British Kinematography (1947)

r 117 Sensitometric Control in the Motion Picture Laboratory Motion picture films are exposed, in the picture camera or on the printer, to a range of intensities for a constant time, of the order of 1/50 — 1/200 second, although in some variable density sound recording cameras the moving ribbons of the light valve cause a variation in time and not in intensity at exposure times of the order of 1/20,000 second. If we are to obtain the " characteristic curve " of the material which is really characteristic of the material in practice we have seen that the test exposures must be made under the same conditions as regards intensity and time as those in practical usage. The difficulties in producing intensity-scale sensitometers to operate at the required short exposure times are firstly the much higher intensities of light required, since there is an enormous waste of light in producing a wide range of intensities, and secondly, in providing a means of modulating the intensity over a sufficient range without altering the quality of the light. So-called neutral density wedges cannot readily be made so non-selective that they do not require elaborate calibration with the type of photographic material with which they are to be used, and intensity-scale sensitometers are in consequence much more difficult to specify and calibrate than time-scale sensitometers. The latter type of sensitometer is easy to specify and can be constructed with great precision, and offers considerable advantages for sensitometric control. The purpose of using sensitometric control in the motion picture laboratory is to enable material to be exposed and processed so that a definite desired result may be achieved. This involves developer bath control for which, although chemical analysis and pTL measurements are a valuable aid in establishing replenishment rates, the final check on developer activity should still be by means of a sensitometric strip ; development of material to a given control gamma ; and determination of the speed of different batches of material so that due allowance can be made for batch variation. For developer bath control a time-scale sensitometer offers no disadvantages. The required degree of development of a given type of material may also be specified by exposures made on a time-scale sensitometer where the type of exposure is rigidly specified as with the Eastman IIB sensitometer. Different manufacturers' materials may, however, require to be developed to different IIB control gammas to achieve the same degree of contrast in^ practice. Again, while the difference in speed between different batches of the same type of material of one manufacturer may be adequately determined by means of a IIB sensitometer, since the reciprocity failure differences between different batches is relatively small, it cannot be relied upon to give accurate information on the relative speed of different makers' products when used in practice. This would be especially the case if a IIB sensitometer were used to compare the speed of different materials required to be exposed at the much higher intensity level used in sound recording. Provided these limitations are realised the IIB sensitometer has much to recommend it as a standard instrument for sensitometric control. One interesting development which some laboratories have been using is what is generally referred to as a " printthrough gamma " strip. In the Denham Laboratories this consists of a wedge of three or four steps and a portrait, the whole occupying the 35 mm. picture frame, made on duplicating negative film by means of an optical printer. Three frames of this strip are attached at the beginning of the negative and printed at a given light change, and are used to control print quality, the IIB sensitometer being used for bath control. In this paper an attempt has been made to indicate the nature of reciprocity failure shown by photographic emulsions and to discuss its implications in the