Color Balance for Television

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  • MacAdam: Color Balance for Television

    CONCLUSIONThe operational aspects of this equipment, including

    exact functions, operational performance, accessoryunits, accuracy and stability are beyond the scope ofthis paper. However, the following information is ofinterest.A number of complete equipments, including all nec-

    essary accessories such as mounting racks, antennas,indicators, control boxes, etc., have been fabricated atthe National Bureau of Standards to permit evaluation(including flight tests) of both the equipments and thenew techniques and materials used.

    Environmental tests have indicated that the equip-ment will give satisfactory service under the severe con-ditions of altitude and temperature originally specifiedfor the equipment. No component failures which can beattributed to these unusually severe environmental con-ditions have occurred to date.An entire equipment has been subjected to severe

    shock and vibration tests, including five impact shocks

    of 30 g in opposite directions along each of the threemutually perpendicular axes (a total of thirty impacts).Damage to the equipment as a result of this severe testwas trivial, and appropriate minor design changes havesince been made.

    ACKNOWLEDGMENTTechnical assistance in developing this equipment is

    acknowledged to C. 0. Lindseth, L. Landsman, E. J.Hebert, Jr., R. F. May, R. J. Luks, and R. J. Hens ofthe National Bureau of Standards, to T. Anderson ofAirtron, Inc., and to G. Mathis of Raytheon Manufac-turing Company. Major administrative assistance onbehalf of the sponsor is acknowledged to Capt. C. A.Cole, Jr., USMC, and Capt. M. W. Cairns, USMC, bothof the Navy Bureau of Aeronautics. This work was per-formed at the National Bureau of Standards and theauthor extends his appreciation to both the NationalBureau of Standards and the Navy Bureau of Aero-nautics for making it possible.

    Color Balance for Television*D. L. MACADAMt

    Summary-The visual phenomenon of color adaptation must bemimicked by any successful process of color photography or colortelevision. This means that a light gray object should be reproducedso as to appear light gray, regardless of variations of the illuminatonin the original scene, within a wide range of qualities. In color pho-tography, films of two different classes are commonly provided, onefor daylight and the other for incandescent tungsten light.

    In the present standard system of color television, the color sub-carrer should have zero amplitude for a light gray object, regardlessof the chromaticity of the illnation, whether daylight or incan-descent tungsten light or arc light.

    Adjustment of receivers to correspond to the adaptation of theviewer is equally important. Although most receivers will be used inliving rooms lighted by incandescent tungsten lamps, receivers aresometimes used in subdued daylight. This fact creates a dilemma,which is familiar to the designers of illuminators for color trans-parencies. A satisfactory compromise has been found: a color tem-perature of 4,000 degrees Kelvin.

    COLOR VISIONT HE ANATOMICAL structures and physiological

    functions responsible for color vision are known inonly a fragmentary manner. Although many theo-

    ries have been proposed during the last century and ahalf, none of them can claim to be more than specula-

    * Original manuscript received by the IRE, May 14, 1954; re-vised manuscript received, October 12, 1954. Communication No.1657, Kodak Research Labs.

    t Research Labs., Eastman Kodak Co., Rochester 4, N. Y.

    tion.1 The following analogy is not intended to suggesta theory, or even a model of color vision.

    Color vision acts like a system consisting of threephotoelectric cells, covered with differently colored fil-ters, attached to a very wonderful interpreting device.The interpreting device is such that we appreciate thecolor as a unity and not as a composite of the threeseparate responses. The three-cell model and the three-response manner of functioning are suggested by, andare adequate to account for, the results of optical experi-ments designed to determine the visual equivalence ofphysically nonequivalent stimuli.

    These facts are well-known, and color television, aswell as color photography, is based on them. Neithercolor television nor color photography would be feasi-ble, at least in its present state, if color vision behavedas if it involved more than three kinds of receptors.Apparently less widely recognized is the fact that the

    visual system also has something like automatic gaincontrols in the three receptor-response channels.24 This

    I Opt. Soc. Am. Com. Colorimetry, "The Science of Color," T. Y.Crowell Co., New York, N. Y. (particularly ch. 3); 1953.

    2 R. M. Evans, "On some aspects of white, gray, and black,"Jour. Opt. Soc. Am., vol. 39, pp. 774-779; 1949.

    ' W. L. Brewer, "Fundamental response functions and binocularcolor matching," Jour. Opt. Soc. Am., vol. 44, pp. 207-212; 1954.

    4R. M. Evans and W. L. Brewer, 'Observer adaptation require-ments in color photography and color television," Jour. Soc. Mot.Pict. and Telev. Eng., vol. 63, pp. 5-9; 1954.

    1955 11


    is indicated qualitatively by the general experience thata white shirt appears white in tungsten light as well asin daylight. Because of the limited brightness range ofcolor television, we shall avoid reference to white ma-terials, and shall refer instead to light gray materials,5such as light gray business suits, having luminous re-flectances about 50 per cent.Gray suits do not look blue in daylight nor yellow in

    incandescent tungsten light, despite the fact that day-light appears very blue and incandescent light quite yel-low when equally intense samples of the two are com-pared side by side. The visual phenomenon responsiblefor this constancy of apparent color of reflecting mate-rials, in particular the gray suit, is called adaptation.

    QUALITIES OF ILLUMINATIONFig. 1 shows the range of variation of chromaticities of

    common light sources. The curve within the diagram isthe locus of the chromaticities produced by blackbodiesat the various temperatures indicated. Most commonlight sources have chromaticities lying very close to this



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    Fig. 1-Chromaticity diagram showing locus of chromaticities ofPlanckian radiators, lines of constant correlated color tempera-ture, and CIE standard sources A and C.

    locus. The temperature corresponding most closely toany such source is called its color temperature. The colortemperature corresponding to any chromaticity nearthe blackbody locus may be interpolated between iso-temperature lines, which cross the blackbody locus inFig. 1. Household illumination from incandescent lampsvaries in color temperature from 2,500 to about 3,000degrees Kelvin. Photoflood and photoflash lamps havecolor temperatures ranging from about 3,400 to 4,200degrees. The color temperature of direct sunlight isabout 5,000 degrees. That of overcast sky or total day-light is about 6,500 degrees, and the color temperature

    6 R. S. O'Brien, "CBS color-television staging and lighting prac-tices," Jour. Soc. Mot. Pict. and Telev. Eng., vol. 63, pp. 41-50; 1954.

    of blue sky can range from 7,000 to 20,000 degrees orhigher. The color temperature of the CIE standardilluminant C is about 6,750 degrees Kelvin.We are very rarely conscious of differences in the

    quality of illumination, particularly in the range from2,800 to 6,000 degrees. Within this range, a gray suitappears gray, regardless of the variation of the qualityof the light in which it is viewed. There is an obviousreason why, for the sake of the survival of the race, thismust be so. Men are more concerned with recognizingthings than with recognizing nuances of the color of lightin their surroundings.To produce this important effect, however, each of

    our visual color receptors must have associated with itsomething that acts like an automatic gain control,which maintains approximately constant the averageoutput of each of the three color-receptor systems.Thus, as we go out of a house, in which tungsten lampsproduce illumination relatively rich in red light, to day-light, which is relatively rich in blue, the blue receptorsor the associated nervous system must reduce theirsensitivity. Likewise, the sensitivity of the red receptorsmust somewhat increase, so as to keep approximatelyconstant the red, green, and blue responses to the aver-age quality of the illumination, and to gray objects seenin that illumination.

    All other colors are perceived relative to white or gray,and therefore our perceptions of colored objects are ap-proximately invariant, regardless of natural variationsof illumination. This visual phenomenon is called colorconstancy.

    MIMICRY OF ADAPTATIONFilms for color photography do not, in general, have

    such adaptation, and therefore we have several kinds offilms, some for use in incandescent tungsten light, andothers for use in daylight. Professional photographersand some advanced amateurs use color filters for betteradaptation of their films to illumination of variousqualities. Eastman Color Negative Safety Film, Type5248, does have sufficient latitude, in each of its layers,so that, if properly exposed, it can be used in eitherincandescent tungsten light or daylight.' However,color negatives prepared under these two conditions, orany condition in between, must be printed differently,to compensate for differing qualities of the illuminationin the several scenes. This compensation in printingoperation corresponds to visual adaptation, so that grayobjects in the


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