Index for Determining a Quality of a Color

Abstract

The present invention relates to a method for determining a characteristic number for characterizing the quality of a shade setting of a paint in relation to a color reference, characterized in that colorimetric coordinates of the paint and of the color reference are determined with a spectrophotometer for a number of measurement geometries and wherein respective color differences are calculated and standardized from the colorimetric coordinates of the paint and of the color reference for each measurement geometry of the number of measurement geometries, and where a group of characteristic values calculated from the respective standardized color differences is assigned a scale value for determining the characteristic number by means of an assignment rule to be provided in advance.

Claims

1. A method for determining a characteristic number for characterizing the quality of a shade setting of a paint in relation to a color reference, the method comprising: determining colorimetric coordinates of the paint and of the color reference with a spectrophotometer for a number of measurement geometries and using at least one illuminant; calculating and standardizing color differences from the colorimetric coordinates of the paint and of the color reference for at least one of: (i) each measurement geometry of the number of measurement geometries, and (ii) each at least one illuminant used; and assigning a scale value for determining the characteristic number to a group of characteristic values calculated from the respective standardized color differences using an assignment rule provided in advance.

2. The method as claimed in claim 1, further comprising: standardizing at least one of respective sparkle differences and graininess differences between the paint and the color reference, measured with a goniospectrophotometer; and using the at least one of the respective sparkle difference and graininess differences in a calculation of the characteristic number, where the assignment rule is extended for the corresponding at least one of the sparkle differences and graininess differences between the paint and the color reference.

3. The method as claimed in claim 1, further comprising calculating the color differences for at least one of achromatic and chromatic colorimetric coordinates with respective color difference formulae.

4. The method as claimed in claim 1, further comprising using a sum of all the standardized color differences between the paint and the color reference for calculating respective characteristic values.

5. The method as claimed in claim 1, wherein the assignment rule for solid-color paints comprises rules which specify that a characteristic value, as a maximum value of at least one of color differences and gray-stage differences between a respective solid-color paint and a corresponding color reference, each measured under different illuminants, for a particular measurement geometry, is assigned to a scale value 1 on a basis of a color difference formula based on an L*C*h.sup.∘ color space, when the characteristic value is greater than or equal to 6, and that the characteristic value is assigned to a scale value 2 when the characteristic value is less than 6, and that the characteristic value is assigned to a scale value 3 when the characteristic value is less than 4.5, and that the characteristic value is assigned to a scale value 4 when the characteristic value is less than 3, and that the characteristic value is assigned to a scale value 5 when the characteristic value is less than 2, and that the characteristic value is assigned to a scale value 6 when the characteristic value is less than 1.7, and that the characteristic value is assigned to a scale value 7 when the characteristic value is less than 1.4, and that the characteristic value is assigned to a scale value 8 when the characteristic value is less than 1.0, and wherein the assignment rule for effect paints comprises rules which specify that assignment to a scale value 1 is made when a first effect characteristic value, formed on a basis of a sum of all the color differences between an effect paint and an effect paint reference for a number of measurement geometries between 25° and 75°, is greater than or equal to 12 and respective effect color differences between the effect paint and the effect paint reference, determined for the measurement geometries of 25°, 45°, and 75°, are greater than or equal to 6 when considered each on their own, and that assignment to a scale value 2 is made when the first effect characteristic value is less than 12 and each of the effect color differences determined for the measurement geometries of 25°, 45°, and 75° is less than 6, and that assignment to a scale value 3 is made when the first effect characteristic value is less than 10 and each of the effect color differences determined for the measurement geometries of 25°, 45°, and 75° is less than 4.5, and that assignment to a scale value 4 is made when the first effect characteristic value is less than 6 and each of the effect color differences determined for the measurement geometries of 25°, 45°, 75° is less than 3, and that assignment to a scale value 5 is made when the first effect characteristic value is less than 3.9 and each of the effect color differences determined for the measurement geometries of 25°, 45°, 75° is less than 2, and that assignment to a scale value 6 is made when a second effect characteristic value, formed for a number of measurement geometries between 15° and 110°, is less than 6.5 and each of the effect color differences determined for the measurement geometries of 15°, 25°, 45°, 75°, and 110° is less than 2, and that assignment to a scale value 7 is made when the second effect characteristic value is less than 6.5, each of the effect color differences determined for the measurement geometries of 15°, 25°, 45°, 75°, and 110° is less than 1.73, and each of a sparkle difference between the effect paint and the effect paint reference, determined for the measurement geometries of 15°, 45°, 75°, is less than 1.73 and a graininess difference between the effect paint and the effect paint reference is less than 1.73, and that assignment to a scale value 8 takes place when the second effect characteristic value is less than 6.5 and each of the effect color differences formed for the measurement geometries of −15°, 15°, 25°, 45°, 75°, and 110° is less than 1.41, and each sparkle difference between the effect paint and the effect paint reference, determined for the measurement geometries of 15°, 45° and 75°, is less than 1.41, and the graininess difference between the effect paint and the effect paint reference is less than 1.41.

6. A computer program with program code means to implement all steps of a method as claimed in claim 1, wherein the computer program is executed on at least one of a computer and a corresponding computing unit.

7. The computer program as claimed in claim 6, stored on a computer-readable data medium.

8. A method for determining a characteristic number for characterizing the quality of a shade setting of a vehicle paint in relation to a color reference applied to a vehicle, the method comprising: determining colorimetric coordinates of the paint and of the color reference with a spectrophotometer for a number of measurement geometries and using at least one illuminant; calculating and standardizing color differences from the colorimetric coordinates of the paint and of the color reference for at least one of: (i) each measurement geometry of the number of measurement geometries, and (ii) each at least one illuminant used; assigning a scale value for determining the characteristic number to a group of characteristic values calculated from the respective standardized color differences using an assignment rule provided in advance; and determining a color adjustment quality of the vehicle paint based on the scale value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIG. 1 shows an assignment table with assignment rules for assignment of respective color differences to a respective scale value of the characteristic number Q in accordance with one possible embodiment of the method presented.

[0041] FIG. 2 shows embodiments of the inventively envisaged characteristic number Q in relation to an average of respective color differences between five different paints and one color reference.

[0042] FIG. 3 shows a graphical representation of the color differences from FIG. 2.

[0043] FIG. 4 shows embodiments of the inventively envisaged characteristic number Q for different solid-color paints.

[0044] FIG. 5 shows a graphical representation of color differences of the different solid-color paints from FIG. 4.

[0045] FIG. 1 shows a Table 10 with three columns 13, 15, and 17. The first column, 13, indicates scale values of a characteristic number Q, which span a rating scale from 1 to 8, with 1 indicating the poorest quality, i.e., the worst possible match of a respective paint with a corresponding color reference, and 8 the best quality, i.e., the best possible match of a respective paint with a corresponding color reference.

[0046] Column 15 indicates assignment rules for calculated color differences from spectral measurements of an effect paint and of a corresponding color reference to a respective scale value of the characteristic number Q in the form of clear concrete equations. In order to assign a quality of a correspondence of an effect paint to a respective color reference of a characteristic number Q, there are at least four and at most ten conditions each defined by a specific equation, that must be met. The term “equation” here is to be interpreted broadly, namely both as “equation” in the narrower sense, as a statement concerning the identity of two terms, and also as “inequation” in the narrower sense, as a comparison of magnitude between two terms.

[0047] It is specified in line 1 of column 15, for example, that when a sum of all the calculated color differences for measurement geometries between 25° and 75°, i.e.,

[00002]$\underset{25}{\overset{75}{.Math.}}.Math.<\mathrm{dE}>,$

is greater than or equal to a value of 12, and each of the measurement geometries 25°, 45°, and 75° when considered alone is greater than or equal to a value of 6, the respective paint is assigned to the characteristic number Q with the scale value 1. Since the characteristic number Q with the scale value 1 represents the lowest quality stage here, assignment to the characteristic number Q=1 is also made if, for example, the sum of all the measured color differences for measurement geometries between 25° and 75°, i.e.,

[00003]$\underset{25}{\overset{75}{.Math.}}.Math.<\mathrm{dE}>,$

is greater than or equal to 12 but, for example, the color difference for one measurement geometry of the measurement geometries 25°, 45°, and 75° when considered alone is less than 6.

[0048] The sum

[00004]$\underset{25}{\overset{75}{.Math.}}.Math.<\mathrm{dE}>$

corresponds in general to a summation of the color differences for the individual measurement geometries 25°, 45°, and 75°. Correspondingly,

[00005]$\underset{15}{\overset{110}{.Math.}}.Math.<\mathrm{dE}>$

corresponds to a summation of the color differences for the individual measurement geometries 15°, 25°, 45°, 75°, and 110°.

[0049] At all higher quality stages, i.e. Q=2, 3, 4, 5, 6, 7, and 8, all quality criteria and all equations assigned to the respective quality stages must be met in each case by the color differences determined, for a corresponding paint to be assigned to the respective quality stage or to the corresponding characteristic number Q=2, 3, 4, 5, 6, 7 or 8.

[0050] Since in the case of effect paints there are also sparkle differences and graininess differences to be taken into account, provision is made, especially in the case of high characteristic numbers, corresponding to a high quality of a match between a respective effect paint and a corresponding color reference, for intensified conditions, i.e., additional conditions, to apply. This means that in order, for example, to satisfy respective criteria for an assignment to the characteristic number Q with a value of 7, for a respective effect paint, no color difference dE calculated in each case on the basis of measurement geometries between 15° and 110° relative to a corresponding color reference may have a value greater than or equal to 1.73. Correspondingly, each color difference dE calculated for the measurement geometries 15°, 25°, 45°, 75°, and 110° must be smaller than 1.73. In addition, a sparkle difference dS between the respective effect paint and the corresponding color reference may not be greater than or equal to a value of 1.73 for any of the measurement geometries 15°, 45°, and 75°. Moreover, a graininess difference dG between the respective effect paint and the corresponding color reference may not be greater than or equal to a value of 1.73.

[0051] Since solid-color paints frequently exhibit a metamerism effect, i.e., a different color or gray level on illumination with different illuminants, independent assignment rules are envisaged for solid-color paints, as shown in column 17.

[0052] For the assignment of a respective solid-color paint to a respective scale value with the characteristic number Q, a maximum value of a color difference or of a gray level difference is calculated between a respective solid-color paint and a corresponding color reference, each measured under different illuminants, as for example under illuminant D64 and illuminant TL84, for a measurement geometry of, for example, 45° in each case, on the basis of a color difference formula CIE94 based on an L*C*h.sup.∘ color space. This means that the solid-color paint is assigned, for example, to a characteristic number Q with a scale value of 1, when the respective solid-color paint has a maximum value among the determined color differences between the respective solid-color paint and the corresponding color reference, based on spectral measurements under illuminants D64 and TL84, of greater than or equal to a value of 6.

[0053] For assignment to other values of the characteristic number Q, corresponding criteria are defined in columns 15 (for effect paints) and 17 (for solid-color paints) of Table 10.

[0054] FIG. 2 shows a Table 20 which in a first column 21 indicates different effect paints and in column 23 indicates an average color difference calculated for a color reference over different measurement geometries of 15°, 25°, 45°, 75°, and 110°, and also indicates, in column 25, a corresponding scale value of the characteristic number Q. The characteristic number Q is, in particular, ordinally scaled, and so respective scale values of the characteristic number Q are always integral.

[0055] All of the paints listed in column 21 were compared with an individual color reference, in order to identify the paint having the best match to the single color reference.

[0056] Whereas for checking in accordance with the prior art it is possible only to compare the color differences determined at different measurement geometries, possibly individually for different illuminants, a qualitative match of a respective paint from column 21 to the color reference can be assessed on the basis of the characteristic number Q, as shown in column 25, while simultaneously considering a plurality of quality criteria, such as, for example, a plurality of color differences determined under different illuminants and/or different measurement geometries.

[0057] Since paint “05”, i.e., the paint from the fifth line of Table 20, shows the highest scale value for Q, namely 6, the paint 5, out of all the paints shown in Table 20, is the best match with the color reference.

[0058] Column 23 shows average color differences calculated via an arithmetic mean. It should be emphasized in this context that none of the average color differences specified in column 23 allows a conclusive assessment to be made about the quality of a match between a respective paint and the color reference. Whereas traditional measured values, such as an average color difference, for example, allow only a comparison on the basis of one condition, i.e., one position in the color space, it is possible via the characteristic number Q to jointly assess and quantify a plurality of criteria, such as, for example, color difference in the color space, sparkle difference, and behavior under different illuminants.

[0059] An increase in respective scale values of the characteristic number Q, as are shown in column 25, such increase having been determined on the basis of measurement values under different illuminants, may be accompanied by a reduction in corresponding average color differences MDE, as shown in column 23. Correspondingly, the values in columns 23 and 25 are generally inversely proportional to one another.

[0060] FIG. 3 shows an experimental profile for discovering a paint with optimal correspondence to the color reference, from FIG. 2, on the basis of a comparison of color differences.

[0061] A diagram 31, comprising an abscissa 33, which indicates a respective paint 1 to 5, and an ordinate 35, which indicates color differences dE calculated for respective measurement geometries of 15°, 25°, 45°, 750, and 110° between the respective paint and the color difference, is a graphical representation of the paints described in FIG. 2 and of their respective color differences with respect to a corresponding color reference.

[0062] In order to compare the five different paints in FIG. 2 with one another and to identify the paint which matches best with the color reference, it is necessary, traditionally, to compare the color differences of the respective paints 1 to 5, plotted in the diagram 31, for each of the measurement geometries of 15°, 25°, 45°, 75°, and 110°, manually by a skilled person. In this context it is difficult to differentiate in particular between paints 2 and 5, since both paints show comparable color differences and match both better and worse with the color reference, relative to the other respective paint, for different measurement geometries. It is therefore almost impossible to find an objective basis for a selection between paint 2 or paint 5, whereas the characteristic number Q, as shown in FIG. 2, identifies paint 5 without doubt as the paint best matching with the color reference, and evaluates paint 2 as even poorer than paint 4.

[0063] By means of the inventive characteristic number Q, accordingly, a respective paint best matching with a color reference can be identified quickly and without expert knowledge by a comparison of merely one respective characteristic number Q per paint.

[0064] FIG. 4 shows a Table 40 which, in analogy to FIG. 2, describes five different paints in column 41 and shows corresponding characteristic numbers Q in column 43. Since the paints described in Table 40 are solid-color paints, measured only at a measurement geometry of 45°, no averaged color difference has been calculated. This means that there is only one color difference for each paint per illuminant, in this case illuminant D65, available for assessment.

[0065] FIG. 5 shows a diagram 51 which corresponds to Table 40 in FIG. 4 and which comprises an abscissa 53, which indicates a respective paint 1 to 5, and an ordinate 55, which indicates color differences dE calculated between respective paint and a color reference. In order to compare the different solid-color paints in respect of any metamerism effect as well, different measurements under different illuminants are necessary, which must be assessed in each case via an independent diagram and/or via different color differences, in order to identify a paint best matching the respective color reference.

[0066] Since the inventive characteristic number Q unites different measurement values, determined under illuminants which are different, for example, it is possible to compare a solid-color paint, even with respect to a metamerism effect, with a respective color reference, quickly and easily, on the basis of just a single characteristic number Q per paint.