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1950 CBS LIGHTING PRACTICES 245
Fig. 1. Halo
The heavy black fringe surrounding the young lady in A results from a shower of low-velocity secondary electrons emitted from the high-light areas on the image orthicon target. These electrons land on the surrounding dark area completely discharging the nearby portions. In B, a lighter background has been substituted and the halo has been greatly reduced. In this case, the secondary emission from the background area is now sufficient to alter the field configuration in the vicinity of the target causing the excess high-light area electrons to land properly on the collector mesh.
NOTE: The dashed-line ellipse is a mask placed on the picture monitors to bound the area within which essential picture information should be held to prevent subsequent cropping by receiver masks or the film recording process.
occur, is followed by the statement and discussion of some of the more important working rules which have been formulated. The intention is to indicate the nature of the approach which has been made toward control of production practices.
TECHNICAL LIMITATIONS OF TELEVISION
Aspects of the present-day television facilities which may constitute technical limitations are : total contrast range, shape of the contrast gradient or luminance transfer characteristic, interaction among adjacent picture areas, and detail resolving capability. The characteristics of the image orthicon pickup tubes, of the electrical transmission system and of the reproducing cathode-ray tubes all may contribute to the over-all distortion in picture quality. For a network originating studio, it is particularly important that careful control be exercised as network transmission facilities and television film recordings used for program distribution certainly cannot be expected to minimize picture defects produced in the studio.
Limited total contrast range constitutes one of the most basic problems. The ranges of luminance values which can be handled by several familiar systems are approximately as follows :
The human eye — for a particular luminance adaptation 100 to 1
35-Mm motion pictures — typical projection. ... 40 to 1
Live, direct-view television — ideal conditions 40 to 1
Live, direct-view television — typical conditions ... 20 to 1
Kodachrome-type color film processes 15 to 1
It is important that, in the television scene to be transmitted, all subject matter have a luminance within the approximate 20-to-l range handled by the system.
Distortion in luminance transfer characteristics may have the effect of increasing the apparent contrast between areas of a scene.