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Sept., 1935] ILLUMINATION AND SCREEN SIZE 229
selected for correct source area and rated in volts or amperes for the proper filament temperature can be procured from the lamp manufacturers. Ordinarily, however, results from the run-of-the-mill product are of chief interest.
In addition, dependable results require the use of an accurate voltmeter or ammeter, depending upon whether the lamp is rated in volts or amperes. Because the present high-powered lamps draw a fairly heavy load and often cause a drop in the line voltage, resistance adjustments are made and meter readings are taken only when the full projector load is connected across the line. Photometers of all types require frequent checking, especially if the filament temperature of the projection lamp differs from the temperature for which the illuminometer was calibrated.
When the output of a large number of projectors must be ascertained, an integrator-box with a ground-glass or opal-glass screen is placed in front of the projection lens. At the other end of the box is inserted the light-sensitive cell. Or, if a photometer of the visual type is employed, a diffusing glass is placed in each end of the box and readings are taken from the glass farthest from the projection lens. With either type of photometer, readings from the box are compared with values obtained by the conventional point-by-point method at the screen, and a conversion factor is established. Insertion of neutral gray filters of the proper density permits reading from the box directly in screen lumens, although the illuminometer may be calibrated in foot-candles. One objection to the integrator-box method is the impossibility of measuring uniformity over the screen area. On the other hand, it is especially useful when several lamps or lenses of different types are under test.
Some laboratories use more complicated equipment, such as hemispheres into which the beam is projected. Such equipment enables skilled operators to read illumination with great precision, but the ordinary methods just described are accurate enough for this purpose if proper corrections are made.
Tests are usually made with the shutter operating, and this should be stated when the results are given. Except for still-picture projection, the light that passes the rotating shutter is of major interest. Occasionally, however, the efficiency of only the optical system is desired, in which case both the heat screen and the main shutter are blocked open and kept stationary.