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PRODUCTION QUALITY CONTROL OF PRINTS 515
duplication of color film, such as Kodachrome, can afford to be without control aids. Figure 122 shows a color-temperature-voltage-current relationship of a typical 500-w., 105-v. T-10 printing lamp. Other valuable lamp data are found in the bulletin Mazda Lamps published by General Electric.
Exposure is most readily measured directly at the printer aperture with the machine stationary ; the better grade illuminometers such as the Weston (priced approximately at 100 dollars) are suitable. Indirect methods, such as the use of black-and-white film as a control, are not practicable in most commercial laboratories as they introduce additional variables whose variations are usually unknown.* In using illuminometers and similar direct-measuring instruments, it is necessary to make up suitable jig-adapters so that readings of the instrument will be reproduced without significant personal error. Provision should also be made for periodically calibrating the measuring instruments. It is necessary that the variation in slip of belt-driven printers be known. It is still better to use direct-gear drive with a 3-phase synchronous motor or equivalent to eliminate machine speed variation as a possible source of exposure variation.
The regulation of the current supply for the printing lamp is very important. It is obviously a sheer waste of time and money to calibrate lamps to ± 10 K when the motor generator or other supply has poor regulation, and other loads are indiscriminately "placed on the line" and "taken off." A simple check is to connect a recording voltmeter to a printer under suspicion, using that printer without current changes meanwhile. Often the local power company will be glad to lend a suitable meter if one is available.
Prom the user's point of view, Kodachrome duplicating film (EK 5265) is quite consistent. The major variation that occurs is the variation from lot to lot, similar to that occurring in black-and-white materials. With fresh film, the variation in sensitivity from one lot to another appears to be less than one of the smallest compensating filter steps (CC23, CC33, or CC43). The variation in layer sensitivity from one layer to another appears to be likewise small and of comparable order. For film that has aged slightly (for example, less than 6 months old when stored at a constant temperature not over 70 F), there is a slight loss in * Such indirect methods can be satisfactory if the sensitometric control on black and white is of a superior order compared with usual commercial control. In such cases, however, the direct methods are still more convenient and more precise.