American cinematographer (July 1933)

July 1933 0 American Cinematographer 87 Control in the Processing of Motion Picture Film by * Emery Huse, A. S. C. presses on the film under test an accurately predetermined scale of exposures which may be maintained constant from test to test over long periods of time. The exposure scale consists of 21 steps produced by exposures equal in illumina- tion and ranging from 1 to 1024 in relative times, each exposure being 1.414 (square root of 2) times as long as the next shorter. This constant factorial difference between steps permits the density readings to be spaced at equal in- tervals along the log exposure axis in constructing an H and D (density-log E) curve. Figure 2 shows an actual sensi- tometric record made with this instrument. Tables 1 and 2 herewith submitted show the actual setup of the instrument for the exposure of positive and negative films respectively. TABLE 1 Positive Setup Lamp . 72 watt, 6 volt, locomotive headlight, calibrated tor 2600°K. Filter 78 B correcting to 3000°K. Intensity .... 27 meter-candles t max 4.99 seconds at 50 cycles log E max. 2.13 TABLE 2 Negative Setup Lamp . . 36 watt, 6 volt, locomotive headlight, calibrated for 2360°K. Filter 79 correcting to 5400°K. Intensity 75 meter-candles t max 4.99 seconds at 50 cycles. log E max ... 0.57 It is with this instrument that practically all sensitometric control in the processing of motion picture film in Hollywood is accomplished. At the time of this writing there are nine of these instruments in use there. It is interesting to note at this point that at the Annual Awards Banquet of the Academy of Motion Picture Arts and Sciences in November, 1932, this instrument was given official recognition by being awarded an honorable mention by the committee on awards on Scientific and Technical Achievements. After the development of the sensitometric strip made on the Type lib sensitometer it is necessary, in order to attain the desired technical results, to find a means of measuring the densities of the various deposits. A photo- metric instrument of one type or another is used for this work. Such an instrument in common use in most of the ''West Coast Manager, Motion Picture Film Division, Eastman Ko- dak Company. laboratories in Hollywood is the Eastman densitometer which has been described by Capstaff and Purdy (2). At the present time there are approximately 25 of these instruments in use in Hollywood, both in laboratories and in sound de- partments. Some of these departments make use of polari- zation photometers such as those made by Schmidt and Haensch or by the Bausch and Lomb Optical Company. The Eastman densitometer, which is shown in Figure 3, is designed to fulfil several conditions, namely: the ability to read densities from 0.00 to 3.00; to measure very small areas ( V 2 sq. mm.) utilizing the same source of light for the illumination of the density to be measured and furnishing the light for the comparison beam; calibrated to read direct diffused density; and designed to be portable, compact and inexpensive. It has been shown in actual practice that this instrument fulfils these requirements. The West Coast Laboratory of the Eastman Kodak Com- pany maintains a continual service in checking the densi- tometers in the field for their physical condition as well as their photometric ability, calibrations being made against standard densities originally calibrated in the Research Laboratories in Rochester. Inasmuch as the actual conditions of exposure of the sensitometric strip are known, i.e., the time of exposure and the intensity and quality of the exposing radiation, it is possible with the values of density available to construct the characteristic density-log E curve. As the exposure in- creases so the density increases until upon completion of the plotting of the curve a graph, such as is shown in Figure 4, is obtained. There are three distinct portions to this curve, namely: the toe, which is that portion indicated between A and B; the straight line, between B and C; and the shoulder, between C and D. It is quite well known that these three portions of the characteristic curve are referred to respectively as the regions of under exposure, correct ex- posure, and over exposure. It is of value, therefore, in the application of sensitometry to motion picture film processing to know the characteristic curve resulting from the develop- ment of sensitometric exposures in the negative, positive, Continued on next page