American cinematographer (July 1937)

282 American Cinematographer • July, 1937 REEVES COMBINES LIGHT TESTER AND SENSITOMETER, TOO W ITHIN the past month Art Reeves, maker of recording and laboratory equipment, has announced a combination sensitometer and light tester known as the Reeves Sensitester. Designed primarily as a really accurate light test machine, the new device has been found sufficiently accurate to serve the purpose of a dependable, practical sensitometer as well. In the final design now in pro- duction this phase has been developed to the point where, under test, the ma- chine has been found to match close- ly the results had with standard sensi- tometers. Both the sensitometer and the more familiar light tester work to a similar end: the making of a graduated se- ries of exposures on a strip of sensi- tive material, the graduations running from a known minimum by fixed steps to a known maximum. In a light tester these exposure steps are proportioned to correspond to the printing light settings of the printer to be used in printing from the negative being tested. In the sensitometer the steps increase by a fixed geometric progression. In this case inaccuracies which would be negligible in a light tester can become intol- erably large. Most sensitometers are of what is termed the “time scale” type, in which the ex- posing light is constant, but is allowed to act for varying intervals on the film. Fluctua- tions in the current supply of either the light source or the ex- posing mechanism can introduce great variations in the accuracy of such time scale sensitometers. Simple Principles Accordingly elaborate precautions are taken to assure uniformity in light and timing. It has been agreed by the leading experts that an “in- tensity scale” sensitometer, in which all steps received the same exposure time, but different intensities of light, would be ideal, but no consistent method of metering the light seemed practically available. The Reeves light tester. however, worked upon a very simple principle, or pair of principles, in achieving this end. It metered the light through a series of diaphragms and controlled the time by the coun- terweighted pendulum of the metro- nome—two almost ideally consistent principles. From this start it was natural to develop a light tester suf- ficiently accurate to serve also as a sensitometer. The new sensitometer is the result. When used as a film light tester the negative to be tested is threaded across the machine from one rewind to another. The positive film upon which the light test is to be printed is Artreeves Sensitester threaded from an inclosed feed maga- zine, over a sprocket and under a pressure pad or platen, past another sprocket and into an inclosed take-up magazine. When making a test the horizontal bar extending across the front of the machine is depressed. This lowers the platen and magazine assembly, bringing the two films into contact across the exposure plane. At this moment the exposure is made. As the control bar is raised the exposed section of film is automatically wound into the take-up magazine and a fresh section of film bi'ought into place. Timing Controlled The exposing light is metered through a series of adjustable dia- phragms which may be pre-set to match the characteristics of any print- er. This gives 11 graduated expos- ure steps corresponding to printer lights 1 to 21. In addition, the ma- chine also prints the marginal foot- age number, eliminating any chance for confusion of similar takes. The timing of the exposure is con- trolled by an adaptation of the metro- nome’s counterweighted pendulum principle. As the position of the counterweight is adjustable, the tim- ing of the pendulum’s swing—and hence the exposure—is adjustable. As the platen is brought down the metronome arm is released. As it starts its swing it switches on the exposing light. As it finishes its W swing it switches off the light and is itself locked in place, ready for the next ^ // test. Combining these two prin- ciples of proved accuracy makes it possible to utilize the same machine as an ac- curate sensitometer. Since the unvarying metronome pendulum times the expos- ure, and all the exposure steps are made at the same time, no outside factor can alter this timing. Since the light for all steps comes from a common source at one time, and is metered through a fixed series of diaphragms, the relative ex- posures of the various steps cannot be upset. Doubling Up For this purpose, a supplementary series of fixed diaphragms is built into the machine. Each of them admits light in a fixed, logarithmically pro- gressing ratio. All that is necessary to convert the machine from use as a light tester in- to a sensitometer is to pull a control Continued on Pape 288