color matching in dentistry. part i. the three-dimensional nature of color

5
needed in the dental shade guides and to manufacture the materials used with these guides. However, the manipulation of the guides and materials at the chair side is a perceptual challenge to the ability of the den- tist to see and to recognize the three dimensions of color. The names of these dimensions may vary, but not the dimensions themselves. Colors of objects (apart from spatial and temporal aspects) will vary, but in no way that cannot be described in terms of combi- nations of these three dimensions. 13 As early as 1611, color was presented as a three- dimensional entity by Sigried Forsius. 14 Since that time, there have been numerous systems devised and numerous approaches used to explain this tridimen- sional property. 15-19 The three-dimensional nature of form—length, width, and height—is a familiar con- cept; the three-dimensional nature of color (color being the abstract property that it is) is not as readily understood. The purpose of this article (Part I) is to explore the three-dimensional nature of color and the correct ter- minology. Part II will investigate the practical application of this concept to color-matching problems in dentistry. MUNSELL COLOR ORDER SYSTEM Many color order systems are available, but for a variety of reasons, including worldwide recognition, consistency, flexibility, and simplicity, the Munsell Color Order System is the system of choice for color matching in dentistry. The attempt to achieve equal visual (perceptual) spacing in this system further rec- ommends it. The color tree (Plate I) is a representation of the tridimensional organization of the colors within the Munsell system. The Munsell color solid can be likened to a sphere or to a cylinder, 20,21 as it is an irregular three-dimensional figure that has characteris- tics of both. Although usually described as a sphere (Munsell’s original concept), for the purpose of this article, it will be treated as a cylinder (Fig. 1). The rela- tionship of one color to another becomes apparent when the organization of the colors within the three- dimensional solid is understood. A colorless or achromatic axis extends through the center of the cylinder, pure white at the top, pure black at the bot- tom. A series of grays, progressing from black to white in equal visual steps, connects these extremities (Plate II, A, and Fig. 1). Colors (Hues) are arranged around this axis, and within each Hue, the colors are arranged Color matching in dentistry. Part I. The three-dimensional nature of color Robert C. Sproull, DDS* William Beaumont General Hospital, El Paso, Texas Forty years ago, Clark 1 said, “Color, like form, has three dimensions, but they are not in general use. Many of us have not been taught their names, nor the scales of their measurement. In other words, we as dentists are not educationally equipped to approach a color problem.” This statement is unfortunately still valid. 2 Today’s approach to a color-matching problem is somewhat different from what it was then. Research and education have made possible a planned approach and a better understanding of the problems involved. Professional color matchers today have a greater appre- ciation of the role of the human observer, the differences in light sources, the analysis of surfaces, the effect of the surrounding color—adjacent or back- ground—and the many other aspects of color matching that enter into the final evaluation by the brain. Nevertheless, the basic truth of Clark’s statement has been strengthened, not weakened, by the passage of time. Color is complex and encompasses both subjective and objective phenomena, but there are laws of color just as there are other laws in nature. 3 For a good anal- ogy concerning some of the problems of color, we can consider the entity known as heat. “Heat” is objective and can be measured, but “hot” and “cold” are sub- jective attributes and cannot be objectively measured. 4 Our perception of color is accepted as subjective, 5-9 and problems in its measurement can be anticipated. In an effort to translate from the physical facts of color, such as measurement of reflectance as a function of wavelength, to the psychologic (i.e., perceptual) facts of color, the science of color measurement (colorimetry) has established an international psychophysical method of color specification** which includes a “standard observer” and standardized light sources. 10-12 This approach has supplied an operating base for our attempts to measure objectively this subjective phe- nomenon. Color matching in dentistry must depend upon col- orimetry in order to correctly specify the colors NOVEMBER 2001 THE JOURNAL OF PROSTHETIC DENTISTRY 453 Read before the American College of Prosthodontists in Las Vegas. *Chief, Fixed Prosthodontic Service **N. W. Hale, Munsell Color Company, Kollmorgen Corp.: Personal Communication, May 30, 1972. Reprinted from J Prosthet Dent 1973;29:416-24. doi:10.1067/mpr.2001.119827 CLASSICAL ARTICLE

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Page 1: Color matching in dentistry. Part I. The three-dimensional nature of color

needed in the dental shade guides and to manufacturethe materials used with these guides. However, themanipulation of the guides and materials at the chairside is a perceptual challenge to the ability of the den-tist to see and to recognize the three dimensions ofcolor. The names of these dimensions may vary, butnot the dimensions themselves. Colors of objects(apart from spatial and temporal aspects) will vary, butin no way that cannot be described in terms of combi-nations of these three dimensions.13

As early as 1611, color was presented as a three-dimensional entity by Sigried Forsius.14 Since thattime, there have been numerous systems devised andnumerous approaches used to explain this tridimen-sional property.15-19 The three-dimensional nature ofform—length, width, and height—is a familiar con-cept; the three-dimensional nature of color (colorbeing the abstract property that it is) is not as readilyunderstood.

The purpose of this article (Part I) is to explore thethree-dimensional nature of color and the correct ter-minology. Part II will investigate the practical applicationof this concept to color-matching problems in dentistry.

MUNSELL COLOR ORDER SYSTEM

Many color order systems are available, but for avariety of reasons, including worldwide recognition,consistency, flexibility, and simplicity, the MunsellColor Order System is the system of choice for colormatching in dentistry. The attempt to achieve equalvisual (perceptual) spacing in this system further rec-ommends it. The color tree (Plate I) is a representationof the tridimensional organization of the colors withinthe Munsell system. The Munsell color solid can belikened to a sphere or to a cylinder,20,21 as it is anirregular three-dimensional figure that has characteris-tics of both. Although usually described as a sphere(Munsell’s original concept), for the purpose of thisarticle, it will be treated as a cylinder (Fig. 1). The rela-tionship of one color to another becomes apparentwhen the organization of the colors within the three-dimensional solid is understood. A colorless orachromatic axis extends through the center of thecylinder, pure white at the top, pure black at the bot-tom. A series of grays, progressing from black to whitein equal visual steps, connects these extremities (PlateII, A, and Fig. 1). Colors (Hues) are arranged aroundthis axis, and within each Hue, the colors are arranged

Color matching in dentistry. Part I. The three-dimensional nature of color

Robert C. Sproull, DDS*William Beaumont General Hospital, El Paso, Texas

Forty years ago, Clark1 said, “Color, like form, hasthree dimensions, but they are not in general use.Many of us have not been taught their names, nor thescales of their measurement. In other words, we asdentists are not educationally equipped to approach acolor problem.” This statement is unfortunately stillvalid.2

Today’s approach to a color-matching problem issomewhat different from what it was then. Researchand education have made possible a planned approachand a better understanding of the problems involved.Professional color matchers today have a greater appre-ciation of the role of the human observer, thedifferences in light sources, the analysis of surfaces, theeffect of the surrounding color—adjacent or back-ground—and the many other aspects of color matchingthat enter into the final evaluation by the brain.Nevertheless, the basic truth of Clark’s statement hasbeen strengthened, not weakened, by the passage oftime.

Color is complex and encompasses both subjectiveand objective phenomena, but there are laws of colorjust as there are other laws in nature.3 For a good anal-ogy concerning some of the problems of color, we canconsider the entity known as heat. “Heat” is objectiveand can be measured, but “hot” and “cold” are sub-jective attributes and cannot be objectively measured.4Our perception of color is accepted as subjective,5-9

and problems in its measurement can be anticipated. Inan effort to translate from the physical facts of color,such as measurement of reflectance as a function ofwavelength, to the psychologic (i.e., perceptual) facts ofcolor, the science of color measurement (colorimetry)has established an international psychophysical methodof color specification** which includes a “standardobserver” and standardized light sources.10-12 Thisapproach has supplied an operating base for ourattempts to measure objectively this subjective phe-nomenon.

Color matching in dentistry must depend upon col-orimetry in order to correctly specify the colors

NOVEMBER 2001 THE JOURNAL OF PROSTHETIC DENTISTRY 453

Read before the American College of Prosthodontists in Las Vegas.*Chief, Fixed Prosthodontic Service**N. W. Hale, Munsell Color Company, Kollmorgen Corp.: Personal

Communication, May 30, 1972.Reprinted from J Prosthet Dent 1973;29:416-24.doi:10.1067/mpr.2001.119827

CLASSICAL ARTICLE

Page 2: Color matching in dentistry. Part I. The three-dimensional nature of color

in scales according to their lightness/darkness (Value)and their purity or strength (Chroma). (In the MunsellSystem, Hue, Value, and Chroma are capitalized.)Light colors are toward the top of the cylinder; darkcolors are toward the bottom. The colors are purest onthe outer skin of the cylinder, and they become pro-gressively grayer as they approach the gray Value axis.Within each of these scales of Hue, Value and Chroma,the intervals were chosen to represent equal visualspacing under a standard light source.

The cylinder may be considered as a series of wheelsstacked one upon the other, each wheel of ascendinglightness as we progress to the top of the cylinder (Fig.1). The hub of each wheel represents the Value axis.The Hues are arranged sequentially around the rim.

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The spokes represent the gradations of Chroma fromthe colorless axis to the purest Hues at the rim. Inactuality, as can be seen by examining Plate I, the Huesproject unevenly beyond the surface of the solid, butthis is one of the advantages of the system. As tech-nology permits the creation of purer colors, they caneasily be added to the periphery. These unequal exten-sions of the Chroma spokes obviously make some ofthe wheels flat or lopsided, but basically we have acylinder, even if it is bent out of shape. The line of dotsextending through and beyond the cylinder to top leftand bottom right outline the distortion of our basiccylinder to conform to the actual outline of theMunsell perceptual color solid for the yellow and pur-ple-blue Hues (Fig. 1).

Fig. 1. An idealized representation of the three-dimensional Munsell Color Order System.The dimension of Hue is represented by the ten colors arranged around the central axis.Chroma is related to the spokes of the wheels; the colors are purest at the periphery andbecome progressively grayer as they approach the central achromatic Value axis. Valuerefers to the lightness/darkness of a color, and the nine Value levels are represented by thenine wheels. The darkest colors are in the bottom wheel, the lightest colors are in the topwheel. The dots illustrate the distortion of the cylinder to reflect the actual outline form ofthe color solid for the yellow and purple-blue Hues. The pie-shaped wedge of color of thenatural teeth is seen within the upper left of the solid. A representation of the Hayashi guideand how it relates to this pie-shaped wedge is seen to the upper left of the cylinder. Thechroma spokes have been represented only in the Value wheels Nos. 1 and 9.

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DIMENSIONS OF COLOR

Hue, Value, and Chroma, the dimensions of color,are just as descriptive in describing color as length,width, and breadth are in describing form, once thelanguage is understood by those using it. Since it is soimportant in working with color to understand thor-oughly the three-dimensional concept of color,22 amore explicit description of each is presented.

Hue. Hue, the first dimension, is the easiest tounderstand, and in Munsell’s23 words, “it is that qual-ity by which we distinguish one color family fromanother, as red from yellow, green from blue or pur-ple.” The color wheel is a familiar form of thisdimension and consists of the Hues that are arranged

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sequentially around the central axis of the MunsellColor Solid (Plate II, C). To refer to a Hue in theMunsell system, the initials are used: R for red, YR foryellow-red, Y for yellow, and so on. Each Hue is sub-divided into ten segments, equally spaced visually (bypsychologic criteria) from each other. Fig. 2 representsthe color wheel cut and placed in a horizontal strip.

Value and Chroma are more difficult to understandand are often confused with one another. Specialattention must be focused on these dimensions.

Value. Value “is that quality by which we distin-guish a light color from a dark color,”23 and this isrelated to the achromatic (colorless) polar axis goingthrough the Munsell color solid (Fig. 1 and Plate II,

Plate I. The Munsell color tree, Munsell Color Company, Division of Kollmorgen Corp.Plate II. (A) Value scale—theoretical white and black included in addition to the nine Valuesteps. (B) Page from the Munsell Book of Color, Baltimore, 1966, Munsell Color Company,Division of Kollmorgen Corp., p. 5R. (C) Color wheel. (Photograph provided by Munsell, aDivision of GretagMacbeth, LLC.)

A B C

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A). The Value of a color is determined by which grayon the Value scale it matches in lightness/darkness.The black of the Value scale is assigned a Value of zero,the white a Value of 10. An infinite number of grada-tions of gray is possible as we go from black to white,but only nine Value (gray) steps are used in theMunsell system. Pure white (10) and pure black (0)are unattainable. Fractional numbers are used when afiner evaluation is needed. “Low” Values refer to darkcolors; “high” Values to light colors.

We perceive Value differences when we watch ablack-and-white television picture. The actual scene isfull of color, but only the lightness/darkness (Value)of a color is transmitted; a blue, red, or yellow couldall be transmitted as the same indistinguishable gray ifthey are of the same Value (a part of the same Value“wheel”). Colors of high Value would be transmittedas light grays, and those of low Value as dark grays,regardless of the Hue or Chroma. It could be said thatthe Value of a color is the gray it would match if it wereseen on a black-and-white television screen. Tab c inPlate II, B, and all the tabs directly to its left have thesame Value and would, therefore, transmit as the sameindistinguishable gray.

Chroma. Chroma, the remaining dimension, “isthat quality by which we distinguish a strong colorfrom a weak one; the departure of a color sensationfrom that of white or gray; the intensity of a distinctiveHue; color intensity.”24 Chroma describes the amountof Hue in a color. The gradations of Chroma werereferred to as the spokes of our wheels (Fig. 1). Theconcept of painting a box will help to clarify thisdimension. Suppose it is desired to paint one side of abox pure red. If an amount of gray paint is added tothe bucket before the second side is painted, the redon the second side will be perceived as less than a purered; the Chroma will be reduced. If additional graypaint is added to the bucket before each additional sideis painted, the paint will come closer and closer tobeing perceived as a gray. Plate II, B, is from theMunsell Book of Color, and studying this illustration will

aid in the understanding of these points. If the red tothe extreme right at Value level 4 (tab c) is consideredas the original color of the paint, the reds to the left ofthis would represent the paints of reduced Chroma.Adding a gray always reduces the Choroma and theo-retically will not affect the Hue. The change in Valueof the original color depends upon the Value of thegray added to it. If a gray of higher Value than that ofthe original color is used, the resulting color will be ofthe same Hue, lessened Chroma, and higher Value(tab a could represent such a result). If a gray of thesame Value is used, only the Chroma will be affected(lessened) (e.g., tab d). If a gray of a lower Value isused, the Chroma will be lessened, and the Value willbe lowered (e.g., tab b). Emphasis is placed on thispoint in order to dispel the confusing statements seenin some dental literature to the effect that Valuedepends upon the relative amount of gray in a Hueand that adding gray always lowers the Value.

The need to refer to gray in describing both Valueand Chroma is a major factor in the confusion con-cerning these two dimensions. To think of Value inrelation to the television picture and Chroma in rela-tion to the painting of the box will provide a simple,easily recalled memory aid.

IDENTIFICATION OF MUNSELLCOLORS

Munsell25 explained the system of designating col-ors in these words: “The notation used in this systemplaces Hue (expressed by an initial, as mentioned ear-lier) at the left; Value (expressed by a number) aboveand to the right of the Hue symbol; and Chroma, alsoexpressed by a number, below and to the right of theslanting line.” The over-all format in common usage isH V/C. The H is preceded by a number to indicatethe precise subdivision of the Hue. The designationfor tab a would be 5R 7/8; tab b, 5R 2/8; tab c, 5R4/14, and tab d, 5R 4/8 (Plate II). Such designationsprovide an exact identification of a color that is inter-nationally understood.

IDEAL COLOR SPACE

The goal sought in arranging colors according tothe psychologic attributes of Hue, Value, and Chromais to create an “ideal” color space with a constant, per-ceptually equal difference between any one color (asdefined by its Hue, Value, and Chroma) and any of thecolors surrounding it. For each color, the next closestmatches are easily located since each color is consid-ered the center of a sphere surrounded by the nextclosest color matches in Hue, Value, and/orChroma.26 If the arrangement of such a system is suc-cessful, a linear relationship will exist between thedistance traveled in such color space and the color dif-ferences that result.26

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Fig. 2. The color wheel cut and placed in a horizontal strip.The Hue ranges of natural teeth and three studies are shown:(1) spectrophotometric analysis: (2) Hayasi study; (3) Clarkstudy.

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The Munsell Color Order System was designed tofulfill the requirements of ideal color space. The factthat the ideal has not been completely achieved (andperhaps is impossible to achieve)27 does not detractfrom the worth of such a system. To work with color,thoughts on color must first be organized to the pointthat differences between surfaces can be recognizedfor what they are and described in objective terms.

SUMMARY

It is possible to arrange color in a manner thatmakes this three-dimensional enigma understandable.The Munsell Color Order System is recommended asthe system of choice for dentists who wish to gain aworking knowledge of color. Once the dimensions ofcolor (Hue, Value, and Chroma) become familiarterms, dentists will have the color language and thenecessary tools to approach color matching problemsin a logical manner.

REFERENCES

1. Clark, E. B.: Seventy-fourth Annual Session of the American DentalAssociation, Buffalo, N.Y., Sept 15, 1932.

2. Sproull R. C.: A Survey of Color Education in the Dental Schools of theWorld, El Paso, Texas, 1967, U.S. Army Research Report.

3. Bond, F.: Color: How to See and Use It, San Francisco, 1954, CameraCraft Publishing Company, p. 7.

4. Ronchi, V.: The Nature of Light, Cambridge, Mass., 1970, HarvardUniversity Press, Inc. p. 265.

5. Billmeyer, F. W., Jr., and Saltzman, M.: Principles of Color Technology,New York, 1966, John Wiley & Sons, Inc., p. 2.

6. Birren, F.: Color: A Survey in Words and Pictures, New Hyde Park, N. Y.,1963, University Books, Inc. pp. 83, 84.

7. Judd, D. B. and Wyszecki, G.: Color in Business, Science and Industry,ed. 2, New York, 1963, John Wiley & Sons, Inc., pp. 5, 24-26.

8. Aronoff, J., et al.: Psychology Today, Del Mar, Calif., 1970, CRM Books,p. 309.

9. Wright, W.D.: The Rays are Not Coloured, New York, 1968, AmericanElsevier Publishing Company, Inc., pp. 2, 19, 20.

10. Burnham, R. W., Hanes, R. M., and Bartleson, C. J.: Color: A Guide toBasic Facts and Concepts, New York, 1963, John Wiley & Sons, Inc., pp.123-150.

11. Billmeyer, F. W., Jr., and Salzman, M.: Principles of Color Technology,New York, 1966, John Wiley & Sons, Inc., pp. 53-89.

12. Judd, D. B., and Wyszecki, G.: Color in Business, Science and Industry,ed. 2, New York, 1963, John Wiley & Sons, Inc. pp. 8-71.

13. Burnham, R. W., Hanes, R. M., and Bartleson, C. J.: Color: A Guide toBasic Facts and Concepts, New York, 1963, John Wiley & Sons, Inc., p. 14.

14. Feller, R. L., and Stenius, A. S.: On the Color Space of Sigfrid Forsius,1611, Color Eng. 8: 48-51, 1970.

15. Burnham, R. W., Hanes, R. M., and Bartleson, C. J.: A Guide to BasicFacts and Concepts, New York, 1963, John Wiley & Sons, Inc., pp. 163-172.

16. Billmeyer, F. W., Jr., and Saltzman, M.: Principles of Color Technology,New York, 1966, John Wiley & Sons, Inc., pp. 25-52.

17. Mackinney, G., and Little, A. C.: Color of Foods, Westport, Conn., 1962,The AVI Publishing Company, Inc., pp. 28-74.

18. Birren, F.: Color, a Survey in Words and Pictures, New Hyde Park, N.Y.,1963, University Books, Inc., pp. 141-152.

19. Judd, D. B., and Wyszecki, G.: Color in Business, Science and Industry,ed. 2, New York, 1963, John Wiley & Sons, Inc., pp. 221-243.

20. Judd, D. B., and Wyszecki, G.: Color in Business, Science and Industry,ed. 2, New York, 1963, John Wiley & Sons, Inc., pp. 226.

21. Nickerson, D.: Color Measurement, U. S. Department of AgricultureMisc. Pub. 58, March, 1946, p. 9.

22. Clark, E. B.: An Analysis of Tooth Color, J. Am. Dent. Assoc. 18:2093,1931.

23. Munsell, A. H.: A Color Notation, ed. 11, Baltimore, 1961, MunsellColor Company, Inc., p. 15.

24. Munsell, A. H.: A Color Notation, ed. 11, Baltimore, 1961, MunsellColor Company, Inc., p. 16.

25. Munsell, A. H.: A Color Notation, ed. 11, Baltimore, 1961, MunsellColor Company, Inc., p. 20.

26. Judd, D. B.: Ideal Color Space, Color Eng. 8:38, 1970.27. Judd, D. B.: Ideal Color Space, Color Eng. 8:44, 1970.

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