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

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1950 CBS LIGHTING PRACTICES 247 Fig. 2. Image Orthicon Ghost The displaced image of the hand shown in A results from high-velocity secondary electrons emitted from the high-light area on the image orthicon target. These electrons travel as a group back toward the photo-cathode, eventually decelerating and returning to the target, producing a positive signal by knocking off secondary electrons as in the case of an ordinary electron image signal. The displacement results from travel through the axial magnetic field present. Here, too, it is possible to minimize the landing of this ghost image by maintaining less contrast between the high-light and the background areas. However, in B, the hand has simply been moved to the center of the raster, where the ghost travels along the axis of the tube and falls back on the high-light area without displacement. Slight defocusing of image focus would also reduce the ghost but at a sacrifice in resolution. Through a typical direct-view receiver, for example, a 2-to-l scene brightness difference may under some conditions appear as a 6-to-l difference due to expansion introduced by curvature of the reproducing cathode-ray tube voltage-to-luminance transfer characteristic. In the studio, the transfer characteristic relating scene luminance to output voltage for the image orthicon tube is similar in shape to the familiar H&D curve for film but is influenced by the level of illumination incident on the image orthicon photo-cathode, by the ratio between high-light and average scene luminance and by adjustment of image orthicon target voltage. It is possible to have compression of one range of luminance values and expansion of another — all in the same scene. This problem again calls for careful control of over-all scene luminance distribution and for careful exposure and camera adjustments. These troubles are caused in part by the electron redistribution process inherent in present-day operation of the image orthicon pickup tube. Several spurious effects which arise in the tube or associated equipment are even more objectionable at times because of their distinctive appearance. These effects of interaction between adjacent areas include : Halo: A black area surrounding a bright high-light, resulting from a rain of low-velocity electrons emitted from the high-light area of the image orthicon target and particularly severe on highly polished jewlery, white clothing, and bald heads (see Fig. 1). Image orthicon ghost: A spurious, displaced image of a high-light area, most noticeable with severe contrast between high-light and background, resulting from high-velocity secondary electrons emitted from the high-light area on the image orthicon target (see Fig. 2). Clouding: An electronic fogging or mottling of large dark areas, similar in effect to lens flare, particularly severe where excessive contrast exists between large dark and large light areas and aggravated