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156
BRITISH KINEMATOGRAPHY
Vol. 17, No. 5
emulsions. The intrinsic luminance of the scanning spot is exceedingly high and may attain a highlight luminance of several thousand foot-lamberts.
The exposure of the negative film is usually determined prior to the adjustment of the shutter. The shutter is adjusted for minimum banding at the aperture setting to give proper' exposure on the negative. The procedure followed in obtaining a correct negative exposure is to use a plain raster on the Kinescope, such as would be obtained by the use of the blanking signal without picture modulation. The brightness of the raster is varied by means of the video gain control. The beam current may be measured with a microammeter. Since the brightness of the tube is dependent upon the beam current and the accelerating voltage, this beam-current measurement serves as a measure of the luminance.
A series of exposures may then be made by varying the beam current in logarithmic steps. After the film is processed to the recommended gamma, the density may then be read on a densitometer and plotted against the logarithm
EASTMAN FINE GRAIN SOUND RECORDING SAFETY FILM, TYPE 5373 Hb Exposure
Fig. 14. H and D Curve of Eastman
Fine Grain Sound Recording Safety
Film, Type 5373, as used in Kinescope
recording.
Log exposure
EASTMAN FINE GRAIN SOUNO RECORDING SAFETY FILM, TYPE 5373 HIGH INTENSITY EXPOSURE Exposure 30 microseconds
Fig. 15. H and D Curve of Eastman
Fine Grain Sound Recording Safety
Film, Type 5373, when subjected to
high-intensity exposure.
Log exposure
of the beam current. For a negative material which has been developed to a gamma value of 0.65, the negative density range normally made use of in the recording of a picture image is from 0.20 to 1.4. A beam current which gives an average density of around 0.8 to 0.9 might, therefore, be considered as providing an average luminance level corresponding to that of a picture tube image which will give an approximately correct exposure. Using this beam current as a starting point, a series of exposures over a smaller range may then be made with picture modulation on the tube, in order to arrive at an average exposure value which will be satisfactory for various types of subject matter.
Film Stocks.
Either a negative or a positive picture may be obtained on the face of the Kinescope simply by reversing the phase of the signal presented to its input grid. The procedure generally used is to photograph a positive-picture tube