Color matching in dentistry. Part II. Practical applications of the organization of color

Miscellaneous

Color matching in dentistry. Part II. Practical applications of the organization of color* Robert William

C. Sproull, Beaumont

D.D.S.** General Hospital,

El Paso, Texas

An any visual color-matching procedure, the questions that must constantly be answered are just how great a color difference exists between the samples to be matched and the standards and/or just how great a difference is acceptable in a given situation. Tooth shade guides offer a series of standards simulating the natural teeth, and the dentist must decide which standard offers the most acceptable color match with the tooth or teeth in question and if such a match will please the patient. The success or failure of this procedure is closely related to the adequacy of the shade guide Plus the familiarity of the dentist with basic color-matching procedures. (Parenthetically, the word “shade” is one of the most overworked terms in color; Judd and Wyszeckil listed seven meanings for “shade.” A more desirable term would be “color standard,” but in deference to common usage, the term “shade guide” is used in this article,) Color matching with available tooth shade guides is subject to frustration and error. Culpeppe? pointed out the inconsistencies among individual dentists in matching natural tooth shades and the inability of some dentists to duplicate their own shade selections reliably from one time to another. Some of the errors in color matching mentioned by Culpepper must be attributed to the human variables always present, but other errors, as he pointed out, must be attributed to the inadequacy of available guides. However, not even an ideal shade guide, if one existed, would eliminate all problems. Davidson and Freide3 pointed out the extreme variability of human observers in evaluating matches of wool flannel. In that study, samples with very large differences were not always rejected, and samples in which there were zero differences were not always accepted. Davidson’s remark, “The eye is a very good mechanism, the trouble is that it is hooked up to the brain,” has much to commend it. Nevertheless, the better the guide, the better the chance of success when the total population of those using the guide is considered. Read *Part **Chief,

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before the American College of Prosthodontists I, J. PROSTHET. DENT. 29: 416-424, 1973. Fixed

Prosthodontic

Service.

in Las Vegas,

Nev.

Volume Sumbrr

29 j

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6.5/

0 Fig.

1. Hue

Fig.

2. Value

and Chroma and Chroma

REQUIREMENTS

range

of natural

range

teeth

of natural

FOR A TOOTH

from

teeth

SHADE

cthroma-

a spectrophotometric

from

a spectrophotometric

study. study

GUIDE

Prime requirements for any color guide include ( 1) a logical arrangement in color space and (2) an adequate distribution in color space. A shade guide based on the Munsell Color Order System could fulfill these requirements. With such a guide, if the proper limit for the color space of the natural teeth has been determined and small enough intervals in Hue, Value, and Chroma have been established, a match can be rapidly chosen. (An “ideal” color space is one in which each color is the center of a sphere of color, and the next closest matches in color surround it.) In order to arrive at a correct match, one must evaluate Hue, Value, and/or Chroma differences and proceed in the proper direction. A guide that does not fulfill the prime requirements of an adequate and logical arrangement in color space generate certain basic problems: ( 1) It takes too lomg to decide where to begin; (2) it is impossible to rapidly or logically check the chosen match for accuracy; (3) the volume of color space occupied by the materials to be matched may be impossible to reach with the guide. COLOR

SPACE

OF NATURAL

TEETH

AND

SHADE

GUIDES

In order to evaluate the range of the required color volume for natural teeth and the color volume and arrangement offered by existing guides, a General Electric Recording Spectrophotometer4 was used to measure the colors, and the data were converted to Munsell notations. The details of these proceedings will be described in a later article. Natural teeth. The data for natural teeth in this study obtained with the use of spectrophotometer are graphically portrayed in Figs. 1 and 2. This limited study indicated an approximate Hue range of 7.5 YR to 2.7 Y. a Value range of 5.8/ to

1

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J. Prosthet. Dent. May, 1973

Fig. 3. Hue and Chroma range of shade guide A from a spectrophotometric study. Fig. 4. Value and Chroma range of shade guide A from a spectrophotometric study.

8.5/, and a Chroma range of /1.5 to /5.6. Hayashi,s from a study of the range of color of 81 natural teeth, using the Munsell Color Order System, established a Hue range of 8.75 YR to 3.75 Y, a Value range of 6/ to 8/, and a Chroma range of /l to /5. Clark,‘j in a study of over 6,000 teeth, visually established a Hue range of 6 YR to 9.3 Y, a Value range of 4/ to 8/, and a Chroma range of /O to /7. A comparison of the Hue range of the three studies is seen in Fig. 2 of Part I.5 Clark8 stated that the extreme ranges of Hue were seldom seen; only 1.1 per cent of the teeth in his study tended toward greenish yellow and 4.3 per cent toward reddish yellow. Clark found that, as tooth colors grow darker (lower in Value), they weaken in saturation (Chroma) ; this is opposite to the trend seen in the limited spectrophotometric study. Both studies agreed that, as the teeth became redder, they became darker. Further spectrophotometric studies are indicated, but for the purposes of this article, the Hue ranges of the spectrophotometric and Hayashi studies will be used. The Value and Chroma ranges of the studies are fairly close. An acceptable tooth shade guide, based on the volume of color space of the natural teeth, must ( 1) embrace the coordinates established by natural teeth, (2) be logically arranged, and (3) have sufficient data points to fill the total volume. Available shade guides. Figs. 3 through 6 are graphic portrayals of the organization of several dental shade guides. The only guide in which there is an orderly and continuous progression from one shade guide tab to another is shown in Figs. 3 and 4, but this guide does not offer adequate Chroma and Value ranges in the available Hues to meet the criteria. Figs. 5 and 6 represent the arrangement found in most other guides, in which there is almost no logical order in Hue, Value, or Chroma. From study of these figures (and from measurement of other shade guides), some conclusions can be reached : ( 1) A vailable guides do not extend through the volume of color space required; (2) an orderly or systematic arrangement or relationship between tabs is usually lacking; (3) there will be clustering or duplication of colors in certain areas of color space and voids in other regions.

Color

matching

in dentistry

+-

0 Fig.

5. Hue

Fig.

6. Value

and and

Chroma

range

Chroma

range

of shade

guide

of shade

guide

w

B from

7. Hue

and

Chroma

a spectrophotometric

B from Hue

1 OYR

Fig.

eChroma-

study.

a spectrophotometric

study.

Y 1Y

range

of the

Hayashi

guide.

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Sproull

/2

/3 eChroma

/4

/5

/b

--,

Fig. 8. Value and Chroma range of the Hayashi guide.

RECOMMENDED SOLUTION FOR DENTAL SHADE GUIDES A shade guide should be developed that is composedof the proper materials and arranged according to the Munsell System. It should have sufficient volume and detail to permit free travel within the “envelope of color” of the natural teeth. A guide so constructed would eliminate the shortcomings of existing guides and make possible the planned approach, the flexibility, and the coverage needed to put dental color matching on a logical basis. Although the guides measured in this study proved inadequate, two guides have been proposed in the past that approach the requirements of an ideal guide based on the Munsell System. One was proposed and developed by E. Bruce Clark in the early 1930’s; the other has been recently proposed by Toshio Hayashi. Both need modifications, but they should be closely studied, and their concepts should be put into usable form. Hayashi guide. The simplicity and logic of Hayashi’s guide are graphically illustrated in Figs. 7 and 8. Its limits are based on his measurementsof the limits in color space of the teeth of the Japanesepeople, and the color space intervals are as follows : 1. Five Hues-8.75 YR to 3.75 Y in 1.25 intervals. 2. Five Values for each Hue-6/ to 8/ in 0.5 intervals. 3. Five Chromas for each Hue-/l to /5 in unit intervals. The boundaries established by these limits can be likened to a pie-shaped wedge of color space,and the relation of this wedge to the total volume of color is seenin Fig. 1 of Part I.’ The 1.25 intervals in Hue present us with five different “pages” (8.75 YR, 10 YR, 1.25 Y, 2.50 Y, and 3.75 Y-Fig. 1 of Part I). Twenty-five color tabs are arranged on each “page” in five stepsof Value (6/, 6.5/, 7/, 7.5/, and 8/) and

Color 160

matching IS0

in dentistry

140

Y30

Y20

Fig. known

range of the Clark guide. from the text of the article.

10. Value and Chroma but can be approximated

The

YlO

Yb7

Y57

147

Y37

Y27

Y17

Yb8

YSS

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i

I

Fig. 9. Hue and Chroma but can be approximated

561 561

exact

range of the Clark guide. The from the text of the article.

Munsell exact

notations Munsell

are

notations

unknown are

ILII-

five steps in Chroma (/l, /2, /3, 1’4, and /5). This guide, which has been published in Japan, uses carefully controlled colored paper tabs to represent each of the equal11 spaced 125 locations in color space.” The use of a guide with 125 color tabs seems at first glance to be a formidable undertaking, but once the method of use is understood, its simplicity becomes apparent. 1. Determine the aproximate Value of the tooth or teeth to be matched. This reduces the number of tabs that must be considered from 125 to 25. 2. Determine the approximate Chroma or saturation of the tooth or teeth to be matched. This reduces the number of tabs that must bc considered from 25 to 5. 3. Uetermine, from the live Hue tabs of the Rosen Value and Chroma, the one that appears to be the closest match. 4. If this tab does not appear to be as close as one Ivould like, probe in thret>dimensional color space for a closer match. The next closest matches must lie in the vicinity of the initial choice, and probing is simply a matter of anal)-ying Hue. \‘aluc. and Chroma differences and moving in the appropriate direction. Clark Tooth Colour System. Clark’s system is based upon his analysis of the colors of morr than 6,000 teeth. The total number of colors available with his system was 703, but his actual shade guide, the Tooth Colour Indicator, had 60 tabs. The arrangement of the Clark guide is graphically illustrated in Figs. 9 and 10. Clark felt a need for only three basic Hues, but required 19 \‘alue steps and six Chroma steps. (The exact Munsell notations for the guide are not known. but they can be appr~xi-

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J. I’mrthrt. Dent. May. 1973

mated by reviewing the limits of the teeth analyzed by Clark.) Each tab had a gingival color and an incisal color; Figs. 9 and 10 reflect the location of the gingival colors. Of the 703 colors, 342 were gingival, and 361 were incisal colors. The 19 additional incisal colors were formulations without Hue and reflected 19 Value steps of gray for those incisal colors without a trace of Hue. Clark’s 703 colors were available from 13 porcelain powders, and the formulation of each shade was published in a special formulary. ClarkOp*-I1 gave detailed directions for the use of the Tooth Colour Indicator and the Clark Tooth Colour System. His advice concerning the positioning of the patient, illumination, evaluation of colors of the teeth, the proper choice of colors to obtain the proper blend, and other prerequisites for successfulrestorations is pertinent today. Clark believed that Value (referred to as “brilliance” at that time) was the most important dimension to control. Chroma (“saturation” in his writings) was considered the next most important dimension. Hue differences in his experience were sounimportant that he seldom used other than the yellow hues and his Tooth Colour Indicator contained only this hue. DISCUSSION

A comparison of the Clark and Hayashi guides shows both similarities and differences. The similarity lies in their opinions of the relative importance of the dimensionsof color. Clark* felt that color differences in Value are recognized more easily than are differences in the other dimensions,and he established 19 stepsin the Value range of his guide. Hayashi, by establishing 0.5 step intervals in Value as opposed to full step intervals in Chroma and 1.25 step intervals in Hue, reflected a similar feeling for Value. Personal observations confirm that Value differences are most readily apparent to both patients and dentists (whether they are familiar with the dimensions of color or not). In the Munsell System itself, one unit of Value is roughly equivalent to two units of Chroma or three units of Hue for samplesof /5 Chroma.‘” Hayashi placed all of his 125 colors in the shade guide itself, whereas Clark placed only 60 of the 703 colors in the guide and arrived at the other colors by interpolation (evaluating Hue, Value, or Chroma differences and modifying the formula accordingly). Additional measurementsof natural teeth might confirm the trend seen in the initial spectrophotometric measurements which indicates that, as teeth go from YR to Y, they become lighter and less saturated. If this could be proved true, certain colors in the Hayashi and Clark guides could be eliminated, i.e., colors which are YR at 8/ Value or Y at 6/ Value. Additional (more closely spaced) colors might prove advantageous in certain well-traveled regions of the color spaceof teeth. One of the peculiarities of color perception is that Hue differences at /I Chroma are not nearly as noticeable as Hue differences at /5 Chroma. As the Clark and the Hayashi guides are now constructed, this means that /l Chroma tabs in adjacent Hue planes are visually much closer at. /l than at /5 Chroma. A shift

Color

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in density of tabs, reducing the number in low Chromas and increasing the number in higher Chromas, seems indicated.* The Hayashi guide would have to be converted to porcelain because of some basic problems in color matching. The specular components (glare factors) of paper and natural enamel are so dissimilar as to make a paper guide extremely difficult to use? whereas the specular components of natural enamel and procelain are compatible. Another problem relates to spectral metamerism’“-lH and refers to the phenomenon which causes a seemingly accurate color match to be revealed as inaccurate when the conditions under which the original match was obtained are changed. It is closely related to one of the physical aspects of color sensation and can best be understood when the spectrophotometric curves of two surfaces are compared. When these curves are different, a change in the light source? the human observer, the angle of viewing, the distance of viewing, and numerous other conditions can cause the seemingly accurate color match to disappear. The spectrophotometric curves of the paper tabs of the Hayashi guide and of similarly colored tooth surfaces are completely different, and therefore the matches are highly metameric.? It is possible to obtain spectrophotometric curl-es of enamel and of matching procelains that are quite similar when the correct pigments are chosen. The Clark guide was not measured with a spectrophotometer, but experience in measuring other porcelains would indicate that the curves could be compatible. This should not be construed as a criticism of Hayashi’s work-he is aware of the needs for modification$ and considers his guide a prototype. Commercially available procelains \vould have to be developed with which to construct the restorations to match the tabs of the Munsell guide. The logistics for such porcelain are not impossible; five Hues and five Values of gray \vould makes available 125 basic colors such as are in the Hayashi guide. Clark used 1?1 porcelains for his 703 colors, but stated it could be done with five. ‘To discard traditional shade guides in favor of a Munsell type guide would admittedly create financial and logistic problems. However. the alternative to applying the principles explored by Clark and Hayashi is fraught with equally monumental problems for practicing dentists. The ever-increasing demand for esthetics by an increasingly sophisticated dental clientele requires dentists to abandon antiquated and unsuitable guides and to take advantage of modern color technology. The beauty, simplicity, and logical approach to dentistry’s color-matching problems offered by a shade ,guide based on the Munsell Color Order Svstem speak for themselves. APPROACH

NEEDED

WITH

PRESENT

GUIDES

The lack of a Clark or Hayashi type shade guide in porcelain enhances rather than negates the need to understand the three-dimensional nature of color. Con-

30,

*Hale, 1972.

N. W.,

fHemmendinger, $Hayashi, 1967.

Munsell H.,

T.,

Tokyo

Color and

Company,

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Medical

and

Kollmorgan N. J.:

Dental

Corp.:

Personal

Personal

communication,

University:

Personal

communication, Oct.

communication,

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10. 1971. Sept.

IO.

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sider the quandary with which dentists are constantly faced: If a shade 69 is the closest match that can be found to the color of a tooth, but it is obviously not close enough, what courses of action are open within the framework of available materials? Custom guide. One approach with the porcelain-fused-to-gold systems is to construct a custom shade guide with a series of gray and colored modifiers for each shade. The original twelve or so shade tabs of a guide can readily be expanded to 60 or more by the use of these gray and pigmented modifiers. This offers the advantage of additional coverage of the color space occupied by natural teeth, but even greater confusion will result, because the lack of organization of the original shade guide is merely being perpetuated and multiplied. If a shade 69 with an orange modifier is closer than a plain 69, but it still is not correct, where is the next closest match? It might vvell be a dozen tabs away, for there is no assurance in such a guide that similarity in geographical location indicates similarity in color. It might be possible to make a custom guide based on the Munsell System, but the spectrophotometric analysis of the resultant tabs needed to assure correct spacing an d avoidance of metamerism is beyond the means of most dental offices. Modification of chosen shade. An approach that should be used by a dentistceramist team conversant with color language is to specify a shade 69, but with a modification of one or more of the dimensions of color, e.g., “a shade 69 but raise the Value.” This approach can be effective but is, of course, subject to error. How much should the Value be raised? It is a subjective approach without actual tabs to check the results, but is the procedure of choice with the guides available today? Modification (staining) of the Complete restoration. The dentist should choose the shade as above and then modify the color match with porcelain stains while the patient is seated in the office and available for constant reference. The more proficient the dentist becomes in analyzing which dimensions of color need changing, the more pleasing will be the results. Savings in time will also result as the dentist learns which color modifications are impossible. Value that is too low and/or Chroma too strong are situations which frequently are impossible to adjust. Modification of stain kits-addition of grays. The porcelain stains needed to modify the color match come in a variety of kits. These stains are indispensable with available porcelain systems, since many times the location in color space of the teeth to be matched cannot be reached directly. One of the prerequisites for successful manipulation of colors in three-dimensional space is the ability to closely control the dimension(s) needing change. With presently available kits, this is impossible; a minimum of five values of gray would be required for such control. To modify an error of too great a Chroma and to maintain the Value are possible with the present kits only if the Value of the restoration happens to be of the same Value as the gray in the kit. Grays are also needed for Value modifications. The addition of gray to modify Value will also reduce the Chroma, but this can be compensated for by introducing additional Hue if needed. At the present time, Value modifications are extremely difficult if the available gray does not happen to be at the proper Value level needed for the change. Grays from complementary colors. Grays can be obtained by the use of comple-

Color

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mentary colors as recommended in some techniques, but the result will be a complex gray that greatly enhances the problem of spectral metamerism. Artists like to uw complements. because they feel that neutrals or near neutrals made in this manner arc morr “lively,” but this is not what dentists want in their restorations. ‘33~ “lively” grays tend to change with each change in observer or light source. In addition to the metameric problems introduced, it may ~~11 be that the correct \:aluc of gray is impossible to obtain M.ith the available complementary colors: Fi\,ri Values of gray are needed in porcelain stain kits.

CONCLUSIONS To bring the approach to color matching in dentistry to an acceptable level, positive steps are needed. 1. I>ental schools should require a comprehensive course in color in the prcdental or dental curriculum. 2. Advice from color engineers and color experts should be sought. and such individuals should be included on the dental faculties. 3. Increased research on color problems in dentistry should be encouraged. 4. Shade guides should be revised to resemble the Clark and Hayashi type guides, and commercially available porcelain should be developed to match these guides. 5. Porcelain stain kits should include five Value steps of gray.

SUMMARY The three-dimensional nature of color and the importance of understanding this concept in approaching a color-matching problem were set forth in Part I.’ The practical application of the three-dimensional Munsell Color Order System to the design of an ideal dental shade guide and the correct way to use this guide were explained in Part II. Until such a guide and the porcelain to go with it bccomc available, existing guides must be used. A means of extending their utility was reviewed. Recommendations \vere made for elevating color-matching procedures in dentistry to an acceptable level. The author wishes to express grateful appreciation to Lieutenant Colonel Martin IA. Nusynowitz and to the Research and Development Command without whose support this article could not have been written and published, to Dr. Henry Hemmendinger whose guidante and advice directed our efforts and who donated the Munsell Color Company, Division of Kollmorgen Plate I and a review of the article for accuracy.

so many hours Corp., for their

of his own time, and to permission to reproduce

References 1.

Judd, D. B., and Wyszecki, G.: Color in Business, Science and Industry, ed. 2. New York, 1963, John Wiley & Sons, Inc., p. 353. 2. Culpepper, W. D.: A Comparative Study of Shade-Matching Procedures, J. PROSTHET. DENT. 24: 166-173, 1970. 3. Davidson, H. R., and Freide, E.: J. Opt. Sot. Am. 43: 581, 1953. 4. Hemmendinger, H.: Color Measurement Laboratory, Belvedere. N. J., 1971. 5. Hayashi, T.: Medical Color Standard. V. Tooth Crown, Tokyo, 1967, Japan Color Research Institute. 6. Clark, E. B.: Tooth Color Selection, J. Am. Dent, Assoc. 20: 1065-1073, 1933.

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Sproull, R. C.: Color Matching in Dentistry. Part I. The Three-Dimensional Nature of Color, J. PROSTHET. DENT. 29: 416-424, 1973. Clark, E. B.: Selection of Tooth Color for the Edentulous Patient,, J. Am. Dent. Assoc. 25: 787-793, 1947. Clark, E. B.: An Analysis of Tooth Color, J. Am. Dent. Assoc. 18: 2093-2103, 1931. Clark, E. B.: The Clark Tooth Color System, Parts I & II, Dent. Mag. Oral Top. 50: 139-152, 1933. Clark, E. B.: The Clark Tooth Color System, Part II, Dent. Mag. Oral Top. 50: 249-258, 1933. Wright, W. D.: The Measurement of Colour, ed. 3, Londoq, 1964, Hilgar & Watts, Ltd., p. 185. Billmeyer, F. W., Jr.,, and Saltzman, M.: Principles of Color Technology, New York, 1966, John Wiley & Sons, Inc., pp. 20-23. Allen, E.: Metamerism-A Study of Dimension, Color Eng. 6: 38-43, 1968. Nimeroff, I.: A Survey of Papers on Degree of Metamerism, Color Eng. 6: 44-46, 1968. Broches, A.: The Problem of Color Metamerism From an Industrial Viewpoint, Color Eng. 6: 48-54, 1968. Little, A.: Shades of Meaning, Color Eng. 7: 10, 1969. Allen, E.: Some New Advances in the Study of Metamerism, Color Eng. 7: 35-40, 1969. 2405 GAIRLOCH DR. EL PASO, TEXAS 79925