FR2768513A1 - Comparative colourmetric analysis and equipment - Google Patents

Abstract

The chromaticity coordinates of colourmetric reference elements are plotted on a CIE diagram to identify reference wavelengths. The ratio of reflected energy from an object at these two wavelengths may then be used to identify a particular colourmetric reference element. The procedure may be automated by electronic equipment A color which is already known approximately may be defined precisely by calculating the chromaticity coordinates of selected colourmetric reference elements and plotting them as a line on a CIE diagram. The intersection of the line with the diagram boundary defines two reference wavelengths and the ratio of the reflected energy in those wavelengths from the object being measured allows a point to be located on the CIE diagram straight line and the corresponding colourmetric reference element to be identified. These procedures may be automated by using an equipment which consists of a motor control microcomputer (1), step by step operation motor and interface (2,3), filter disc (4), light source (6), photodiode (7), operational amplifier (8), object (9), display (10), light and color microcomputer (11) and optical fibers (61,62,63)

Description

 The invention relates to a method for comparative colorimetric analysis as well as a corresponding analysis device for implementing this method.

 Comparative colorimetric analysis, hereinafter also called colorimetric comparison, finds numerous applications in the industrial field and in other fields. Let us cite for the industrial field the application of a paint, on a vehicle where a newly painted part must have the same color as the part still coated with the initial paint, or the search for a fabric or a skin whose color should be as close as possible to that of a fabric or skin in need of repair. We can also cite the possibility of evaluating the age of an object using the color change it has undergone through aging. Finally, let us cite the field of the manufacture of dental prostheses where it is a question of giving a prosthesis a color which is as close as possible to the color of the neighboring tooth or teeth.

 Currently, the color of a dental prosthesis is defined by the dentist using a color chart representing different shades or colors of white teeth. The practitioner places the color chart, shade by shade, next to the reference tooth, adjacent to the prosthesis to be manufactured, until the shade of the color chart corresponds, by visual comparison, to the color of the reference tooth. The result of this comparison, and in particular the precision with which the comparison is made, depends mainly on the vision of the practitioner, the lighting of the tooth and the color chart. The comparison is made even more difficult if it is necessary to interpolate between two colors very close to the color chart. The quality, even the accuracy, of the comparison is also influenced by the condition of the surfaces to be compared.

Indeed, most often, the reference tooth is in the mouth, that is to say, it is moist and therefore shiny. Its colorimetric appearance, with equal color, therefore does not correspond to that of the dry surface of the color chart plates. And even in the case of a dental appliance repair,
The appearance of the teeth when the appliance is in the mouth, may be different from that which it has when it came out of the mouth and is therefore dry. In addition, this difference can be a function of the material in which the prosthesis will be produced.

 Another difficulty which influences the precision with which the colorimetric comparison is carried out, relates to the lighting under which the tooth or any other object to be compared, is usually seen. Thus, the artificial light of a television studio or the light of the projectors used during a film shooting can be quite different from the light used by the practitioner to make the comparison.

 Recognition and definition of colors and hues were facilitated by a chromaticity diagram recommended by the International Commission on Lighting (CIE) in 1931.

 According to this diagram, in which x and y are chromaticity coordinates in a two-dimensional orthonormal graph, each color is represented by a point within an area limited by the spectral location and the line of the purples, said later limited by a limit curve.

 To establish this colorimetric diagram, three distribution coefficients were defined as corresponding to the basic reference stimuli of an average observer.

A given color, analyzed by three cells, each of which is corrected by a filter reproducing one of these coefficients, provides corresponding fluxes X, Y, Z (tristimuli). It is represented in the CIE graph, 1931 which is two-dimensional, by the chromaticity coordinates x and y according to the definitions:
X x = ------- and
X + Y + Z
Y y =
X + Y + Z
Obtaining the chromaticity x, y coordinates therefore usually involves the use of three optical measurement cells. The precision with which a given color is identified depends on the more or less perfect agreement between the theoretical curves and the real curves of the combinations of optical cells / filters used, or even the use of spectrophotometers.

 These devices are generally expensive and bulky and require delicate and often quite long implementation.

 The difficulties mentioned above are further increased in applications such as the colorimetric comparison of a tooth with a color chart, when the surface of the object to be compared does not have a uniform color and it is then necessary to carry out the comparison. on several areas of the object.

 The object of the invention is to find a solution which makes it possible to remedy the drawbacks described above.

 Advantageously, this solution must allow a comparative colorimetric analysis in such a way that it is possible to identify a color with respect to a predefined color chart or shade guide, whatever the field of application, and in particular to render the colorimetric comparison as independent as possible from the subjective faculties of the operator of the comparison.

 The object of the invention is achieved by a method of comparative colorimetric analysis of a surface of an object with respect to a predetermined set of reference colorimetric elements using a colorimetric diagram established by the CIE in 1931, this diagram comprising an area limited by a limit curve. This limit curve consists of the spectral location of monochromatic radiation and the straight line of purples. Each color is represented by a point located inside the area.

 The chromatic diagram of the CIE 1931 has been the subject of various publications. In particular, it is described in the OPTICAL PHYSIOLOGICAL treatise by Yves Le Grand (Tome II Lumière et couleurs; Masson & Cie, Editeurs, 1972).

 The method of the invention does not require the exploration of the entire colorimetric domain if it is known in advance that the color is located in a particular domain of the chromatic diagram.

 Indeed, in many applications, it is possible to know in advance certain properties of the colors analyzed and it is then necessary to find the particular color of the object which serves as a reference. Thus, for example, in the field of manufacturing dental prostheses, a tooth will always be white, the white going from pure white to off-white with a shade more or less yellow or more or less blue. In a comparable way, a piece of wood will always be brown, brown can vary from beige to dark brown. A painted object will keep its original base color, the variation due to aging generally bringing more and more clear colors. And a fabric will also keep its original color, the variation due to aging can however bring lighter or darker shades depending on the nature of the dyes used.

 This concentration of the colorimetric aspects of the objects, according to their nature, around a basic color and the fact that this color is compared with others, makes it possible to simplify the measurement and by that the comparative colorimetric analysis of the surface of an object. In particular, it has been found that a detailed analysis of a color often amounts to positioning it on one or more lines in the CIE 1931 diagram.

The process of the invention has the following characteristics:
- the reference colorimetric elements are characterized by their chromaticity coordinates and each is represented by a point placed on said diagram of the
CIE;
the set of points thus represented is approached by a segment carried by a straight line;
- said straight line intersects the limit curve of the diagram of the
CIE at two intersection points defining two reference wavelengths;
- the surface of the object is illuminated by a given light source and the ratio between the energies reflected by the object is measured in two wavelength ranges centered respectively on one and the other of the two reference wavelengths;
- the color of the object is assimilated to that of the colorimetric element whose corresponding point is located at distances from the intersection points such that the ratio between the distances is close to the inverse of the ratio between the energies reflected by object.

The invention also relates to the following characteristics, considered in isolation or in all their technically possible combinations:
- the set of points representing the reference colorimetric elements is represented on the diagram and approached by one or more segments carried by different lines;
- one or more pairs of reference wavelengths are defined;
- several pairs of spectral zones are defined, each spectral zone of which is concentrated around a reference wavelength.

 The pairs of spectral zones are defined beforehand for each application. Thus, for example, the analysis of the white color in the field of the manufacture of dental prostheses leads by the measurement of the elements of a color chart to two blue and yellow spectral zones.

 - In addition, the brightness of the object to be compared is measured.

 This arrangement makes it possible to take into account the fact that the colorimetric appearance of an object, with equal color, can vary according to the state of the surface of the object, for example according to whether this surface is dry or wet, or smooth or more or less rough.

 The brightness of an object having an influence on the light intensity of the light reflected by an object, regardless of its wavelength, this intensity is measured through a neutral filter, i.e. d '' a filter with uniform spectral attenuation.

 Advantageously, the object is illuminated and the reflected light is measured in the absence, or sheltered, of ambient light.

 This arrangement has the particular advantage of making the comparison with a precisely defined quantity of light.

 - The light illuminating the object to be compared is modulated in intensity.

 In general, the object to be compared is lit by continuous light, that is to say by uninterrupted light and having a constant light intensity. However, it may be useful to vary this intensity, for example by transforming the incident light into pulsed light, the modulation being obtained by intervention at any level of the lighting system whatsoever, for example on the light source. .

 - The incident light illuminating the object to be compared is white light chosen from natural light, artificial daylight and light specific to the patient's activities.

 This arrangement takes into account the fact that the color of a dental prosthesis, which must correspond to that of the neighboring tooth or teeth, may vary depending on the light with which it is lit in the mouth.

 In the context of the present invention, to facilitate reading of the following description of the invention, the term color will include colors and their tints. Thus, for example, the terms white and approximately white used with reference to a tooth, designate all the usual colors or shades of a tooth.

 - The object of the comparison is a tooth in the mouth of a patient. This tooth is located next to a place where a dental prosthesis having the same colorimetric appearance as the comparison tooth should be inserted or disposed.

 - Parameters representative of the predetermined set of reference colorimetric elements are introduced into a memory. Advantageously, this introduction is carried out prior to a comparative colorimetric analysis.

 This prior introduction of the reference colorimetric elements can be carried out essentially according to two different modes. The first mode of introduction consists in illuminating, directly before the comparative colorimetric analysis of an object, reference objects, carrying several colors close to the color to be analyzed, by a given light, capturing the lights reflected by the reference object according to the different colors and generate electrical signals representing their chromaticity coordinates.

 The second method of introduction consists in introducing into the apparatus, in electronic or computer form, like a data library, several sets of colorimetric elements, each representing a range of different colors. This library can be supplemented, for example, by sets of color elements representing different colors, but taking into account certain influences on the color aspect produced by the nature of the lighting.

 The object of the invention is also achieved by a device for implementing the method described above.

 This device for the comparative colorimetric analysis of a surface of an object with respect to a predetermined set of reference colorimetric elements, comprises a light generator, means making it possible to direct the light in the form of a light beam on the surface of the object, at least one optical sensor for separately capturing two components of the light reflected by said object and for generating electrical signals as a function of the reflected light, means for receiving the electrical signals and for comparing them with the 'predetermined set of reference colorimetric elements, and means intended to emit a result signal representative of the comparison.

The invention also relates to the characteristics below considered in isolation or in all their technically possible combinations:
- The set of colorimetric elements is represented by electrical signals introduced into a memory connected to the comparison means.

 - The device comprises a single optical sensor and two filters intended to allow two components of the light reflected by said object to pass to the sensor.

 This arrangement makes it possible to overcome a drift (mainly by a heating effect) of the sensor, since we use relationships between two values obtained by the same sensor.

 - The means for directing the light beam emitted by the generator comprise at least one optical fiber.

 - The device includes a bundle of optical fibers.

The end of the beam intended to be directed towards the object to be analyzed is advantageously provided with a chamfered cover relative to the axis of the beam, and sheltering the surface to be illuminated with ambient light.

Other characteristics and advantages of the invention will emerge from the description of an embodiment given below with reference to the drawings. In these drawings:
Figure 1 represents the chromaticity diagram established by the International Commission on Lighting (CIE) in 1931.

 FIG. 2 represents the principle of a device for comparative colorimetric analysis according to the invention.

FIG. 3 represents the device of FIG. 2, adapted to a measurement with three sensors, and
FIG. 4 represents the diagram of a device for comparative colorimetric analysis according to an advantageous embodiment of the invention.

Figure 1 represents the chromaticity diagram established by the International Commission on Lighting (CIE) in 1931. On this diagram, we can essentially distinguish six different zones referenced by letters. Five of these different zones represent chromatic domains (colors) whose dominant wavelengths are located respectively in red (R), orange (O), yellow (J), green (V) and blue (B). The sixth area represents white (W). The set of colors is limited by the limit curve
CL.

 By approaching the chromatic domain in which the color of the object to be compared is located, by one or more lines (fig. 3), the operator performing the comparative colorimetric analysis will choose the reference wavelength pair (s) A1 and 2 appropriate to which it will measure the energies reflected by the object.

 Indeed, the color of a reference colorimetric element is represented by a point M located on said diagram of the CIE. A predetermined set of reference colorimetric elements is approached by a segment carried by a straight line D. The wavelengths Ai and 2 are those of the intersection of the straight line thus defined with the limit curve of the diagram in two points of intersection P, and P2.

 To obtain the color of the object to be compared, its surface is illuminated by a given light source and the ratio between the energies reflected by the object in two wavelength ranges centered respectively by one is measured. and the other of the two reference wavelengths.

Then, we assimilate the color of the object to that of the reference colorimetric element whose corresponding point M is located at distances d1 and d2 from the points of intersection P1 and P2 such that the ratio between the distances is close to the inverse of the relationship between the energies reflected by the object.

 Advantageously, the operator has a library constituted by parameters representative of a set or of several predetermined sets of reference colorimetric elements and of the corresponding reference wavelengths. This library can be of different natures. We cite as a non-exhaustive example, on the one hand, a collection of color charts and on the other hand, a database. In the case of a collection of color charts, each color chart includes a plurality of plates each of which represents a shade or shade of a color or a color of a range of neighboring colors.

 In the case of a database, parameters representative of different colors of different chromatic domains and, advantageously, also parameters representative of reference wavelengths, are introduced into a memory to which the operator of the analysis comparative has access, for example by means of a comparative analysis device.

 The device of the invention, intended for the colorimetric comparison of the surfaces of two objects, is constituted according to the principle represented in FIG. 2. A light source 6 sends light having a given chromatic spectrum through an optical fiber 61 , thus forming an incident light beam on the surface of an object 9.

The light reflected by the surface of the object 9 is transmitted through a receiving optical fiber 62, passing through a filter 5 with a uniform spectral response in the visible spectrum, to an optical sensor 7. The sensor 7 generates a electrical signal corresponding to the brightness of the light having passed through the filter 5. The electrical signal is received by an amplifier 8 for an appropriate electronic processing resulting in an output signal which can be transmitted to a display element 10 such as for example a measuring instrument 10 calibrated in any unit of measurement allowing this value to be used, or a device displaying the name of the color analyzed.

 According to this principle, we first define the reference wavelengths Ài À2 and the filters centered approximately on these wavelengths. For the examples below, the light source 6 is assumed to send white light, and the reference wavelengths are located in the blue and yellow domains.

 A colorimetric comparison device taking into account two chromatic components of the reflected light can be designed essentially according to the variant shown in FIG. 3. According to this variant, the device comprises three optical reception fibers including two reception fibers 62 for measuring the chromatic components and a receiving fiber 63 to obtain a reference measurement. While the two receiving optical fibers 62 each terminate on a filter intended to allow the blue or yellow component to pass respectively, the receiving optical fiber 63 terminates either directly on the sensor 7 or on a filter which makes it possible to obtain a spectral response as close as possible to that of the eye in photopic vision and which prevents the passage of infrared and ultraviolet. Such a filter is qualified in the examples which follow, as blank. This reference measurement makes it possible to take into account variations in the brightness of the light source 6 and thus compensate for the effects of variation due to aging of the lamp, to variations in voltage or to any other disturbance, or quite simply to an exchange of the lamp.

 The absence of a white filter between the end of the receiving fiber 63 and the corresponding sensor 7 is conceivable, if the latter has - by construction - a spectral response corresponding to that of a white filter.

 Each of the sensors 7 generates a signal corresponding respectively to one of the chromatic components, to the other chromatic component or to white light. These electrical signals are each sent to a corresponding operational amplifier which generates an output signal. This output signal is sent, passing through a measuring or display instrument 10 or directly to a signal processing device allowing appropriate processing of these signals.

 In practice, the device described above will initially be used to measure, define and record the respective values of two chromatic components of each color of a reference palette or of a given color chart. This reference palette can be used to know, for example, the state of degradation or aging of an object, caused by time, by light or by any aggression such as chemical. To this end, the palette presents gradual stages of degradation and allows, with a single and rapid measurement, to determine the state of the object.

 Similarly, the colors of the color chart intended for the manufacture of dental prostheses are first measured.

Next, the color of a comparison tooth in the mouth of a patient is measured and the result obtained is compared with the predetermined values recorded in a memory of the comparison means. The result of this comparison attributes the color of the comparison tooth to one of the colors of the color chart or shade guide, or to a value interpolated between two colors of the color chart. When making a dental prosthesis, the manufacturer refers to this identification of the color and manufactures the dental prosthesis accordingly.

 The output signals of the amplifiers 8 can be used directly, for example by transmitting them, without display on a display instrument 10, to a pigment mixer for the manufacture of the tooth. This is valid as well in the case where the comparison between the tooth and the color chart is carried out in the laboratory of manufacture of the dental prosthesis as in other cases where a transmission of these data at distance is necessary.

 According to an advantageous embodiment of the invention, the comparison device is designed around a single optical sensor 7 and with a support, such as a disc 4, making it possible to successively place several filters, for example a first color filter "and a second color filter, a white filter and a neutral filter, that is to say having a uniform spectral attenuation, in front of the sensor. It goes without saying, without departing from the scope of the present invention, on the one hand, the support can have other forms, in particular non-rotating and that, on the other hand, All of these filters can be replaced by a single filter whose transmission or reflection characteristics can be modified by appropriate means.

 According to this design shown in FIG. 4, a microcomputer 1 controls a stepping motor 2 via an interface 3. The motor 2 actuates the disc 4 carrying four filters 5. The filters 5 are arranged at the periphery of the disc 4, for example in order, blue filter 51, yellow filter 52, white filter 53 and neutral filter 54.

 It is understood that the white filter 53 can at the same time be a filter reducing the light level of white light. As a variant, the filter 53 can be omitted, if the sensor 7 is already corrected by design. In this case, there is a simple notch in the periphery of the disc 4.

 With regard to the use of a neutral filter in certain applications, it should be noted that it would be insufficient to take into account only the color of the object, when it is an object whose surface is made shiny for example by humidity, such as a tooth in the mouth, or whose shine depends, at least in part, on the material of which the object is made. A ceramic tooth has a different shine than a resin tooth, etc.

 The light generator 6, for example a halogen lamp whose color temperature is of the order of 3200 to 3300 K, sends the light by an optical fiber 61 to the surface of the object 9, the comparison tooth, and simultaneously sends white light through the optical fiber 63 to the disc 4.

 The light reflected by tooth 9 is transmitted by optical fiber 62 to disk 4.

 In a first step, the sensor 7, for example a silicon photodiode, receives directly, but preferably attenuated by the white filter 53, the light from the lamp 6 by the optical fiber 63 and generates a corresponding electrical signal. The microcomputer It records and stores the result of this measurement.

 Then, the first color filter, in the example chosen, the blue filter 51, is presented in front of the sensor 7. The sensor 7 receives the light reflected by the object 9 through the optical fiber 62 and filters it and generates a signal corresponding electric. The microcomputer 11 records the electrical signal generated by the photodiode 7 and processed by the operational amplifier 8, in a memory.

 In a third step, the second color filter, in the example chosen, the yellow filter 52, is placed in front of the photodiode 7. The sensor 7 receives the light reflected by the object 9 through the optical fiber 62 and the filter and generates a corresponding electrical signal. The microcomputer 11 records and stores the third measurement result. Following the measurements made, the microcomputer 1 1 establishes the yellow / blue ratio of the light intensities of the two components representing the color of the comparison tooth.

 In a fourth step, the neutral filter 54 is placed in front of the sensor 7, when the three white, blue and yellow measurements have been made. The sensor 7 receives the light reflected by the object 9 through the optical fiber 62 and filters it and generates a corresponding electrical signal. The microcomputer records and stores the result of the measurement of the brightness or luminance of the object.

 As a variant, before establishing the yellow / blue ratio to determine the color of a tooth to be analyzed, the intensity of the light emitted by the light generator is measured through a filter which may be attenuating or reduced by reduction of the section of the corresponding light pipe, and the brightness of the tooth through the same filter or an additional filter, to determine the luminance value, i.e. the brightness or the brightness of the tooth, expressed by the ratio of these two values. Then, the energy Ài and the energy + X2 reflected by the object to be analyzed are measured in the two wavelength domains centered respectively by one and the other of the two reference wavelengths À1 and 2 of the blue and yellow components and forms the relationship between these two values. The result obtained by these measurements is compared with the reference values of the color chart previously recorded.

The result of this comparison is displayed, for example in the form of the tooth reference according to the criteria of a standard color chart or shade guide. This reference can also be transmitted remotely to the laboratory for manufacturing the dental prosthesis to be displayed there or, alternatively, to be introduced there into a control device intended to control an automatic mixer of the ingredients necessary for the manufacture of the dental prosthesis. .

Independently of the chosen embodiment of the colorimetric analysis device according to the invention, the ends of the optical fiber 61 directing the white light on the tooth 9 and of the optical fiber or fibers 62 work, repeatedly, for each zone of the graduation
The reference signs inserted after the technical characteristics mentioned in the claims, have the sole purpose of facilitating the understanding of the latter, and in no way limit their scope.

Claims (12)

 1. Method for comparative colorimetric analysis of a surface of an object in relation to a predetermined set of reference colorimetric elements using a diagram established by the CIE (International Commission on Lighting) in 1931 and comprising a limited area by a limit curve, characterized in that:  - the reference colorimetric elements are characterized by their chromaticity coordinates and each is represented by a point (M) placed on said diagram of the CIE;  - the set of points (M) thus represented is approached by a segment carried by a straight line (D);  - said straight line (D) intersects the limit curve of the CIE diagram at two intersection points (P1, P2) defining two reference wavelengths (À1, 2);  - the surface of the object is illuminated by a given light source and the ratio between the energies ((t) À1, é reflected by the object in two wavelength domains centered respectively by the one and the other of the two reference wavelengths (λ1, 2);  - the color of the object is assimilated to that of the reference colorimetric element whose corresponding point (M) is located at distances (d1, d2) from the points of intersection (P1, P2), such as the ratio between the distances (d1, d2) is close to the inverse of the ratio between the energies reflected (Ài, h,) by the object.  2. Method for comparative colorimetric analysis according to claim 1, characterized in that:  - The set of points (M) shown is approached by one or more segments carried by different lines, and  - one or more pairs of reference wavelengths (A1, A2) are defined.  3. Method according to claim 1 or 2, characterized in that, in addition, the brightness of the object is measured.  4. Method according to any one of claims 1 to 3, characterized in that one performs the illumination of the object and the measurement of the reflected light in the absence of ambient light.  5. Method according to any one of claims 1 to 4, characterized in that the light illuminating the object to be compared is modulated in intensity.  6. Method according to any one of claims 1 to 5, characterized in that parameters representative of the predetermined set of reference colorimetric elements are introduced into a memory and that the reference colorimetric elements were obtained from 'reference objects whose surface has been illuminated by a light and whose chromaticity coordinates have been measured.  7. Method according to any one of claims 1 to 6, characterized in that the object is a tooth in the mouth of a patient next to which must be disposed a dental prosthesis having the same colorimetric appearance as the tooth.  8. Device for the comparative colorimetric analysis of a surface of an object with respect to a predetermined set of reference colorimetric elements, the device comprising:  - a light generator (6),  - means (61) for directing the light in the form of a light beam on the surface of the object,  - at least one optical sensor (7) for separately capturing two spectral components of the light reflected by the object and for generating electrical signals as a function of the reflected light,  - means (8) for receiving the electrical signals and for comparing them with parameters representative of the predetermined set of reference colorimetric elements, and  - means (10) intended to emit a result signal representative of the comparison.  9. Device according to claim 8, characterized in that it comprises means for measuring the brightness of the object.  10. Device according to claim 8 or 9, characterized in that it comprises a single optical sensor (7) and two filters (51, 52) intended to allow the two components of the reflected light to pass to the sensor (7).  11. Device according to any one of claims 8 to 10, characterized in that the means (61) for directing the light on the surface of the object comprise at least one optical fiber.  12. Device according to any one of claims 8 to 10, characterized in that it comprises a bundle (60) of optical fibers whose end intended to be directed on the surface of the object, is provided with a chamfered cover (64) sheltering the surface to be lit with ambient light.