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366 HATCH October
centimeter per minute is equal to 0.449 watt per square inch, full-scale reading of the indicating meter is obtained when projecting 0.898 watt per square inch to the receiver plane (2 X 0.449). Converting 1 watt per square millimeter at the aperture to watts per square inch gives a value of 645 watts per square inch. With a lens-transmission efficiency of 85 per cent, the magnification constant necessary to convert 645 watts per square inch at the aperture to a full-scale meter reading of 0.898 watt per square inch at the receiver plane, is as follows :
M = J645 * °-85 =247 \ 0.898
CALIBRATION
An approximately correct calibration, utilizing the design constants, will result, if the receiver element plane is maintained at a distance of I3l/z inches from the pinhole aperture, and the distance between the pinhole aperture and lens varied to obtain either a sharply defined spot in the receiver plane, or a spot diameter 24.7 times the pinhole diameter. The accuracy of the receiver and millivoltmeter unit is =•=5 per cent of full scale any place on the scale, which means an accuracy of ±0.05 watt per square millimeter at the aperture.
A more accurate calibration of the over-all instrument can be made by adjusting the lens setting, so that the readings obtained agree with the laboratory method already mentioned. With the meter calibrated for several points against the more exact laboratory method of measuring the aperture energy, and with the application of the resulting correction factors, an over-all accuracy of about 2 per cent can be expected.
METHOD OF USE
To obtain a measurement of total center-aperture energy, simply remove the projection lens and insert the aperture-energy meter into the lens holder so that the aluminum block enters through the picture aperture. Make sure that the projector-shutter blades are out of the light path and prop the fire shutter open. After checking the arc current and the setting of the positive carbon in relation to the lamphouse optical system, open the lamp dowser long enough for the motor needle to come to a steady position. Immediately upon reading flic value, the dowser should be closed. The heat capacity of the pinhole aperture is small, and it will become a radiator of some proportion, if subjected to the radiant energy for any but the short period