PHOTOGRAPHIC MATERIALS 



139 



Motion-picture positive film is coated with a bromide emulsion which has approxi- 

 mately the speed of the slower process negative emulsions. It is not color sensitized, 

 as it is always exposed to a negative in monochrome, and it is desirable to be able to 

 handle it under bright orange or green safe lights. The maximum gamma is rather 

 high, and the behavior during development differs somewhat from that of most nega- 

 tive emulsions in that, as the development time is increased, not only is the slope of the 

 characteristic curve (gamma) increased but the curve is shifted to the left, as shown in 

 Fig. 12. Thus the correct exposure depends very markedly on the degree of develop- 

 ment to be given. 



The toe portion of the curve is of more importance than in the case of negative 

 materials, since it is desirable that the brightest high lights should have as low a density 

 as possible, in order to allow a bright screen image. This introduces some distortion 

 of tone values since the high-light tones, falling on the toe of the curve, will have a 



40 



5.0 



-20 



1.0 



1.0 2.0 3.0 4.0 



Log Exposure 



Fig. 12. — Characteristic curves for a typical motion-picture positive emulsion. 



lower contrast than the middle tones and shadows. Thus the attempt is made to 

 keep the toe of the curve as short as possible. 



The graininess is much lower and the resolving power much higher than for the 

 ordinary negative materials. For this reason and because reasonably high contrast is 

 readily obtained, positive film is used to some extent in microcopying work. 



Positive emulsions of extra-high contrast are available for special work. 



Lantern-slide plates of the ordinary type have emulsions somewhat similar to that 

 of positive film. They are available, however, in several degrees of contrast. As with 

 other positive materials the more contrasty emulsions are somewhat slower. The 

 normal tone of the developed image is a neutral black, but slightly warm or cold tone 

 images can be obtained by proper modification of the developer. 



A special type of lantern-slide plates, coated with a slower chlorobromide emulsion 

 of very fine grain, allows a wide variation in image tone. Bj' proper control of the 

 development conditions, with corresponding variations in exposures, the image tone 

 can be varied from a neutral black through a warm brown to a sepia, and in some cases 

 even to a red. 



