JP6639192B2 - Color difference measurement method - Google Patents

Description

  The present invention relates to a method for measuring a color difference of a plurality of parts.

  For example, when applying the same color to a steel plate body and a resin bumper in the painting process of an automobile, even in the same color, the composition of the paint and the coating conditions (such as baking temperature) are different. It is often said. In such a case, it is difficult to completely match the colors of both members. In particular, when a paint containing a glittering material such as a metallic coating or a pearl coating is used, the appearance of the color depends on the arrangement of the glittering material ( (The degree of reflection) is greatly different. If the colors of the body and the bumper are slightly different, the color becomes discontinuous at a portion where the two members are adjacent to each other, which may impair the appearance. Therefore, it is necessary to adjust the composition of the paint and the coating conditions while checking the color difference (color difference) between the two members.

  Confirmation of such a color difference is usually performed visually by an operator. However, since it is not easy to visually confirm the color difference accurately, only a part of the skilled workers can perform the operation, and the working efficiency is very poor. In addition, since the sensitivity of the color difference tends to vary for each worker, the reliability of the confirmation result cannot be said to be sufficient.

  Therefore, methods for quantitatively evaluating the color difference have been studied. For example, Patent Literatures 1 to 3 below show a method of receiving reflected light in directions at a plurality of angles with respect to incident light and evaluating a paint color (color difference) based on the amount of received light of each reflected light. Have been.

JP 2009-264924 A JP 2006-292578 A JP 2012-173272 A

  As described above, by measuring the amount of reflected light received at a plurality of light receiving angles, the reliability of the color difference measurement result can be increased. In this case, it is preferable to measure the amount of reflected light received at as many light-receiving angles as possible, but since the number of light-receiving angles that can be actually measured is limited, there is a limit in improving the reliability. Was not able to be measured accurately. In particular, if the colors look different due to differences in the coating method or management conditions, the conventional method may not be able to detect the visual discomfort even if the color difference between the objects to be measured is extremely small. is there.

  The present inventors have verified whether a color difference visually confirmed is detected as a color difference by the method using the reflected light. First, the color difference between a plurality of portions Q1, Q2, and Q3 painted with the same paint was visually evaluated. As a result, it was confirmed that both of Q2 and Q3 had a color difference that should be corrected with respect to the reference point Q1.

Next, the color difference between Q1 and Q2 and the color difference between Q1 and Q3 were measured from the amounts of reflected light at a plurality of light receiving angles at the portions Q1, Q2, and Q3. Specifically, calculates the value of ^L * ^a * ^{b *} from the received light amount of the reflected light at a plurality of light-receiving angles at each site Q1, Q2, Q3 (details will be described later), the reference point Q1 ^L * a ^* b ^* was calculated difference Q2, Q3 of the ^L * ^a * ^{b *} values to the value.

As a result, the difference between the values of L ^* a ^* b ^* of Q3 with respect to the reference point Q1 was sufficiently large. Therefore, the color difference between Q1 and Q3 is sufficiently expressed as the difference between the L ^* a ^* b ^* values of both parts, and it can be said that the color difference can be detected by a method based on the amount of reflected light received. On the other hand, the difference between the L ^* a ^* b ^* values of Q2 with respect to the reference point Q1 was very small. Therefore, the color difference between Q1 and Q2 does not sufficiently appear as a difference between the L ^* a ^* b ^* values of both portions, and may not be detected by a method based on the amount of reflected light received.

  As described above, when visually confirming the color difference of a plurality of parts coated with the same paint, even if a color difference of a degree to be corrected can be confirmed, the measurement method using the amount of reflected light received is not sufficient. There is a possibility that the color difference cannot be detected and a defective coating is missed. In particular, in the case of paint containing a specific glittering material, the appearance of colors changes depending on the viewing direction, but in recent years paints have become more complicated and color differences must be checked in all possible directions. Cannot be matched with the visual sensation.

  An object of the present invention is to provide a method capable of accurately measuring (detecting) color differences at a plurality of sites.

  In order to achieve the above object, the present invention is a method for measuring a color difference of a plurality of parts, and irradiates each part with incident light and forms a different light receiving angle with respect to its regular reflection light. Receiving reflected light in a plurality of directions and measuring tristimulus values X, Y, and Z of the reflected light at each light receiving angle; and determining a relationship between the received light angle and each stimulus value X, Y, Z at each part. Calculating a color difference between the plurality of portions based on values of the stimulus values X, Y, and Z at arbitrary light receiving angles on the approximate curve. provide.

The stimulus values X, Y, and Z (JIS Z8701: 1999) are the reflection intensities of light with different intensities for each spectral wavelength, and are sensory characteristics like L ^* a ^* b ^* and L ^* u ^* v ^* described later. Is not reflected, and is proportional to the light reflection intensity. For this reason, the change of the stimulus values X, Y, and Z with respect to the light receiving angle tends to approximate a curve (such as a curve drawn using an exponential function). The present inventors have paid attention to this point, and have calculated an approximate curve of each stimulus value X, Y, Z and the light receiving angle. As a result, it is possible to obtain the stimulus values X, Y, and Z at not only the light receiving angles at which the reflected light is actually received, but also at any light receiving angles therebetween. By using the stimulus values X, Y, and Z, a color difference at a light-receiving angle at which light is not actually received can also be acquired, so that a color difference of a plurality of parts that were missed by the conventional method can be detected.

By the way, the difference (distance) between two points in the color space of the XYZ color system may not match the color difference feeling by the naked eye. Therefore, instead of using the tristimulus values X, Y, and Z of each part as they are for calculating the color difference, the tristimulus values X, Y, and Z are converted into coordinates in a uniform color space, and the color difference is calculated using the coordinates. Is preferred. The uniform color space is a coordinate space set so that the distance between two points on the coordinates and the sense of color difference with the naked eye match as much as possible. As a typical example, the L ^* a ^* b ^* color space (JIS) Z8781-4: 2013) and L ^* u ^* v ^* color space (JIS Z8781-5: 2013).

The L ^* a ^* b ^* color space is constructed from a subtractive color mixing idea, and is suitable for, for example, measuring the color difference of a painted portion. On the other hand, the L ^* u ^* v ^* color space is constructed from an idea of additive color mixture, and is suitable for measuring the color difference of a light-emitting body such as a liquid crystal display. As described above, the L ^* a ^* b ^* color space and the L ^* u ^* v ^* color space are constructed from different ideas, and each has advantages and disadvantages. A suitable color space is selectively used.

For example, when measuring the color difference of a painted surface of an automobile, an L ^* a ^* b ^* color space is usually used. However, according to the verification of the present inventors, it is possible to measure the color difference more accurately by using the L ^* u ^* v ^* color space for the paint color using a paint containing a glittering material such as pearl paint or metallic paint. It became clear that there were cases. Therefore, the L ^* a ^* b ^* color space is used in combination with the L ^* u ^* v ^* color space, and the largest color difference between the two spaces is adopted. Color difference can be accurately measured.

Based on such knowledge, the color difference measuring method further converts each stimulus value X, Y, Z at the arbitrary light receiving angle on the approximate curve into coordinates in the L ^* a ^* b ^* color space. Calculating the color difference ΔE _ab of the plurality of parts in the L ^* a ^* b ^* color space, and converting the stimulus values X, Y, and Z at the arbitrary light receiving angles on the approximate curve to L ^* u ^* v. ^* it is converted into the coordinates of a color ^space, L ^* u * ^{v *} and calculating a color difference Delta] E _uv of the plurality of sites in the color ^space, L ^* a * ^{b *} color difference in the color space Delta] E _ab and ^L * ^{u *} v ^* calculating a color difference ΔE of the plurality of parts based on the color difference ΔE _uv in the color space.

  As described above, in the color difference measurement method of the present invention, not only the light receiving angle at which the reflected light is actually received, but also the approximate curve representing the relationship between the light receiving angle and each stimulus value X, Y, Z is calculated. The color difference at an arbitrary light receiving angle can be calculated. As a result, the color difference that was missed in the conventional method can be extracted as a stimulus that should be originally present, so that the color difference of a plurality of portions can be accurately measured.

FIG. 2 is a perspective view schematically illustrating a multi-angle spectrophotometer. FIG. 2 is a side view of the multi-angle spectrophotometer. FIG. 3 is a side view of the multi-angle spectrophotometer. FIG. 2 is a plan view of the multi-angle spectrophotometer. FIG. 4 is a flowchart of a color difference measurement method (color difference evaluation method) according to the embodiment of the present invention. It is the figure which plotted ((theta), Y). It is the figure which approximated the plot of FIG. 6 with the exponential curve. (A) is a curve of (θ, L ^* ) at a plurality of sites, and (b) shows a difference between the curves. (A) is a curve of (θ, a ^* ) at a plurality of sites, and (b) shows a difference between the curves. (A) is a curve of (θ, b ^* ) at a plurality of sites, and (b) shows a difference between the curves. (A) is a curve of (θ, u ^* ) at a plurality of sites, and (b) shows a difference between the curves. (A) is a curve of (θ, v ^* ) at a plurality of sites, and (b) shows a difference between the curves.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

  The color difference measuring method according to one embodiment of the present invention is used, for example, when evaluating the color differences of a plurality of portions on a painted surface of an automobile. Specifically, for example, a color difference between a steel plate body and a resin bumper painted in the same color and a color difference of a plurality of portions on the same part (for example, the body) are measured, and the color difference is within an allowable range. Evaluate whether there is.

  The color difference measurement method according to the present embodiment includes the multi-angle spectrophotometer shown in FIG. 1 and an operation unit (computer) that performs various operations using the amount of received light (eg, spectral distribution) measured by the multi-angle spectrophotometer. This is performed using a color difference measuring device.

Multi-angle spectrophotometer colorimeter has irradiated portion 1a of irradiating the incident light _{L in,} and 1b, the light receiving unit 2a for receiving the reflected light _{L ref,} 2b, and 2c. In FIG 1 to FIG 3, a plane containing the incident light _{L in} and specularly reflected light _{L ref0} the main plane _{P 0,} it represents an arc.

Irradiation unit 1a, 1b is provided in the main plane P _0, it is provided at a position to irradiate the incident light L _in respect stbm O from different angles. Specifically, the irradiation unit 1a irradiates the perpendicular light V passing through the measurement target site O with the incident light Lin _(a) from a 45 ° direction (see FIG. 2). The incident light Lin _(b) is irradiated from the 15 ° direction (see FIG. 3).

Light-receiving portion 2a is provided in the main plane P _0, it is provided at a position for receiving the direction of the reflected light L _ref which forms a predetermined acceptance angle θ with respect to the perpendicular line V. In the present embodiment, both of the incident light L _in side and specularly reflected light L _ref0 side of vertical line V, each plurality of light receiving portions 2a provided.

Receiving unit 2b, 2c is provided at a position for receiving the direction of the reflected light that intersects the main plane P _0. Receiving unit 2b includes a vertical line V, provided in a plane _{P 1} which is perpendicular to the main plane _{P 0.} Receiving portion 2c is provided in a plane _{P 2} which is tilted plane _{P 1} in the positive reflected light _{L ref0} side. As shown in FIG. 4, the light receiving unit 2b receives the reflected light L _ref in the 90 ° direction in plan view with respect to the regular reflected light L _ref0 , and the light receiving unit 2c receives the reflected light L _ref0 in plan view. _Receives the reflected light L _ref in the 45 ° direction. In the illustrated example, the light receiving unit 2b and the light-receiving portion 2c is provided at symmetrical positions with respect to the main plane P _0, respectively.

The color difference measuring method according to the present embodiment is performed through steps (1) to (10) shown in FIG. Hereinafter, each step will be described in detail. In the present embodiment, will be described when the (see FIG. 2) measuring the color difference using the irradiation portion 1a of the 45 ° direction and a light receiving portion 2a in the main plane P _0.

In step (1), the irradiation unit 1a irradiates the measurement target site O with the incident light Lin _(a) , and the plurality of light beams L _ref reflected at the measurement target site O are received by the plurality of light receiving units 2a. The spectral distribution of each reflected light L _ref is obtained.

  In step (2), the spectral distribution acquired by each light receiving unit 2a is converted into tristimulus values X, Y, and Z in the XYZ color system. Note that a specific method of converting the spectral reflectance into tristimulus values X, Y, and Z is as described in JIS Z8701: 1999, and thus the description is omitted.

In step (3), the stimulus values X, Y, and Z with respect to the light receiving angle θ of each light receiving unit 2a are plotted on a graph. Then, plots of (θ, X), (θ, Y), and (θ, Z) at each part are approximated by curves. FIG. 6 is a plot of (θ, Y), and FIG. 7 shows a curve obtained by approximating the plot of FIG. 6 with an exponential curve. As described above, the plot of the stimulus values X, Y, and Z with respect to the light receiving angle θ can be approximated with an exponential curve with high accuracy. Note that the light receiving angle θ in the present embodiment refers to the angle of each reflected light L _{ref with} respect to the regular reflected light L _ref0 in the main plane P ₀ (see FIG. 2).

In step (4), L ^* , a ^* , b ^* , u ^* , and v ^* at an arbitrary light receiving angle θ are calculated from the approximate curves of the stimulus values X, Y, and Z. Specifically, L ^* , a ^* , b ^* , u ^* , and v ^* are calculated from the values of the stimulus values X, Y, and Z at a number of light receiving angles (for example, every 1 °) on the approximate curve of each part. calculate. FIGS. 8A to 12A show the relationship between the light receiving angle θ and L ^* , a ^* , b ^* , u ^* , and v ^* at each of the plurality of portions O ₁ , O ₂ , and O _3. FIG. The specific method of calculating L ^* , a ^* , b ^* , u ^* , v ^* from the stimulus values X, Y, Z is as described in JIS Z8781-4: 2013 and JIS Z8781-5: 2013. Since there is, description is omitted.

In step (5), the range of the light receiving angle θ is set so that L ^* falls within a predetermined range. L ^* is because an index representing the brightness, the L ^* is too large or too small, there is a possibility that can not be accurately measured color difference. For example, the range of the light receiving angle θ is set so as to exclude a range in which L ^* is excessive, that is, a range in which L ^* is excessively bright. In the present embodiment, the light receiving angle θ is set to 25 ° or more so that L ^* ≦ 100. Further, the range of the light receiving angle θ is set such that the range where L ^* is too small, that is, the range where the darkness is excessively low is excluded. In the present embodiment, the light receiving angle θ is set to 60 ° or less so that L ^* ≧ 40. The range of the light receiving angle θ may be set so as to define only one of the upper limit and the lower limit of L ^* . Alternatively, if there is no particular need, step (5) may be omitted.

In step (6), differences ΔL ^* , Δa ^* , Δb ^* , Δu ^* , Δv ^* of L ^* , a ^* , b ^* , u ^* , v ^* among a plurality of parts are calculated. FIG 8 (b) ~ FIG 12 _(b), between O 1 -O _{2, and,} [Delta] L between _{^{O 1 -O 3 *, Δa *}} , Δb *, Δu *, shows the curve of Delta] v ^*. From these _curves, between the ^{_{O 1 -O 2 ΔL *, Δa}} *, Δb *, Δu *, while not so large none of Delta] v _^*, between the O 1 -O _3, in particular Delta] u ^*, Delta] v ^* A large difference is recognized.

In step ^{(7), ΔL *, Δa} *, from the values of ^{Δb *, L * a * b} * between two points in the color space (in this _embodiment, while O 1 -O _2, and between _O 1 -O ₃₎ calculates a color difference Delta] E _ab (inter-coordinate distances), ΔL ^*, Δu ^*, is calculated from the value of ^{Δv *, L * u * v} * color difference between two points in the color space Delta] E _uv (coordinate distance) . The method of calculating the color differences ΔE _ab and ΔE _uv is also as described in the above JIS. In step (8), obtained in the step range of the light receiving angle set in (5) (25 ° to 60 °), the maximum value Delta] E _uv maxima Delta] E _{ab (max)} and Delta] E _uv of Delta] E _ab a _(max) I do. In step (9), the larger one of ΔE _{ab (max)} and ΔE _{uv (max)} is adopted as the color difference ΔE between the two points.

  In step (10), the quality of the coating is determined by checking whether or not the color difference ΔE between the two points measured as described above is within a predetermined range. Specifically, if the color difference ΔE between the two points is within a predetermined range, the paint colors of these portions are determined to be normal (same color). On the other hand, if the color difference ΔE between the two points is outside the predetermined range, it is determined that the paint colors of these portions are abnormal. In this case, the paint is applied after adjusting the composition of the paint and the coating conditions, and the color difference ΔE is measured again by the above-described procedure. By repeating the above steps until the color difference ΔE falls within the predetermined range, it is possible to set the optimum paint composition and coating conditions.

  The present invention is not limited to the above embodiment. Hereinafter, other embodiments of the present invention will be described, but the points overlapping with the above-described embodiments will be omitted.

In the above embodiments, to get the maximum value ΔE _{uv (max)} of the maximum value ΔE _{ab (max)} and Delta] E _uv of Delta] E _ab in the range of predetermined light receiving angle (25 ° ~60 °) within, of these Although the case where the larger one is the color difference ΔE is shown, the present invention is not limited to this. For example, it is possible to acquire the integral value of ΔE _{ab and} the integral value of ΔE _uv within a predetermined light receiving angle range, and calculate the color difference ΔE based on these integral values. Specifically, it is possible the larger the integrated value of the integrated value and the Delta] E _uv of Delta] E _ab or a color difference Delta] E, the sum of the integral value of the integration value and the Delta] E _uv of Delta] E _ab or a color difference Delta] E.

In the above-described embodiment, the system _in which the incident light _Lin side from the specular reflected light _{Lref0 (a) is set} to the positive direction of the light receiving angle θ using the irradiation unit 1a in the 45 ° direction (θ ₍₄₅₊₎ in FIG. 2 ₎ . Although the calculation of each parameter is performed using the illustrated system, the present invention is not limited to this. For example, by obtaining ΔE _{ab (max)} and ΔE _{uv (max)} in a plurality of systems and setting the largest one of them as ΔE, it is possible to more accurately measure the color difference.

For example, in addition to the above system, each parameter may be calculated by a system using irradiation units having different incident angles. Specifically, as shown in FIG. 3, (system shown in FIG. 3 theta ₍₁₅₊₎₎ strains of the incident light _{L in} side and positive acceptance angle from 15 ° direction of the specular reflection light _{L ref0 (15)} Thus, each parameter can be calculated.

Alternatively, in addition to the above system, the calculation of each parameter may be performed by a system in which the light receiving angle is positive on the side opposite to the incident light from the regular reflection light. Specifically, as shown in FIG. 3, in lines that the side opposite to the incident light _{L in} the positive acceptance angle from 15 ° direction of the specular reflection light _{L ref0 (15)} (in FIG. 3 theta _{(15 over)} (The system shown), each parameter can be calculated.

Alternatively, in addition to the above system, the light receiving unit 2b for receiving the direction of the reflected light L _ref which intersects the main plane P _0, in line with 2c, it may be performed calculation of the parameters.

Further, in the above-described embodiment, the system in which the light irradiated from one irradiation unit is received by the plurality of light receiving units has been described. However, the present invention is not limited thereto, and the light irradiated from the plurality of irradiation units may be received by one light receiving unit. May be adopted. For example, a reflected light of the incident lights Lin (a) and Lin (b) irradiated from the plurality of irradiation units 1a and 1b is received by any one of the light receiving units, and a system using the received light amount is adopted. Is also good. Specifically, for example, received radiation portion 1a, the reflected light from the 1b, the main planes P ₀ and the light receiving unit 2b for receiving the direction of the reflected light L _ref crossing, in any one of 2c strains Can be adopted. In this case, the above steps may be performed by regarding the angles (15 °, 45 °) of the incident light as the light receiving angle θ.

In the above embodiment, after calculating L ^* , a ^* , b ^* , u ^* , and v ^* , the range of the light receiving angle is set so that L ^* is equal to or less than a predetermined value. Absent. For example, first, only L ^* is calculated, a range of the light receiving angle is set so that L ^* is equal to or less than a predetermined value, and then a ^* , b ^* , u ^* , and v ^* are calculated within the range of the light receiving angle. May be.

  Further, in the above embodiment, the case where the color difference of a plurality of parts on one object is measured is described. However, the present invention is not limited to this, and is applied to the case where the color difference of the same part of a plurality of objects is measured. Is also good.

1a, 1b Irradiation sections 2a, 2b, 2c Light receiving section ΔE Color difference ΔE _ab L ^* a ^* b ^* Color difference in color space ΔE _uv L ^* u ^* v ^* Color difference _in color space L _in Incident light L _ref reflected light L _ref0 specular reflection Light P ₀ Main plane V Perpendicular θ Light receiving angle

Claims (1)

A method for measuring a color difference of a plurality of parts,
Each part is irradiated with incident light, receives reflected light in a plurality of directions having different light receiving angles with respect to the regular reflected light, and measures the tristimulus values X, Y, and Z of the reflected light at each light receiving angle. Steps and
Calculating an approximate curve representing a relationship between the light receiving angle and each stimulus value X, Y, Z at each site;
Each stimulus values X in any of the light receiving angle on the approximate curve, Y, converts the Z to the coordinates of the ^L * ^a * ^{b *} color ^space, L ^* a * ^{b *} of the plurality of sites in a color space color difference ΔE calculating _ab ;
Converts the stimulus values X, Y, and Z in any of the light receiving angle on the approximation curve to the coordinates of the L ^* u ^* v ^* color space, L ^* u ^* v ^* color difference between the plurality of sites in a color space Calculating ΔE _uv ,
Calculating the color difference ΔE of the plurality of parts based on the color difference ΔE _ab in the L ^* a ^* b ^* color space and the color difference ΔE _uv in the L ^* u ^* v ^* color space .

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