AU612899B2 - Method for determining the color of an object, particularly a dental prothesis - Google Patents

Description

OPI DATE 02/04/90 APPLN- I D 42175 89 Per AOJP DATE V"Q13%PTF8/05 DEMANDE INTERNATIONALE PUBLIEE E R RTU F G RERN EN MATIERE DE BREVETS (PCT) (51) Classification Internationale des brevets 5 G 0%IJI 3/46, A61C 19/04 (11) Num~ro de publication Internationale: At (43) Date de publication internationale: WO 90/02929 22 mars 1990 (22.03,90) (21) Numniro de la demande internationale: PCT/FR89/00454 (22) Date de dip6t international: 8 septembre 1989 (08.09.89) Donnies relatives i In prioriti: 88/11788 9 septembre 1988 (09.09.88) FR (71) Diposant (pour tous les Eta is d~sign&s sauf US): BERT] N CIE [FR/FR]; B.P. No 3, F-78373 Plaisir C~dex (FR).

(72) Inventeurs; et Inventeurs/Diposants; (UJS seulement) DORDET, Yves, Pierre [FR/FR]; 26, les Terres-Rouges, CD 58, F-13590 Meyreuil DECAUDIN, Jean-Michel [FR/FR]; 24, alle Jean-S~bastien-Bach, F-13880 Velaux DU- RET, Franqois [FR/FR]; Draye-des-Vignes, F-38690 Ie Grand-Lemps (FR).

17A 41 11 n' C A n, NTErr0 nx rr. S a v .ue I E -1 ie 75008 Paris (FR).

(81) Etats disign~s: AU, JP, KR, US.

Publik Avec rapport de recherche internationale.

Avant l'expiration du d~lai pr~u pour la modification des revendications, sera republike si de telles modifications sont repues.

(54)1Title: METHOD FOR DETERMINING THE COLOR OF AN OBJECT, PARTICULARLY A DENTAL PROTHESIS (54) Titre: PROCEDE DE DETERMINATION DE LA COULEUR D'UN OBJET, EN PARTICULIER D'UNE PROTHESE

DENTAIRE

(57 Abstract Meho fr etnnnigth clo o a ojct(1),suh s oohbymesuin tsdifue petrlreletacebymen (57) Absract Metod~ der determinin dhe couleur fan objet (10) telh asun denoth, pr mesuret dife sea reflectance pcty difueaus oe da spectrom de dtecturs (22) e erut lectronicqprocdesingaitement (24, 26, 28) associate wit un micro-proces- h setho32) consistnt in determinger ttal chromatiufertlentce baecoueur dhe oao den thce dun anecierds un spae chniforthin 5 a iomsa 5 b and thcr uartqe onetrmeaqreeiation speten fue diafuden setrerefdetcer ofu thnooanr pho the colors ofa oule chrthinoer odtrietecloftepohss

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A METHOD OF DETERMINING THE COLOR OF AN OBJECT, IN PARTICULAR OF A DENTAL PROTHESIS The invention relates to a method of determining the color to be given to an object, in particular to a dental prothesis, on the basis of determining the color of another object, such as an adjacent tooth in the mouth of a patient.

It is essential for a dental prothesis (for example a crown, a pivot-tooth, a bridge, etc. to be as close as possible in color to the adjacent teeth under all types of lighting that may be encountered.

At present, the color of a prothesis is usually determined by inspection, generally by a dentist comparing the colors in one or more color charts with the teeth of a patient. The I results obtained using this procedure necessarily depend on the ability of the dentist to distinguish differences between colors which are very similar, and the results are therefore not always satisfactory.

It can also happen that a color chosen in this way from a color chart is very close to the color of the adjacent teeth under the illumination provided by a light source of a given i~ttype, but that the color is poorly matched to that of the adjacent teeth when illuminated by a light source of a Pt different type (the phenomenon of metamerisn).

Proposals have already been made to determine the color of an object, e.g. of a tooth, by using an apparatus comprising a spectrometer suitable for measuring the diffuse spectral reflectance of the tooth and by deducing the tristimulus values X, Y, Z of the color of the tooth therefrom, from which values the color can. then be reconstituted using three primary colors.

However, a knowledge of the tristimulus values under a given type of illumination does not serve to determine the values that apply under a different type of illumination. Thus, in order to obtain good results with such apparatuses, it is necessary to measure color-under several different types of illumination, and this is a major drawback that greatly increases the risk of error.

PT

The invention seeks, in particular, to avoid such drawbacks of the prior art.

An obj ect of the invention is to provide a method of determining the color to be given to a dental prothesis on the basis of a single measurement of the color of an adjacent tooth, while guaranteeing that the color of the prothesis will be identical or nearly identical to the color of the adjacent tooth, regard-less of the type of illumination.

Another object of the invention is to provide a method of this type enabling the apparent color of a dental prothesis and of each of the teeth adjacent thereto under various different types of illumination to be determined from a measurement performed under adequate illumination, and enabling such apparent colors to be displayed.

Yet another object of the invention is to provide a method of the above type suitable for determining the nature and the concentration of the pigment, or each of the pigments, which should be added to a resin or some other given color base in order to obtain a prothesis whose color is identical to that of an adjacent tooth.

The present invention provides a method of determining the color of an object such as a dental prothesis on the basis of the color of a different object, such as an adjacent tooth in the mouth of a patient, the method consisting in measuring the diffuse spectral reflectance of the tooth by means of a light beam reflected by the tooth and transmitted to a spectrometer, in determining the tristimulus values of the color of the tooth under given illumination, and in comparing the measured color with colors in a chart whose characteristics are known, thereby making it possible to select the prothesis color which is closest to that of the adjacent tooth, the method being characterized in that it consists in determining the total color distance 6E in a uniform color space such as the system between the tooth and each of the colors in the chart, in determining the mean square difference 8Q between the diffuse spectral reflectance of the tooth and the diffuse ,~j>spectral reflectance of each color in the chart, and in 3 selecting the chart color for which the pair of numbers (6E, 8Q) has the smallest values.

By selecting a small total color difference in the system between the color of a tooth and one of the colors in a color chart, a tint can be determined which is in any event close to that of the adjacent tooth, and by having a low value for the mean square difference between the diffuse spectral refleotances of the tooth and of the prothesis, it can be guaranteed that these colors are nearly identical under all types of illumination.

A single measurement on a tooth adjacent to the future location of the prothesis thus makes it possible to determine the color which should be given to the prothesis in order to obtain the best match In appearance, regardless of the type of illumination used.

Naturally, this measurement can be repeated on all of the teeth adjacent to the location which is to receive the prothesis, so that when the adjacent teeth are different in color it is possible to take an average of these colors in order to determine the color of the prothesis.

According to another characteristic of the invention, the method also consists in taking the difference between the diffuse spectral ref lectances of the tooth and the color selected for the prothesis in order to determine the type and the concentration of the pigment, or each of the pigments that ought to be added to said color in order to obtain a perfect or a near-perfect match between the final color of the prothesis and the color of the adjacent tooth. The composition of a resin to be used in a dental prothesis can thus be determined in a manner which is reliable and completely automatic.

The invention also provides for performing the various color measurements using a substantially equal-energy light source.

This provides useful information over the entire visible spectrum.

Prior to performing a measurement on a tooth, the method 'q of the invention also includes performing a calibration operation in which the diffuse spectral reflectance of a calibrated reference sample is measured, and then taking account of the relationship between said reflectance as measured and a pre-established reflectance value for the reference sample while determining the diffuse spectral reflectance of the tooth.

The invention also provides for measuring the light flux levels emitted by the source during the calibration operation on the reference sample and while taking a reading from the tooth, and in taking account of the relationship between these levels when determining the diffuse spectral reflectance of the tooth.

This avoids any possible variations in the characteristics of the light source influencing the determination of the diffuse spectral reflectance of a tooth.

In practice, the method also consists in recording and storing in a memory the diffuse spectral reflectances of the colors in a color chart, together with their chromaticity coordinates in a given color space, and also the diffuse spectral reflectance of a calibrating reference sample.

Naturally, the invention is applicable to determining the colors of objects other than teeth.

A better understanding of the invention, and further clarification of its characteristics, details, and advantages can be obtained by reading the following description given by way of example and made with reference to the accompanying drawings, in which: Figure 1 is a block diagram of apparatus for performing the method of the invention; Figure 2 is a flow chart of the main steps of the method of the invention; Figure 3 is a craph showing two diffuse spectral reflectance curves, one for a tooth and the other for a color in a color chart; Figure 4 shows the position of the color of a tooth in the space together with the positions of several colors in the color chart; and Figure 5 shows the difference between the spectral absorbancze of a tooth and of a color in the color chart for the purpose of determining which pigments should be added to said color in the chart.

Reference is made initially to Figure 1 in which reference numeral 10 designates a tooth which is adjacent to the future location of a prothes is, and whose color is to be determined.

The apparatus used for determining the color of the tooth comprises a light source 12 associated with an optical system 14 for illuminating the tooth 10 by means of a beam 16 of light rays.

The light rays of the beam 16 are preferably substantially parallel so as to illuminate the tooth 10 perpendicularly and uniformly over a large portion of its surface.

An optical system 18 disposed at 450 to the light beam 16 serves to pick up the light reflected and diffused by a small portion of the illuminated surface of the tooth.

In this way, the measurement performed takes account not only of the light reflected and diffused by the surface layers of the tooth, but also of the light reflected and diffused by deeper layers thereof (since teeth are made of substances which are partially translucent).

The optical system 18 serves to form an image of a portion of the illuminated surface of the tooth at the inlet of a spectrometer 20 whose output co-operates with detection means 22 such as one or mo~re strips of silicon photodiodes.

The light source 12 is also connected to the detection means 22, or to similar means, via an optical system 23 in order to measure the level of light flux emitted by the source 12 during a color measurement.

The signals produced by the detectors 22 are processed by electronic circuits including preamplifier means 24, sampling means 26, and analog-to-digital converter means 28 which are connected via an appropriate interface 30 to a microprocessor 32 which is associated with memories 34, a display screen 36, a printer 38, and a controlling keyboard The main steps of the method are described below with reference to Figure 2.

It is assumed that the characteristics of the colors in a color chart (i their diffuse spectral ref lectances and their chromaticity co-ordinates in the have already been recorded in a memory 34 of the microprocessor 32.

The method then consists initially in measuring the diffuse spectral reflectance Rm of a reference sample in order to calibrate the apparatus. To do this, the reference sample, e.g. a piece of ceramic having calibrated spectral reflectance, is disposed in the intended location facing the optical systems 14 and 18, and it is illuminated by the source 12. The light reflected and diffused by the sample is picked up by the optical system 18, is diffracted by the spectrometer 20, and is then applied to the detection means 22.

Simultaneously, the level of light flux Oe emitted by the source 12 is detected and measured.

Thereafter, the optical systems 14 and 18 of the apparatus are pointed towards a tooth 10 of the patient adjacent to the future location of the prothesis. The tooth is illuminated by the source via the optical system 14 and the reflected light is picked up by the optical system 18, tranmitted to the spectrometer 20, and sensed by the detection means 22. The signals from the photodetectors are then processed by the circuits 24, 26, and 28, and then by the microprocessor 32 which determines the diffuse spectral reflectance Dm of the tooth.

Simultaneously, the level of light flux 'kd emitted by the source 12 is measured.

The diffuse spectral reflectance of the tooth is obtained from the measured values by using the following equation: Re() O'e D( Dm( O'd in which: diffuse spectral reflectance of the tooth; Dm(X) the measured diffuse spectral reflectance of the le t tooth; 2 7 Re(X) diffuse spectral reflectance of the sample; Rm(k) measured diffuse spectral reflectance of the sample; 0e level of light flux on the sample; Od level of light flux on the tooth.

In practice, the method of the invention makes it possible to use only a limited number of photodetectors, e.g. less than since the values of the diffuse spectral reflectance at less than ten different wavelength intervals are sufficient in practice.

Given the diffuse spectral reflectance of the tooth, it is possible to determine the tristimulus values X, Y, Z of the tooth color by means of the following equations: X JAD JdX Y =JA)y(X)dX Z in which: and z(X) are the spectral tristimulus values of the reference observer in C.I.E. colorimetry.

Given the tristimulus values of the color of the tooth, the chromaticity co-ordinates of the color in the space II can be calculated using the following equations: i L* 116(Y/Yn) 1 3 16 1a* 500[(X/Xn) 1 3 (Y/Yn)I/ 3 b* 200[(Y/Yn) 1 3 (Z/Zn) 1 3 where: Xn, Yn, and Zn are the tristimulus values of a nominal white stimulus, and |X/Xn 0.01 Y/Yn 0.01 Z/Zn 0.01 The method of the invention then consists in calculating the total color distance in the space between the color of the tooth and each of the colors in the color chart, together with the mean square difference between the diffuse spectral reflectance of the tooth and of each of the colors in the chart, by means of the following equations: total color distance 6E *,(6L2+6*2 &b* 2 mean square difference 860 An] in which: D the diffuse spectral reflectance of the tooth; the diffuse spectral reflectance of a color in the chart.

Number pairs (6Ei, 8Qi) having different values are thus obtained by comparing the color of the tooth with each of the colors in thek- chart. The color in the chart for which the pair of numbers (6Ei, &Qi) are as small as possible is the color which is closest to that of the tooth, regardless of the type of lighting used.

Figure 3 shows the general appearance of the diffuse spectral reflectance curve D(X) for a tooth and of the diffuse spectral reflectance curve for a color in the color chart.

Figure 4 shows the position of the color D of the tooth and the positions of various colors Cl, C2, C3, C4, C5, and C6 in the chart when plotted in the space It can be seen that the total color distance 8E corresponds to the distance between the point D representative of the color of the tooth and any one of the points C representative of the colors in the chart, and the mean square difference cor-responds approximately to the surface area between the curves and If the operator so desires, when the pair of numbers (6E, 8Q) having the smallest values is selected, a set of views may be displayed on the screen 36 representing the prothesis and the adjacent tooth or teeth and showing their apparent colors under various different types of lighting, and the printer 38 can be used to print out the characteristics of the color of the tooth and of the color selected for the prothesis.

The characteristics of the colors in the chart (diffuse spectral reflectance and chromaticity co-ordinates in the space are previously recorded in a memory 34 of the microprocessor 32 either by performing the same method as described above for determining the color of a too-th, or else by entering the corresponding numerical values directly into the memory, e.g. by means of the keyboard 7 Once the diffuse spectral reflectance of the color of an adj acent tooth and] the diffuse spectral reflectances of the colors in the chart are known and] stored in memory, it is easy, as shown diagrammatically in Figure 5, to determine the appropriate pigment(s) and concentration( s) thereof for adding to a color in the chart in order to obtain a color which is identical to that of the adjacent tooth, regardless of lighting conditions.

Figure 5 shows a portion of the spectral absorbance curve AD(N) of the tooth and a portion of the curve AC(X%) of one of the colors in the chart. The curve ADON) corresponds to the difference between these two curves and represents the spectral absorbance of the pigment that needs to be added to the color in the chart in order to match thb) color of the tooth.

Spectral absorbance and spectral reflectance are related by the equation: -log(D(X)) Once the pigment absorbance AP(X) has been determined in this way, pigment concentration can be deduced using Beer- Lambert equations: Ap(X) 1 0 where: E0.) the extinction coefficient per unit mass of the pigment; and] c pigment concentration.

Thus, on the basis of the measured diffuse spectral reflectance of a tooth adjacent to the future location of a prothesis, and on the basis of its difference from the diffuse spectral reflectance of a color in a color chart, the method of the invention makes it possible automatically to determine which pigments should be added to the color in the chart, and also the appropriate concentrations thereof, in order to obtain an exact match with the color of the tooth. The invention thus makes it possible to provide full and automatic control of a device for metering out the components of the resins used for making dental protheses.

A corollary is that it is no longer necessary to record the chlaracteristics of a very large number of colors in the memo~ry of the microproce-'sor since considerable differences between the ref lectances of a tooth and of a reference color can be compensated by adding one or mo~re pigments.

In order to improve results still. further, the invention also provides for repeating the measurement of diffuse spectral reflectance on other teeth adjacent to the future location of the prothesis. Should the ref lectances measured in this way differ fromi one another, then their average is used for determining the color of the prothesis for obtaining the best possible match with the adjacent teeth.

Measuremenmt may also be performed on different portions of a single tooth, thereby reducing the risk of error.

In general, the method of the invention provides the following advantages: the color to be given to a prothesis can be determined accurately and reliably by performing a single measurement on one tooth adjacent to the future location of the prothesis; the color of the prothesis is matched to that of the adjacent tooth for all types of lighting; and it is possible to determine the appropriate pigments and concentrations thereof that need to be added to a given color in a chart in order to reproduce the exact color of a tooth adjacent to the future location of a prothesis.

Naturally, the method of the invention is not limited to the particular application described, and could be used for matching the colors of objects other than teeth, for example when restoring art objects or when matching items of jewelry such as pearls in a necklace, or when matching cloth.

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