PHOTOGRAPHIC SENSITOMETRY 



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or slots decreasing in angle of arc from the center outward. The largest aperture was 

 180°, the second one was 90°, the third 45°, etc. The relation between the aperture, 

 angle of arc of the aperture, and the relative exposure of their disk is given in Table II. 



Table II. — Exposure Characteristics for Sectored Disk 



Aperture number. 

 Arc angle, degrees 

 Relative exposure, 

 logio Erbl 



1 

 180 

 256 



2 

 90 

 128 

 2.11 



3 



45 

 64 

 1.81 



4 

 22.5 

 32 



1.51 



5 

 11.2 

 16 



1.20 



6 

 5.3 



7 



2.6 

 4 

 0.6 



1.3 



2 



0.3 



It will be seen that logio Ere l is a uniform progression; the absolute value of 

 the exposure of the nth step being twice that of the (n — l)st step. At the present 

 time successive steps are often made equal to ■\/2 or \/2 times the next step in order 

 that more steps and finer gradations may be obtained between successive steps. 



Practical Sensitometers. — Practical sensitometers usually take the form of a rotating 

 disk or cylinder in which slots of various widths are cut. Figure 5 shows the working 



Fig. 5. — Diagram showing the construction of the Eastman Type 116 sensitometer. 



essentials of the Eastman Type 116 sensitometer popularly employed in motion-picture 

 work. A line switch LS turns on current to a standardized lamp L, whose intensity is 

 controlled by adjustments of the rheostat R. The beam from the lamp travels in a 

 cone until it strikes the mirror M, which reflects the beam upward to the motion- 

 picture film/, held in position by the plate PL. A shutter is provided at S, and a filter 

 may be used, if desired, at F. The exposure drum D is driven by the synchronous 

 motor SM through the gear train G and is coupled to the shutter S in such a way that 

 the shutter is opened for only one complete revolution of the drum D. Slots are cut 

 in the drum in such a way that the exposure for each slot is \/2 times that of the 

 preceding slot. The total exposure range of the drum is 1024 to 1. 



