Bulletin of the Psychonomic Society19 75, Vol. 5 (4),325-326

A simple additive colormixerfor exploration ofthe color solid

ROBERT FRIEDHunter Collegeof the City University ofNew York

New York, New York 10021

A simple additive color mixer, requiring only three degrees of mechanical translation and permittingmanipulation of hue, saturation, and intensity of three primary light sources, is described.

A SIMPLE ADDITNE COLOR MIXER FOREXPLORATION OFTHE COWR SOLID

Color mixers are used for classroom demonstration ofthe laws of chromatic addition. They are also useful formeasurement of the chroma of an object of unknownhue : the color mixer is manipulated to obtain a chromamatching that of the unknown. The setting on the mixerthen provides a measure of the location of the unknownchroma on the color surface .

Such devices also have research applications. Theymake possible controlled variation over a continuousrange of hues when chromatic stimuli are used forexperimental purposes. Devices of this type are moreuseful for all these purposes when they are also capableof intensity variation : then the color solid may beexplored in all three dimensions.

The author recently had need of a color mixing deviceto be used to investigate subjects' spontaneous variation ofcolors in the color solid. It was also desirable that thecontrol manipulandum be so designed that it operatedisomorphically with the subjective color solid. For thispurpose , the device illustrated in Figure 1 was designedand constructed.

The manipulandum has three degrees of freedom.Two degrees of translation represent hue and saturation .The third, rotation of the knob on the manipulandum,represents intensity . The equivalence of all huedirections in the color surface is well represented by itsspecific manipulation by rotation. Thus, operation iscontinuous but not indefinitely extended.

This device does not provide continuous precisioncolor mixing. But it does clearly demonstrate how colorsadd. It is also not possible to demonstrate the full rangeof hues; one must stay within the triangular systemwhose apexes are the colors of each of the three sourcesof light. However, this may also have training value sinceadroit selection of hues for the primary sources providesan optimal set of mixture , i.e., one that seem to fill wellthe color solid. Such a selection may be used to define aset of psychological primaries.

CONSTRUCTION

The apparatus consists of a set of three cellulosefilters (labeled A, B and C in Figure I). These aremounted in the pattern of an equilateral triangle . Thefllters, red, blue and yellow (Edmund ScientificCompany, 806, 819, 859) are fixed in place in lensmounts. Behind the plate containing the filters there is alamp carriage (labeled L), which is held in place by thetension of the three springs shown in the schematic. Thelamp carriage is completely sealed except for a circularaperture covered by a milkglass filter .

While the dimensions of the filters and lamp apertureare not critical, it should be noted that the light sourceshould have a diameter such that when its center fallsexactly at the center of the equilateral triangle , whoseapexes are the filters, the light source should overlapapproximately 50% of the area of each filter.

The cont rol knob , K, at the end of the rod , isconnected by a series of links to the lamp carriage. Thecontrol rod permits rotation through 360 deg of arc bymeans of the universal joint, D. Moving the rod throughits maximum vertical displacement also moves the lamp

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.Figure 1. Lens arrangement and mechanical linkage of colormixer,

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326 FRIED

carriage, in a like fashion through its maximum verticaltravel : at the top of the travel, the hue determined bythe filter , B, will be projected on the milkglass screen, S,in the front panel of the apparatus housing. And, at thebottom of its travel, the projected hues will be thoseresulting from the combination of the lights projectedthrough Filters A and C.

Moving the rod through its maximum horizontaldisplacement produces a combination of hues projectedthrough Filters A and B at the extreme right. However,horizontal and vertical movement may be accomplishedsimultaneously . Thus , Hue A may be obtained byleft-down displacement and Hue C by right-downdisplacement of the control knob .

When the control rod is released, both the rod and thelamp carriage return to the center position and theresulting hue combination produces the projection of anoff-white on the screen. Pure white can never be

obtained for several reasons: the lamp color is not purewhite, and the combination of all hues in athree-primaries system invariably produces a numbe r ofcomplimentary components. But, the off-white is not ,subjectively , a bad match.

The potentiometer mounted in the control knob , K, ismounted to the rod by its operating shaft. Thus, theshaft is fixed in position and rotation of the knob resultsin rotation of the potentiometer about the shaft , varyingin tum, the intensity of the lamp (BRS, 4-V ac driven by6.3-Vac filament transformer) .

Movement of the control rod to any position, thecombination of a vertical and horizontal distances fromthe center, results in a hue combination dependent uponthe ftlters overlapped by the source of light.

(Received for publication December 23,1974.)