same color. MoreoA^er, the judgment of the operator enters into the determination 

 of the conditions of balance. Because of the vagaries of human judgment over a 

 period of time, these subjective methods of measurement do not usually give svich 



1 Jones, L. A., "Photographic Sensitometry," /. Soc. Motion Picture Engrs., 18, 324 (1932). 



