Journal of the Society of Motion Picture and Television Engineers (1950-1954)

scope, but it can also monitor the television picture signals being recorded and immediately indicate to the operator discrepancies in level which might need correction. The 7-in. screen is preferred because it presents an expanded scale upon which level variations can be readily detected. An ingenious little device used to advantage by our staff is a spot photometer made in England by Salford Electrical Instruments, Ltd. Although not currently in operational use in NBC Hollywood it has served well to establish the control procedure under discussion and might well serve other interested persons. Brightness values in log foot-Lamberts can be read directly from small areas on the face of the recording kinescope. Since visual matching of the luminances of fields of different colors presents difficulty, it is desirable to measure the photo-actinic output of the phosphor; a blue filter may also be installed in the photometer as an aid in matching the kinescope phosphor color. A calibrated density wedge, incorporated in the instrument, is visually compared through an eyepiece with the brightness produced by the small area being measured. Operation, in this respect, is much like that of a Capstaff-Purdy Densitometer. Since this photometer can read minute areas on the face of the kinescope it can be used to determine the brightness of the various steps of the step generator signal, particularly the two extremes, step 1 and step 10. Kinescope brightness may therefore be set in accordance with predetermined brightness values to produce desired exposure on the film negative. While this is a very useful instrument, it has three operational disadvantages: 1. The human element enters into the readings, hence no two people will get precisely the same results. 2. Due to slight differences in kinescope phosphor color, it is often difficult to decide when the two comparative densities are perfectly matched. 3. Calibration drift occurs rapidly due to insufficient current capacity in the exposure lamp supply. One remedy for this is an auxiliary battery box supply. Use of the spot photometer soon led to the design and construction of a different device eliminating the human element from the kinescope brightness measurements. This was the phototube amplifier shown attached to the hood in Fig. 7 which, although not impressive in appearance or complexity, has introduced a high degree of accuracy into our exposure control measurements. Positioned just outside the recording light path, the type 929 high-vacuum phototube receives light from the kinescope face during scanning and no light during the horizontal and vertical retrace times. If employed only with a blank raster produced by the step generator this phototube and its amplifier produce an electrical waveform having an amplitude proportional to the intensity of the light output from the kinescope. By frequent calibration of the phototube amplifier an exact comparative measurement of light output can be obtained and fed to the calibrated 7-in. oscilloscope (Fig. 8). A low-gain switch position is used for peak-white measurements where much light is produced by the kinescope, and a high-gain switch position is provided for black-level measurements where a very low level of light output is produced by the kinescope. Thus the two extremes of exposure, namely peak white and black level, can be accurately set with no "human element" error such as occurred with the spot photometer. In addition to the electronic instruments thus far mentioned, two conventional instruments well known to the film industry are employed. These are the Eastman Processing Control Sensitometer, an intensity-scale type, and the Eastman Densitometer, known generally as the Capstaff-Purdy Densitometer after its inventors. Lovell and Fraser: Kinescope Exposure Control 229