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Nov., 1945 RECORDING OF PHOTOGRAPHIC DENSITIES 373
ously variable sensitometer strip to be scanned by an aperture which is 175 mils wide by either 16 or 65 mils in the direction of the film travel. A film drive unit was, therefore, developed to meet the above requirements, and arranged to replace the T-shaped film guide normally used with the densitometer. This device is shown by Fig. 1 and installed on the densitometer in Fig. 2. It consists of a film pulling assembly mounted on a base plate which is fastened to the densitometer panel by means of 4 thumbscrews. These may be quickly removed to permit the use of the standard film guide. The film drive is accomplished by a pair of rollers which propel the film by friction and it is designed to operate with film strips 35 mm in width. The upper roller is driven by a small synchronous motor with the proper
FIG. 2. RA-1200 densitometer film moving mechanism and amplifier.
gearing to drive the film strip exactly 10 in. in 30 sec. A small lever on the front of the device is used to lift the upper roller so that the film strip may be inserted from the front. With the strip in the proper position required for scanning, the lever is lowered to contact the film and sufficient spring tension is provided to prevent slippage. The synchronous motor is connected to the same source of power as the Speedomax paper drive motor and both motors are started by the same switch. The accelerating times of the 2 motors are quite short and are sufficiently alike that no measurable error is introduced. It is essential that good contact be maintained between the film emulsion and the scanning aperture plate of the densitometer. This is accomplished by 2 small spring-mounted rollers which bear on the film on both sides of the aperture. To protect the film from dirt or abrasion, a protective guide assembly is inserted in the slot under the