Journal of the Society of Motion Picture Engineers (1930-1949)

576 BERKLEY AND MANSBERG November position of the cathode-ray-tube spot for any vertical or horizontal input voltage. The appearance of such a matrix is shown in Fig. 27. After this calibration, the desired oscillogram is produced on the screen and double-exposed onto the same film. This method has the advantage that if it is done immediately prior to recording, it produces a more or less uniform distribution of electronic charge over the screen. The oscillograph trace is then not disturbed by stray screen charges which may be quite troublesome. This is particularly true in the case of equipment which is warmed up with an uncharged screen and a single transient flashed thereon. A more expensive solution to the screen charge problem can be obtained by the use of the metalized screen cathode-ray tubes mentioned previously which hold the po (a) (b) Fig. 28— Same oscillogram with bright and dark markers. (Note how overshoot is missing because of dark markers in (b).) tential of the screen uniform and constant. In this case, the method of calibration produces good results. IDENTIFICATION Where trace markers are used, bright markers have some advantages. (See Fig. 28.) After making more than three or four oscillograms, the problem of proper identification of the various wave shapes on a single roll of film becomes important. A useful accessory for this purpose is the illuminated data card which may be used to double-expose data or calibrations on the recording, as seen in Fig. 29. EQUIPMENT CHOICE In starting out to photograph a phenomenon, one should make certain preliminary considerations as to the best equipment for the purpose. In order to obtain adequate recordings, the oscillograph should have a screen capable of producing sufficient density at the