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

Record Details:

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1000 8 6 -HIGHLIGHT SIGNAL* " ILLUMINATION:287 "SCANNED AREA OUTPUT CURRENT=0.35M« 0°K INCANDESCENT DF PHOTOCONDUCTIVE LAYERS/2*3' fa \ \ 2 100 e * \ \ \ ^T >^ Sy j5c X. V X 2 10 ^v X. X DARK C JRRENT=. I )2Ma--^ 15 20 25 30 35 40 45 5< SIGNAL-ELECTRODE VOLTS Fig. 8. Illumination requirements of the vidicon photosurface for a highlight-signal current of 0.35 /j,a. fact that the photoconductive layer has a continuous surface. The constant gamma of the surface acts to reproduce each portion of the scene in its true tone values regardless of the illumination of adjacent areas or the overall illumination level. For this reason, the camera needs no white or blackstretching circuits to produce a picture with accurate tone rendition. The gamma of 0.65 also allows the tube to generate a picture with a very high contrast range. A dynamic contrast range of 200 to 1 can be reproduced. Black Level Another desirable characteristic of the vidicon is its ability to furnish accurate black-level information in the video signal. With operation at low signal-electrode voltages, true blacks in the scene represent essentially zero current from the signal electrode. The signal during the scanning retrace also represents zero output current, since the scanning beam is prevented from landing during this time by the application of a blanking pulse to the electron gun. Blanking is accomplished either by applying a negative blanking pulse to the control grid of sufficient amplitude to cut the beam off completely or by the application of a positive pulse to the cathode. The positive pulse applied to the cathode need only be of sufficient amplitude to prevent the beam from landing on the photoconductive surface which has been driven to zero potential during the scanning interval. The signal level obtained during the retrace is used by the camera clamp circuits to set the black level of the video signal (sometimes called the "d-c component"). Light Requirements The curve of Fig. 8 indicates the order of magnitude of illumination required Neuhauser: Vidicon for Film Pickup 149