color and its measurements

7/29/2019 Color and Its Measurements

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J. Soc. CosmeticChemists,19, 649-667 (Sept. 16, 1968)

Color and Its Measurements*MICHAEL BORNSTEIN, Ph.D. t

Synopsis--An account is given for the fundamental principles of diffuse and tristimulus re-flectancemeasurements. The experimental basis or these measurements, ncluding the CIEand Munsell systems or defining color space, s also discussed. These principles are sup-ported with qualitative and quantitative examples for color matching, as well as interactionstudies with reflectance values and the Kubelka-Munk equation. A review of several availa-ble commercial color measuring devices, reference standards, and sample preparation tech-niques are also presented.

INTRODUCTIONThe stability and appearanceof colored cosmetic and pharma-

ceutical dosage orms are dependent on the dye or lake employed, aswell as many other parameters. These may include adjuvant absorp-tion coefficientsof microscopic and macroscopic structures as well asotherphysicaland chemicalproperties 1). Althoughmostformulatorsstill rely on empirical knowledge n the subject of color measurement,many articles have appeared with respect to color instrumentation.Swartz and Cooper (2) have reviewed a number of pharmaceuticalcolorantsand their properties. Lachman and associates ave appliedvarious parameters to color stability of tablet formulations (3-5).Goodhart, Lieberman, and associates6, 7) have also studied he stabil-ity of certified dyes in tablets. Raft (8, 9) has presented eports ofcolor measurements obtained from FD&C colorants.The purposeof this communications to discuss rinciplesand instru-ments used in diffuse and tristimulus reflectance measurements and their

application o the developmentof coloredpharmaceuticaldosage orms.* Presented October 2, 1967, Sixth Annual A. Ph. A. Industrial Pharmaceutical Techno-

logy Meeting, Chicagop Pitman-Moore Division, Dow Chemical Co., Research Center, Zionsville, Ind. 46077.Present address, Eli Lilly & Co., Indianapolis, Ind. 46206.

649


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650 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS

CONCEPTS OF COLORThe sensationof color s the result of radiant energy, between400 and700 mu, impinging he eye (10). Since he phenomenon f color ncludespsychological s well as physicalaspects, olor scalesand instrumentsare often designed o include both parameters. The psychologicalas-pectsof colorare the result of colorcodes tored n the brain, includingassociationof an individual hue such as blue-greenwith a characteristicsensation uchas coolness. Physical aspectsof color result from the factthat visible radiant energy s necessary or vision.The basic principle on which tristimulus color measurement s baseddepends n the assumptionhat mostcolorsmay be produced y a com-bination of red, blue, and green colored ights. Red and green beamscombine to form yellow, whereas blue and green combine to form abluish-green olor. A purple color results rom the combinationof redand blue; a white hue forms when the red, green, and blue primariesunite. By varying the amounts n the three primary beams, all inter-mediate colors can be produced. This phenomenonof uniting threebasic colors to form white is the basis for an additive color mixture.

Onemay also orm a subtractivecolormixture by usingyellow,magenta,and cyan pigments. The resultant color darkenswith the subtractivemixture as more of these filters combine; when all three subtractive pri-maries unite, a black color is formed. Numerous systems have beendeveloped o define color. This discussionwill be primarily concernedwith the CIE and Munsell systems.

CIE SYSTEMThere are two basic oundationson which colorimetry depends. Thefirst is that color can be matched by a suitablemixture of three selectedlight radiationsand that if two colorsare matched by three radiations,

the mixture of these two colors is found additive by suitable opticalmeans. The essential principles of this tristimulus color system werefirst independently eveloped y Ives (ll) andGuild (12). In 1931, heCommission nternationale de l'Eclairge (CIE) standardized he colormixture characteristicsof an "average observer" and developeda stand-ard framework for a color specification.This standard observer represents a series of functions determinedfrom data provided by observersmatching the color at eachwavelengthfrom 400 to 700 m, with mixtures rom three primary light sources.The experimentalset-up for defining he "average" human observermeasures he observer's esponseo the three primary colorsby focusing


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COLOR AND ITS MEASUREMENT 651

1.6

1.2

0400 500 600 700

muFigure 1. Responseof the "standard CIEobserver" to an equal energy light source

SPECTRUM COLORS

GREEN 'x 540

s

REDI BLUE [ 4o .'c' ,oo

400Figure 2. Chromaticity diagram illustratingthe visible spectrum locus measured with 10spectrophotometric readings. The tristimu-lus chromaticity coordinates (x,y) for illumi-nant C are plotted near the center of the chart

the three ights on a screenn the same ocationso hat they may be mixedin properproportions. A monochromaticight source s focused n a spotjust adjacent o the mixture of the three colored ights. The observerviews the screenthrough a cone angle of two degreesand is asked toadjust the three primary colorsuntil the mixture matches he mono-chromatic ight source, ecording he relative amountsof the three lightsnecessary or the match. Figure 1 shows hese relative amounts of red,green,and blue lights between400 and 700 mu neededby the observerfor the match. The monochromatic ight is ideally a "white light,"meaning hat it is an equal energysource ontaining ight from all visiblewavelengths. Most standard light sourcesdo contain light of everycolor, but not in equal amounts. Illuminant A, found in standard in-candescent r tungsten ight sources, as light at all wavelengths,withmuch more energyat longerwavelengths. Illuminant B is a light sourceequivalent o the noonsun. Illuminant C, representing aylight on thenorth side of a building, contains light of all wavelengths with muchmore energy n the blue part of the spectrum. Figure 2 shows he threedimensionsof the CIE color solid in two dimensionsusing x and y chro-marlcity coordinates. These chromaticity coordinates re calculatedbyexpressing he tristimulus values X, Y, Z, as fractions of their totalThese tristimulus values of a sample are either calculated from diffusereflectancemeasurements9) to be discussedater or they may be mea-


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652 JOURNAL OF THE SOCIETY OF COSMETI2 CHEMISTS

DOMINAILL0C'F{gur 3. Chromaticity diagram (x,y) definingi]]um{nant C, dominant wavelength, and purity

sureddirectlyon tristimuluseolorimeters. Only two coordinates eed obe specified ince he total of the x, y, and z chromaticitycoordinatesequals nity. Here the coloraxisor chromaticity oordinatesie on theoutsideof the triangle going rom blue to green o red, whereas he whiteto black axis or luminosity function is the center; this brightness xiswouldbe perpendicularo the screenwith the white at the top andblackat the bottom. Nonspectrum colors represent mixtures having thechromaticitiesepresented t any point along he straight ine oining heextremitiesof the spectrum ocusand are producedby mixing suitableportionsfradiant ngeryakenrom heextremehort-wavelesshan440 m/) and long-wave greater han 680 m/) regionsof the spectrum.Colors epresented y points on straight lines between he achromaticpointC n Fig. 2 and he spectrumocus recalled pectral olors.The CIE systemdivides he characteristicsf light from whichcoloris composednto severalcomponentsncluding uminance,dominantwavdength, and purity. Luminance s the characteristichat differ-entiates the light reflected rom a standard white sample, lluminatedwith a 100-W lamp, from that of light reflected rom this samewhitesamplewhen it is illuminatedby a 200-W lamp with all other thingsbeing equal. The dominantwavelength ay be definedas the wave-length hat appearso bedominantn the ight and s usually hemost n-tensevariety of radiant energy n the stimulus. Purity refers o the de-gree o which he dominantwavelength ppearso predominaten thelight.Figure3 showshat the dominantwavelengths obtained y plottingthe chromaticitycoordinates f the illuminant and sample. A line is


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White

Black

8 9 IO1

23

5

.10

FigureNeutral Saturatedcolour Chroma colour

Hue, value, and chroma coordinates of the Munsell system

drawn from the achromatic or illuminant point through the sample co-ordinates onto the locus of the diagram. This point on the locus iscalled the dominant wavelength. The per cent purity is determinedbythe ratio of the distance from the illuminant point to the sample coordi-nates, A, divided by the distance rom the illuminant point to the spec-trum locus, B, expressedn per cent. Luminosity is expressedn percent Y, obtained directly from tristimulus colorimeters. This diagramin which each point represents he chromaticity, independentof lumi-nance,sm,alled chromaticityiagram.

MUNSELL SYSTEMAnother system for defining color space,called the Munsell system(13), is basedon a color solid llustrated in Fig. 4. The central verticalaxis represents he locusof neutral colorswith white at the top and blackat the bottom. In this system, lightnessof the sample, called value, isdivided into a number of steps rom 0 to 10. The distanceof the sample

from the central vertical axis is a function of the saturation or intensity


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of color and is called chrornaon the Munsell map. Chroma is alsodivided into a seriesof steps, with the neutral axis or gray being 0,whereasa fully saturatedsamplehas a chromaof 12. Color, or hueas tis called in the Munsell system (14), is presentedby different planesaround the vertical axis of the Munsell solid. In this system, he wholesolid s divided into ten equal vertical segmentswith five principal hues(red, yellow, green, blue, and purple) occupying he central planesofalternate segmentswhile intermediate hues (yellow-red, green-yellow,blue-green,purple-blue and red-purple) occupy the remaining planes.Each hue segment s further divided into ten sectionsnumbered 1 to 10,with the main hue segmentalways numbering5.

The hue of a sample s designatedby a number, indicating the sectionof the segment, ollowedby symbolswhich show he colorof the segmentinvolved. For example, 10P indicates he hue section10 of the purplesegment. To complete he specification f the sample, he value quan-tity follows the hue which is then followedby a stroke and then thechroma. Here the colordefinedat 10 P / has he hue 10 P, the value5,and the chroma of 8.

COLOR DIFFERENCE AND TOLERANCE

It is of interest to point out that color differencesare not entirelylinear throughout the entire CIE chromaticity diagram since, f one cal-culates two points on this diagram, another set of points twice as farapart do not necessarily ave a totally linear difference n color (9, 15).In other words, equal distanceson the diagram do not always indicatethe samedegreeof color change.Judd (16) devised he Uniform ChromaticityScale UCS) adoptedbythe National Bureau of Standards NBS). Adams (17) followedwith hischromatic-valuediagram which was acceptedby the American Societyfor Testing Materials (ASTM). MacAdam (18) investigatedcolordif-ferenceswhere the observermade a large number of matches on a seriesof test colors ocated at various points in the chromaticity diagram.The spread n the colorsettingsnecessaryo make a match (Fig. 5) wasused by MacAdam as the criterion of sensitivity to color differences.The distances rom points on the boundary of each ellipse o the centralpoint within the ellipseapproximatelycorrespondo the chromaticitydifference ust perceptiblewith certainty under specified iewing condi-tions; this just perceptible color difference s defined as one MacAdamunit. The orientation of the major axis of individual ellipses s a func-tion of the dominant wavelength and purity changes, but does not


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Y 1.s2. 5300 'X 4

o.4-499 oo

Fgure5. MacAdam's perceptible hromaticitydifferencesn the CIE diagram.The axis of each ellipse has been multiplied by 10 for illustrative purposes

SOURCE

MONOCHROMATOR

PHOTO-SENSOMPLE

Figure 6. Schematicdiagramrepresenting he major components f a spectrophotometer rcolorimeter. Spectrophotometers use prismatic or grating monochrometers where colori-meters would contain filters

changewith lightness; i.e., the shape and orientation remain constantat different uminosities. The ellipses o vary in size with a changeof lightness,becomingsmaller as lightness ncreases,since errors inmakingcolordecisionsecrease nder mproved ight-viewing onditions.COLOR MEASURING INSTRUMENTS

Reflectance pectrophotometersThis type of instrument,diagrammed n Fig. 6, is basedon a beam ofmonochromaticight penetratingnto a sample. This light is scatteredin many directions,s partially absorbed, nd finally re-emergeso the


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400 700 400 700SAMPLE REFLECTANCE (R) SOURCEEMISSION E)

RxE

SPECIFYING THE COLOR OF A SAMPLE

Figure 7. Schematic diagram for interactionfactors to produce color visionsurface. Measurementsmay be plotted as reflectance s. wavelengthusing a relatively nonabsorbingwhite referencestandard. This infor-mation is then used or quantitative Kubelka-Munk equationsor it maybe converted to tristimulus values which will be discussed.

Commercial instruments include the Hardy General Electric Spec-trophotometer,Bausch& Lomb Spectronic505 with reflectanceattach-ment, as well as the Beckman DU Spectrophotometerwith the reflect-ance accessory. The G. E. instrument has a recorderand automaticallyconverts reflectance data to tristimulus values. The Bausch & Lombinstrument has a recorder whereas the Beckman DU is a manual spec-trophotometer.

Tristimulus InstrumentsIt should be pointed out that the CIE system was developed oeliminate eye variationsby using he "standard observer." The princi-ple behind tristimulus colorimeters, llustrated in Fig. 7, is as follows:A light source,E, strikesa samplehaving a reflectance urve,R. Thisinteraction results n a reflectanceenergy, RE, which is specific or eachwavelength; this is also the energy striking the eye. This resulting REpasseshe filters and is multiplied by eachof the hypotheticalcolormix-ture curves,9, and , representing olorvisionof the standardobserverin the CIE system. This produces hree new curves,each having anarea representedby the tristimulus values X, Y, and Z. The abovemanipulationsagain indicate that the CIE systemspecifies color bythree quantities,X, Y, and Z, called tristimulusvalues. These values


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represent the amounts of primary red, green, and blue color that thestandard observer would need to get a match. Ii each of these tri-stimulusvalues s divided by the sum of three, the resulting values x, y,and z, called chromaticity coordinates,give the proportion of the totalstimulus attributed to each primary color. Furthermore, since thesum of the three chromaticity coordinatess unity, the values of x and 3'plotted on the chromaticity diagram may be used alone to specify thecolor. The third achromatic dimension of lightness or darkness isspecifiedby the tristimulus value. The manipulations ncluded intristimulus colorimetry are mathematically presentedbelow:Weighted Ordinate Calculation ofMethod Chromaticity Coordinates

700 XX = ExC'RxX x =oo X + Y + Z700 yY = 'Ex 'RX':?x y =oo X + Y + Z700 aZ = Ex 'RX.zX z =oo X + Y + Z

z = I-- (x-t-y)The Hardy General Electric Spectrophotometer, reviously dis-cussed,provides spectral reflectancecurves in addition to tristimulusvalues. This combination results in nonmetameric matches in color

formulationswhich are colorssimilar in appearance nder all lightsources. Tristimulus instruments alone often produce metamericmatcheswhich ook the sameonly f similarviewingconditions re used.Instrument Development Laboratories (IDL) market the Color-Eyewhich s an abridged pectrophotometeriving10 or 16 visiblewave-length points as well as tristimulus values. The instrument is also de-signedo measureluorescencef a sample ince he tristimulus iltersareplacedbetween he sample nd detector s llustrated n Fig. 8. A com-puter may alsobe purchasedwith the instrument o directly measurecolor difference in MacAdam units.

A competitive tristimulus colorimeter called the Hunterlab D-25Color DifferenceMeter (19),* schematicallyresentedn Fig. 9, in-cludesvacuum phototubes,calibrated ristimulus ilters, and a transis-* HunterAssociatesaboratory,nc., 9529LeeHighway,Fairfax,Va. 22080.


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SPECULARNSERT'-----.._GHTOURCEREFLECTANCE----,,,f --ltSAMPLE /< ....

TNSSSION

ZEROING SLIT

SPLITENS 'ROTATINGFLICKER"MmmO , , ,,,, ,

STOP t

PHOTOMULTIPLIERI

REFLECTANCESTANDARD

DIRECT LICHTSTOPS

STRAY LIGHTSTOPS

TPNSMISSIONSTANDARD

- MICROMETRIC SLIT(VERTICAL ApERATURE)

. TRISTIMULUS andABRIDGED

SPE CTROPHOTOME TERFILTERS

Figure 8. Block diagram of the I.D.L. Color-Eye

torized color measurementcircuit. The readout s presented n X, , Ztristimulus values as well as L, a, b scales 20) which closelycorrelatewith the Munsell system. The L measuresightnessor value, whereasthe a and b readings represent chromaticity coordinates. A computermay be purchased with the instrument to give direct color differencemeasurements in NBS units. The Hunterlab Color Difference Meterdoesnot offer the 10 or 16 wavelengthspectrophotometric oints offeredin the Color-Eye, which are useful where nonmetamericmatchesare re-quired. The Hunter colorimeter may, however, be used in combina-tion with a Beckman DU or other diffuse eflectancespectrophotometerif nonmetameric matches are required and may be applied to routinecontrol of pharmaceuticalcolor formulations.Another commercial instrument, called the Davidson and Hemmend-inger Colorant Mixture Computer (COMIC), enables he colorist tomeasure eflected ight from a sampleof variouswavelengths,usingK/S


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COLOR AND ITS MEASUREMENT (359

Figure 9.

COLOR-MEASUREMENTCIRCUIT

..

a -bL

L _ TRISTIMUUSFILTEIS

CLEARMETHACRYLATE

LITE PIPE

Block diagram of the Hunterlab D-25 Color Difference Meter

values o make a nonmetamericmatch (21). The techniquesnvolved operform thesecalculationswill be discussed.REFERENCE STANDARDS

Sincemost tristimulus data are a comparison f luminosity and chro-maticity, reference standardsmust be used as a means of comparison.An ideal reflectance eferencestandard would diffuselyreflect 100% ofthe visible light impinged on its surfaceat all wavelengths. Magnesiumoxide,magnesium arbonate,barium sulfate,and white vitrolite have ap-plication as reflectance standards. A disadvantage with MgO,MgCOa, and BaSO4 s that they are brittle and yellow with age. Theyellowing process s slowestwith BaSO4. Their advantage, however, isthat these standardshave a diffuse reflectanceof about 97-99% in thevisible wavelengths,makinx reflectance eading almost absolute.


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White Vitrolite, used as an NBS referencestandard, s an opal glassmaterial with a fire-polishedsurface. The absolute eflectance s 90%over most of the visible spectrum, requiring CIE correction factors ateach wavelength. Vitrolite is a permanent standard, is easily cleanedand excellent for routine colorimetric work.

SAMPLEPREPARATIONTECHNIQUESPowders

Sample powders are presented o most tristimulus colorimeters n amanner similar to reference standards. A dish with a 4 in. diameter and/-in. depth s sufficient sa sample older. The powders oftenpackedand smoothedwith a spatula; a glasscoverslipmay be placedover thesample,provided the referencestandardhas a similar cover.

TabletsSince the 2- to 4-in. sample viewing area found in most tristimuluscolorimeters s greater than most tablets, it is suggested hat sampleholdersof in. or smaller be made, and the tablets measured ndividu-ally wherehigh precision s required. The light beam of the instrumentwould have to be adjusted here in order to decrease he beam diameter

hitting the individual tablet.Suspensions

The reflectanceof opaquesuspensions ay be measured rom a plas-tic cylinder with a glassbottom. Caution shouldbe exercised o be surethat the precipitate s well suspended,esulting n uniform colorreadings.Clear Solutions

The color of clear solutions is best measured with transmittance tri-stimuluscolorimetrysince eflectancemeasurements ften lead to exces-sive light scattering. If reflectancemeasurement s required, one mayplace a standardizedwhite ceramicbackground n the sample cell; theclear solution transmits the light onto the background which in turn re-flects this light to the photodetector. The thicknessof the solutionmustbe controlled by adjusting the volume of sample used; a 5-ml glasssampleholder is feasible or darker or highly coloredsolutionswhereasavolume of 10 ml may be used or nearly clear and colorless olutions. Itshould be noted that the light beams travel twice the distance with thistechnique,making the reading approximatelytwice the stimulusvaluesobtained with normal reflectance echniques.


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QUANTITATIVE OMPUTATION FCOLORANDCOLOR ORMULATIONKubelka-Munk Method

Sincea number of different dye combinationsare often possiblewhenmatching a given standard and the use of many FD&C dyes s becominglimited, formulators must often resort to color combinations or matchingpurposes. However, many matches are metameric and therefore requirespectrophotometric s well as tristimulus analysis. Spectrophotometricmatches, used in combination with tristimulus values of a mixture, tendto retain the characteristic eatures of each colorant. An existingprob-lem with spectrophotometriceflectancecurves s that these curves donot directly follow Beer's law since concentration s not directly linearwith reflectance. Kubelka (22) developednumerous ormulas or cor-relating reflectancewith concentrationby making scatteringand surfacedifference orrections. It was found that the ratio of light absorption olight scattering at a given wavelength is proportional to the concentra-tion of the dye in the sample. The relationship shownhere is derivedfrom the Kubelka-Munk equation.( - ) ()

Where R = 1.0 at 100% reflectance.Relationshipof K/S to concentration

K/$ = kc (2)Where

K = light absorbedS = light scatteredk = constantof proportionalityC = concentration of colorantSinceK/S factors or each dye at a particular wavelengthare addi-tive when mixed together, this principle may be usedas a basis or com-puting the amountsof various dyes necessaryo match a given standard.A sampleequation or findingK/S of a colorantmixture s:

(K/$),x. = a(K/S)A + b(K/$) + c(r/$)c + ...w(K/X)whe se (3)Where

a = concentration of colorant Ab = concentration of colorant Bc = concentration of colorant Cw = concentration of white base


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This equationwould be usedas follows: The K/S for each dye and ex-cipient are first obtained by measuring he diffuse eflectance or a knownconcentration of the dye and excipient or white base at a given wave-length. This R value may be substituted in the Kubelka-Munk equa-tion (eq. 1), in order to calculate he K/S values, or these may be ob-tained directly from tableswhich giveK/S values or reflectance eadingbetween0 and 100%. The K/S value of the diluent shouldbe subtractedfrom each K/S of the dye to obtain the correctedK/S values. Forexample,one can calculate he K/S valuesof three dyes, blue, red, andyellow,separatelyat onewavelength. The total K/S valuesat that onewavelengthwill equal the K/S value of a brown, which results rom theproper concentrations f the above three dyes. Therefore, if thevalue of the sample s knownat three wavelengths nd the K/S valuesofthe blue, red, and yellow (at any concentration) are also known at thesamewavelength, one may set up three simultaneous quations o calcu-late the actual concentrationsof blue, red, and yellow used to make thematch. The Davidson and Hemmendinger COMIC Computer per-forms these calculationsat 16 wavelengthsbetween 400 and 700 m/ andalsocalculates ristimulusvalues or the purposeof colormatching (21).

WeightedOrdinate MethodThis method, mathematically illustrated earlier in this presentation,is a meansof calculating ristimulus valuesand chromaticity coordinatesfrom reflectance data. It should also be stressed that x, y, and aretristimulus values resulting from the averagevisual observerand are usedas imaginary primaries under specific llumination for tristimulus cal-culations. The values X, Y, and Z, at any wavelength, correspond othe magnitude of these primaries needed by the standard observer tomatch a color. In addition, chromaticity coordinates, x, y, and z, arethe tristimulus values X, Y, and Z expressed s fractionsof their total,which equal one. These chromaticity coordinates of the tristimulusvalues are then plotted on an x, y chromaticity diagram and the spec-trum locus is located. The weighted ordinate method is a means ofdetermining tristimulus values and sample ightnesseither from spectro-photometric reflectancecurvesor directly using tristimulus colorimetry.Using this method, the relative energy of the light sourceE c (for illumi-nant C) at a particular wavelength, which is found in published colortables (23), is multiplied by the tristimulusvalue c and reflectance aluefor the samewavelengthbetween400 and 700 m/, usually 10 m/ apart.The values are then summedover the total range to yield the X, Y, and


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COLOR AND ITS MEASUREMENT 66;3

100 '

80-

60-

I I I

400 500 540 600 700muFigure 10. Diffuse reflectancespectra of FD&C Red #3 Dye (100 mg) and light magnesiumcarbonate, USP (2.00 g). Key: A, standard dye spectrum: B, new sample lot spectrum

Z tristimulus values. Chromaticity coordinates re then calculatedbyexpressinghe tristimulus values as fractions of their total values. Theluminosity or lightness is determined directly from the Y tristimulusvalue expressedn per cent.Although there are other available methods for calculating tristimu-lus values (23), the weightedordinate techniqueservesas a goodmeansfor deriving color values from diffuse reflectancemeasurements.

IHARMACEUTICAL APPLICATIONQuality Controlof FDiC Dyes

The purity and quality of pharmaceutical dyes may be controlled, asillustrated with the following example: A manufacturer purchasesanew ot of FD&C Red#3 dye. The predeterminedtandard pectrum fthis dye compared to a MgCO3 reference standard is shown in Fig. 10The spectrum for the standard dye is also diluted in a 1:20 ratio withMgCO3, since t is important that the color componentbe highly dilutedwith a relatively nonabsorbingdiluent. This ensures similar grain sizefor the dye and reference standard, causing the reference and samplescatteringcoefficients o canceleach other in comparisonmeasurements.This facilitatesscattering o be independentof wavelength 24).


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The specificationsor 100 mg of standard dye diluted with 2.00 g ofMgCO3, (Fig. 10,A) are as follows' Xmx 540 mu; R = 62.8% (K/S= 0.1102); Tolerance = - 10%. A new lot of FD&C dye (Fig. 10,B)gives he followingspectrophotometric alues after 100 mg of the dye hasbeen diluted with 2.00 g of MgCO3 USP' X .... = 540 m; R = 65.5%(K/S = 0.0909).Since K/S values are proportional to concentration, he purity of thenew lot may be found from the calculationsshown below, assigningapurity of 100% to the standarddye.

K/Ssta. Csta._ (4)K/Xsamrle CsampleWhere

K = absorptioncoefficientS = scatteringcoefficientC = dye concentrationn %0.02 oo%= --; Csampe 81.6%0.0909 CsampleThe new sample ot shouldbe rejected,since t contains18.4c im-purities. It should be emphasized that the colorist must check the

linearity of K/S valueswith concentration,and the experiments houldpreferably be performedwithin a closeconcentration ange n order to en-surebetter accuracy 24).Tristimulus colorimeters may also be used to control the color andlightness of pharmaceutical dyes. This is done by plotting x and ychromaticity values for a known standard dye and establishingpermis-sible color differences or new lots. The sample lightnessmay be con-trolled by comparing the sample's Y tristimulus value to that set forthe standard dye. This is a rapid and easyquality control technique ornew sample ots of FD&C dyes.

Dye-A djuvantChemisorption tudiesSeveral articles have appeared n the literature with respect o colorstability and other changes esulting from the interaction of dyes withpharmaceuticaladjuvants (25-27). Results of a dye-adjuvant chemi-sorptionstudy (25) usingdiffuse eflectance echniques re presented nFigs. 11 and 12. The former represents the results of equilibratingFD&C Red #3 dye with starchUSP in an aqueous ispersion edium,

followedby lyophylization 25).


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COLOR AND ITS MEASUREMENT 005lOO

A

A

250 I 00 350 4.00 5JO 600Figure11. Diffusereflectance pectraof FD&C Red 3 Dye (30 mg) and starch, USP (10.00g). Key: A, control; B, sample; C, sampleexposed o 184-hourartificiallight at 2000 f.c.

lOO

2hO 2[0 300 3150 4)0 50 60 9JoFigure 12. Diffuse reflectance spectra of FD&C Red #3 Dye (30 mg) and lactose, USP(10.00 g). Key: A, control; B, sample; C, sample exposed o 184-hour artificial light at

2000 f.c.

In Fig. 11, ,4 is the spectrophotometriceflectancecurve of the con-trol, consisting f a triturated dry physicalmixture of the dye and adju-rant. Curve B shows his spectrumof the samedye-starchcombinationplaced n a suitable container, with 20 ml of distilled water added. Thissample s then equilibrated for 24 hours at 30 C and the material is driedby lyophilization. Curve C resulted when the equilibrated and driedsample s exposed o 184 hours of artificial light at 2000 f.c. using anEnvira-Lite Cabinet.*

Figure 12, ,4, B, and C representa FD&C Red #3-lactosecontrol,sample,and light-exposed ample, espectively. A comparison f Figs.11 and 12 indicates hat both systemsundergo ntensespectrophoto-metric changes uring equilibration,especiallyat the Xmx f 540 m. Itis of interest to note that the dye-starch nteraction is far weaker than thedye-lactosecomplexsince there is about 78% fading observedat the* Thermal Research, Inc., Thermal Road, Iselin, N.J.


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X ... in the starchsampleas compared o about 4% fading n the lactosesample 25).These studies and others previously conducted on drug-adjuvantinteractions 28) point out the need or dye incompatibility ests n con-junction with other considerationsn the formulation of pharmaceuti-cals.

CONCLUSIONThe fundamental measurement prindples indicate that the use ofinstruments for the control of color will continue to be of great benefit inthe cosmeticand pharmaceutical ndustry. Although future problems

will not be totally solvedwith the aid of instrumentation, the amount ofinformation gainedfrom thesetechniquesmore than justifiesthe useofthese methods for the control of color.(Received December 18, 1967)


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(16)

(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)

Judd, D. B., A maxwell triangle yielding uniform ehromatieity scales, J. Opt. Soc. Am.,25, 24 (1935).Adams, E. Q., X-Z planes on the 1931 I.C.I. system of colorimetry, Ibid., 32,168 (1942).MacAdam, D. L., Visual sensitivities to color differences in daylight, Ibid., 32, 247(1942).Hunter, R. S., Photoelectric color difference meter, Ibid., 48, 985 (1958).Judd, D. B., and Wyszeeki, G., Color n BusinessScienceand Industry, 2nd Ed., JohnWiley and Sons, Inc., New York, N.Y., 1963.Stanziola, R., Practical colour instrumentation for the dyer, Can. Textile J., 1-5 (Sep-tember 17, 1965).Kubdka, P., New contributions to the optics of intensely light scattering materials,J. Opt. Soc. Am., 38, 448, 1067 (1948).Hardy, A. C., Handbook of Colorimetry,The M.I.T. Press, Cambridge, Mass., 1936.Lieu, V. T., and Frodyma, M. M., Selection of the optimum concentration range forreflectance spectrophotometric analysis, Talanta, 13, 1319 (1966).Bornstein, M., Walsh, J. P. Munden, B. J. and Laeh, J. L., Diffuse reflectance studiesof dye-adjuvant ehemisorption, J. Pharm. Sci., 56, 1410 (1967).Laehman, L., Kuramoto, R., and Cooper, J., A study of the interaction between qua-ternary ammonium compounds and several certified dyes, Ibid., 47, 871 (1958).Scott, M. W., Goudie, A. J., and Huetteman, A. J., Accelerated color loss of certifieddyes in the presenceof nonionic surfaetants, Ibid., 49, 467 (1960).Bornstein, M., and Lach, J. L., Diffuse reflectance studies of solid-solid interactions II,Ibid., 55, 1033 (1966).

color and its measurements

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