Method for determining colorimetric value

Abstract: The colorimetric value at a specified temperature is determined by measuring the colorimetric value of a colored material sample at an arbitrary temperature using a color meter such as a spectrophotometer, colorimeter, or densitometer, and by correcting the measured colorimetric value on the basis of the change in colorimetric value with temperature, which change in colorimetric value is computed from the difference in colorimetric values observed on a single colored material sample under at a minimum of two different temperatures. When a spectrophotometer is applied, the spectral reflectance or the spectral transmittance of the colored material sample is measured. On the other hand, spectral reflectance or spectral transmittance is measured in advance for a dye or pigment at a minimum of two different temperatures. The difference (ΔK) of absorption coefficient (K), the difference (ΔS) in the scattering coefficient (S), and the difference (ΔD) in absorbance (D) at each wavelength are computed from the preliminarily observed spectral reflectance or spectral transmittance. The spectral transmittance or spectral reflectance observed at an arbitrary temperature is corrected to define the colorimetric value at a specified temperature using the obtained values of K, S, and D. The method allows the determination of the colorimetric value of a single or a mixed color at a high level of precision using a simple operation under arbitrary temperature conditions without generating observation differences caused by temperature change.

Description: FIELD OF THE INVENTION The present invention relates to a method for correcting the colorimetric value of various colored materials, more particularly to a method for determining colorimetric value by correcting observation difference caused by temperature changes to assure a sustained high accuracy when measuring colorimetric value. BACKGROUND OF THE INVENTION Control of the color of colored products has up to now mainly employed a method using digitized colorimetric value (hereinafter referred to simply as "colorimetric value") such as L*, a*, b*, which conform to the CIE standard colorimetric system, and the Munsell value, which conforms to the Munsell color system. The obtained colorimetric values, however, have the disadvantage of fluctuation thereof depending on the temperature at which the measurement was taken, and thus generating an error. To compensate for this, a method has long been applied whereby colorimetric values are observed under the same temperature conditions, or the observed colorimetric values are compared against a value measured on a standard sample for every measurement. Generally, the change in colorimetric value with temperature gives a color difference (ΔE*) error ranging from about 0.1 to about 0.3 per 1°C of temperature change. Consequently, when greater accuracy of determination is required, measurements must be conducted while maintaining the sample temperature to within 1°C. Control of the sample temperature to within an accuracy of 1°C for every measurement operation requires dependency on expensive equipment and complex procedures. Therefore, the usual alternative method is to measure the colorimetric value both for the standard sample and for the target sample at the same time, as far as possible to remove the effects of errors generated by temperature change. The additional measurement of colorimetric value for the standard sample requires painstaking process control, and requires preventive measures to be taken against discoloration and contamination over the long period of storage of the standard sample. Since materials which degrade during the storage period and generate discoloration cannot be used as the standard sample, a material resistant to this kind of degradation needs to be separately prepared for every measurement cycle. Furthermore, when, for example, a product is continuously produced in a commercial production line, a product for the outdoors, or a building structure is the target of colorimetric value determination, it is impossible to make measurements under completely static temperature conditions. For these types of colorimetric value determinations, a colorimetric value containing an error has to be used in many cases. US 5 266 792 A and JP 04285829 (1992) disclose the use of a correction coefficient for correcting the spectral characteristics of the detector with respect to the temperature. SUMMARY OF THE INVENTION The inventors performed an investigation on the correlation between the temperature of colored materials and their colorimetric values to clarify the detail of temperature dependency of colorimetric values. Through the investigation, the inventors found that the change in temperature and the change of colorimetric value are closely correlated, and proved that any colorimetric value measured under arbitrary temperature conditions can be corrected to a colorimetric value at a specified temperature by utilizing the phenomenon of changing colorimetric value depending on temperature. The present invention was completed on the basis of the findings described above. One object of the present invention is to provide a method to determine colorimetric value reliably and highly accurately without any accompanying observation error caused by a temperature change under arbitrary temperature conditions. Another object of the invention is to provide a method to determine colorimetric values effective for process control with a simple operation. A further object of the invention is to provide a method to determine colorimetric values which allow the estimation of correct mixing rate of dye or pigment in color-matching operation at arbitrary temperatures. To achieve the objects of the invention, the method of the invention as defined in claim 1 determines a colorimetric value of a colored material by measuring the colorimetric value of a colored material sample at an arbitrary temperature using a color meter, and correcting the measured colorimetric value on the basis of a change in colorimetric value resulting from a temperature difference to determine the colorimetric value at a specified temperature, whereby the change in colorimetric value is computed from the difference in colorimetric values measured on a single colored sample at a minimum of two different temperatures. The present invention uses a spectrophotometer as the color meter to measure the spectral reflectance or transmittance, to compute the change of spectral reflectance or transmittance per unit temperature range based on the difference in spectral reflectance or transmittance observed under two different temperature conditions, and to correct the spectral reflectance or transmittance based on the determined change to derive the colorimetric value at a specified temperature. Another mode of the present invention uses a spectrophotometer as the color meter to measure the spectral reflectance, and to correct the measured spectral reflectance by the equations for mixed color mentioned below on the basis of the difference (ΔK, ΔS) of absorption coefficient (K) and scattering coefficient (S) at each wavelength to determine the colorimetric value at a specified temperature, whereby the values of difference (K, S) are computed in advance from the spectral reflectance observed on a dye or pigment at a minimum of two different temperatures. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The color meter used in the present invention is a spectrophotometer, colorimeter, a densitometer, etc. The change which is the correction factor in the present invention is given as the difference in colorimetric value measured on a standard colored sample under at least two temperature conditions. The standard colored sample is not necessarily a specific colored material, and it may be a plastic material colored by various kinds of dye or pigment, or a metallic material with color coating. In concrete terms, the colorimetric values of a colored sample are measured at two arbitrary temperatures, for example, at 30°C and 50°C. The difference between these measured colorimetric values is converted to the change per specific temperature range, which change is then used as the correction factor. In the preferred embodiment, the change is preferably a change per 1°C. The measurement operation of colorimetric value may be carried out by providing a built-in temperature sensor to equipment such as a spectrophotometer, colorimeter, or densitometer to detect the measurement temperature, and may be implemented by entering a specified temperature via the keyboard of the computer. The process control is simplified if the colorimetric values of target samples measured at arbitrary temperature levels are converted into the data at specific temperature levels and if the converted data are stored as a database. The present invention was completed as a color determination method suitable for an industrial control technique by utilizing the correlation between colorimetric value and temperature and by correcting the colorimetric value measured at an arbitrary temperature to one at a specified temperature. That is, according to the present invention, the difference of measured colorimetric values of a colored sample at a minimum of two different temperatures, or the amount of change, is determined as the correction factor per unit temperature range. A colorimetric value of the target material sample is then measured at an arbitrary temperature is converted to the colorimetric value at the target temperature using the correction factor. Thus the observation error caused by temperature change is effectively eliminated. Consequently, a simple operation allows the reliable and accurate determination of colorimetric value, and allows the simulation of the colorimetric value at any arbitrary temperature. The method of the present invention is highly effective as a daily control of colorimetric value determination. A spectrophotometer or a colorimeter is used as the color meter to measure the spectral reflectance or color value, to compute their change per unit temperature range based on the difference in spectral reflectance or color value observed under two different temperature conditions, and to correct the spectral reflectance or color value based on the determined change to derive the colorimetric value at a specified temperature. According to the present invention, the spectral reflectance on a mixed color material can be synthesized by the theory of Kubelka Munk and the theory of Duncan, which are given below. That is, the spectral reflectance of a material is expressed by the function of absorption coefficient and scattering coefficient in eq. (1) derived from the theory of Kubelka Munk. R = 1 + K/S - {(K/S) 2 + (K/S)} 1/2 where R denotes the reflectance at each wavelength, K denotes the absorption coefficient at each wavelength, and S denotes the scattering coefficient at each wavelength. When a dye or pigment consists of a mixture of several components and when the surface covering power is strong, as observed in paint, K/S is expressed by modifying eq.(1) to Duncan's eq.(2). K/S = (ΣKiCi + Ko) / (ΣSiCi + So) where Ki denotes the absorption coefficient of each color material, Si denotes the scattering coefficient of each color material, Ko denotes the absorption coefficient of the medium, So denotes the scattering coefficient of the medium and Ci denotes the mixing rate of each color material. On the other hand, the value of K/S for the case of a material with primary coating, which is experienced in printed matter, is expressed by eq.(3). K/S = ΣεiCi + (K/S) o where εi denotes the value of K/S per unit of color material, Ci denotes the mixing rate of each color material, and (K/S)o denotes the value of K/S of the primary coating. In the case of transparent material, the transmittance (T) of each dye or pigment leads the absorbance (D) using the theory of [ Lambert Beer] expressed as eq.(4) and eq.(5). T = 10 -D D = ΣDiCi + Do where, Di denotes the absorbance per unit of color material, Ci denotes the mixing rate of each color material, and Do denotes the absorbance of the medium. For example, the change in spectral reflectance and transmittance per 1°C allows the computation of the change of absorbance and scattering coefficient for each dye or pigment, or conversely, the change in spectral transmittance allows the computation of the change of absorbance. This means that, the observed value of spectral reflectance and transmittance of a mixed color sample at an arbitrary temperature can be corrected using the computed change as the correction factor to eliminate the effect of temperature conditions. According to the invention, the change used as the correction factor is given as the difference between the absorption coefficient (K) and the scattering coefficient (S) at each wavelength, which are determined from spectral reflectance observed at a minimum of two different temperatures of a dye or pigment, or given as the difference in absorbance at each wavelength, which is derived from spectral transmittance (T) observed at a minimum of two different temperatures on a dye or pigment. In that case, the change in correction factor is preferably the difference per 1°C, and the computation of the correction factor is automatically conducted using the CCM method. The dye or pigment applied to the present invention is not specifically limited, and it may consist of various kinds of materials such as plastics which are colored by a single or mixed dye or pigment, and metallic materials. Examples of dyes are the anthraquinone group, the heterocyclic group, the methine group, the perynone group, and the perylene group. Examples of pigments are inorganic compounds such as titanium oxide, carbon black, the iron oxide group, and the ultramarine group, and organic compounds such as the azo group, copper phthalocyanine group, the perynone group, the perylene group, the quinacridone group, and the pyrrole group. For example, a sample is prepared by coloring a hard polyvinyl chloride resin with a pigment of yellow lead pigment (with addition of 1 phr). The spectral reflectance is measured at 29°C and at 60°C using a spectrophotometer. Then, the absorption coefficient (K) and the scattering coefficient (S) are computed at wavelength intervals of 10 nm. When the difference (ΔK, ΔS) per 1°C for each wavelength is derived from the computed K and S, the values listed in Column "b" of Table 1 are obtained. Next, for the observed values at 29°C, the absorption coefficient (K) and scattering coefficient (S) for each wavelength at 25°C are corrected at each wavelength using the correction factors ΔK and ΔS, and the estimation values listed in Column "a" of Table 1 are obtained. That is, when 25°C is taken as the standard temperature of measurement, the absorption coefficient and the scattering coefficient of yellow lead pigment at the standard temperature are readily derived. Following the same procedure, the case of polyazo yellow as the pigment is expressed in Table 2. Other dyes and pigments are treated using the same procedure. That is, the absorption coefficient (K), scattering coefficient (S), and absorbance (D) at 25°C are determined using the computed changes (ΔK, ΔS, and ΔD) at each wavelength per 1°C as the correction factors. These determined values are entered along with the correction factors (ΔK, ΔS, and ΔD) into computer to store as the database. In this way, color is brought under numerical control. The method of measurement includes the correction of observed spectral reflectance and transmittance for a target color sample at an arbitrary temperature, and the conversion of the corrected values into corresponding Munsell values on the basis of JIS Z8729 "Method of Color Specification using L*, a*, b* system and L*, u*, v* system" and the Attached Table 1 "Standard of Color Atlas using Three Color Qualities" in JIS Z8721 "Color Display on the basis of Three Color Qualities". The above-described method of measurement may be implemented by mounting temperature sensors in the spectrophotometer colorimeter, densitometer or other peripheral devices and computation circuit to sense the target temperature, and is possible to automatically generate colorimetric values at a standard temperature, and is also possible to enter a specific temperature for practical application through the keyboard of the computer to a system comprising a separately located color meter, temperature sensor, and calculator. The present invention was completed by adopting a method of correction and conversion of a measured spectral reflectance and transmittance at an arbitrary temperature into the values at a specified wavelength at specific temperature using the intrinsic correlation between the spectral reflectance and transmittance and the temperature of a dye or pigment. The derived invention is a suitable color control method for use in industrial control. If the blending ratio is not specified and a database of correction factors of dye or pigment is not available, the blending ratio is obtained by determining the colorimetric value of the sample at two different temperature levels and by converting these values to the value at a standard temperature. On the other hand, if the database of correction factors of dyes or pigment is available, the colorimetric value of the sample is determined at a single temperature level, and the value is converted to the value at a standard temperature. The subject to be determined by the present invention may be paint and ink mixed with dye or pigment, or plastics, fiber, and paper colored by paint and ink, or other colored materials found in nature. The method for determining colorimetric value in the present invention can be effectively applied to control hue in computer color matching (CCM). Wavelength (nm) "a" Column (value estimated at 25°C) "b" Column (change per 1°C) K S ΔK ΔS 380 3.29002 0.92676 - 0.000469 0.003283 390 3.11278 1.15303 0.000996 - 0.000508 400 2.95694 0.83849 0.002118 - 0.000719 410 3.02632 0.89754 0.001617 - 0.000293 420 3.09738 0.85726 0.001076 - 0.000591 430 3.17634 0.85442 0.000888 - 0.000519 440 3.19967 0.84830 0.001118 - 0.000518 450 3.17526 0.84722 0.000791 - 0.000355 460 3.11191 0.84653 0.000566 - 0.000261 470 3.04310 0.84621 0.001090 - 0.000385 480 2.93735 0.85656 0.001180 - 0.000418 490 2.77874 0.88700 0.000917 - 0.000429 500 2.49862 0.89904 0.001764 - 0.000489 510 1.72013 0.85543 0.005676 - 0.000340 520 0.77998 0.75975 0.006831 - 0.000306 530 0.28473 0.70877 0.003771 - 0.000500 540 0.11522 0.66600 0.001547 - 0.000757 550 0.06075 0.63866 0.000576 - 0.000530 560 0.03880 0.62649 0.000226 - 0.000320 570 0.02750 0.61683 0.000096 - 0.000127 580 0.02229 0.60102 0.000042 - 0.000037 590 0.01981 0.59075 0.000018 - 0.000023 600 0.01825 0.56338 0.000010 0.000160 610 0.01712 0.56806 0.000005 - 0.000243 620 0.01625 0.56093 0.000005 - 0.000162 630 0.01538 0.55786 0.000005 - 0.000089 640 0.01472 0.56209 0.000004 - 0.000234 650 0.01408 0.55564 0.000005 - 0.000592 660 0.01379 0.53055 0.000003 - 0.000267 670 0.01389 0.52536 0.000003 - 0.000283 680 0.01399 0.52039 0.000002 - 0.000301 690 0.01362 0.49605 0.000000 - 0.000369 700 0.01325 0.47175 - 0.000002 - 0.000453 Wavelength (nm) "a" Column (value estimated at 25°C) "b" Column (change per 1°C) K S ΔK ΔS 380 3.61180 3.57644 - 0.000632 - 0.004358 390 3.36768 4.65334 0.000751 - 0.003614 400 3.16364 3.55484 0.001811 - 0.001690 410 3.18842 3.94067 0.001361 - 0.000657 420 3.21256 3.95203 0.000910 - 0.002441 430 3.25457 3.87999 0.000227 - 0.002544 440 3.21325 3.69975 0.000900 - 0.003551 450 3.13895 3.40270 0.000577 - 0.003600 460 3.00381 2.96234 0.000528 - 0.003492 470 2.88586 2.41565 0.000329 - 0.002842 480 2.67385 1.92838 - 0.000142 - 0.002313 490 2.35882 1.45787 - 0.000228 - 0.001917 500 1.87664 1.01748 0.000289 - 0.001277 510 1.31689 0.65396 0.001000 - 0.000536 520 0.78470 0.42351 0.001643 - 0.000213 530 0.39371 0.31297 0.001519 - 0.000159 540 0.18817 0.24889 0.000969 0.000009 550 0.10335 0.22169 0.000458 0.000020 560 0.07404 0.22753 0.000182 0.000035 570 0.06403 0.21840 0.000065 0.000355 580 0.06193 0.17918 0.000019 0.000897 590 0.06300 0.12984 0.000004 0.001210 600 0.06512 0.13313 0.000005 0.001081 610 0.06789 0.10360 0.000003 0.001144 620 0.07170 0.04728 0.000000 0.001253 630 0.07197 0.05167 0.000123 - 0.000419 640 0.07738 0.05620 0.000547 - 0.001550 650 0.08470 0.05486 0.000150 - 0.001215 660 0.08881 0.05341 0.000199 - 0.000852 670 0.10393 0.04869 0.000001 0.000328 680 0.11254 0.04454 0.000030 0.001365 690 0.11922 0.04758 0.000010 0.000605 700 0.09037 0.05053 0.001018 - 0.000133 EXAMPLE The present invention is described in more detail referring to examples and comparative examples. Nevertheless, the invention is not limited to the content of the examples. Example 1 Color No. P29-110 in the Standard Color Sample Book for Paints (Wide Edition) issued by the Japan Paint Industrial Association (P edition, 1989) was selected as the standard colored sample. The spectral reflectance of the sample was measured at the sample temperatures of 10°C and of 40°C within a wavelength range of from 380 nm to 700 nm at wavelength intervals of 10 nm using a spectrophotometer (Column C; made by Dainippon Seika Kogyo Co., Ltd.). Based on these observed values, the change (ΔR) per 1°C at each wavelength was computed from the difference in spectral reflectance at a 30°C temperature difference. The result is shown in Table 3. Wavelength (nm) Change (ΔR) Wavelength (nm) Change (ΔR) 380 - 0.000015 550 0.000040 390 - 0.000118 560 0.000130 400 - 0.000095 570 - 0.000455 410 - 0.000015 580 - 0.000745 420 0.000050 590 - 0.000805 430 0.000030 600 - 0.000660 440 0.000020 610 - 0.000435 450 0.000025 620 - 0.000240 460 0.000010 630 - 0.000110 470 0.000030 640 - 0.000075 480 0.000030 650 - 0.000030 490 0.000030 660 - 0.000030 500 0.000000 670 - 0.000038 510 - 0.000115 680 - 0.000055 520 - 0.000090 690 - 0.000084 530 0.000010 700 - 0.000125 540 0.000070 Next, the colored sample as the target material was measured to obtain its spectral reflectance under the temperature conditions of 10°C, 20°C, and 30°C using the same spectrophotometer. The change (ΔR) shown in Table 3 was used as the correction factor to compute the estimated reflectance at 20°C from the observed values at 10°C and 30°C. The Munsell value was then computed from the estimated reflectance. The result is given in Table 4. Table 4 also shows the color difference (ΔE) from the Munsell value at 10°C and 30°C on the basis of the Munsell value observed at 20°C. Standard colored sample Observation temperature (°C) Converted Munsell number Color difference (ΔE) Color sample: P2 10 2.26Y 6.86/6.29 0.06 9-110 20 2.24Y 6.85/6.29 Standard 30 2.24Y 6.85/6.29 0.06 The table indicates that the Munsell values corresponding to the temperature change are limited to a range of very slight dispersion and that the values are derived as high precision observed values. Examples 2 - 5 Four colors were selected as the standard colored samples from the Standard Color Sample Book for Paints (Wide Edition) issued by the Japan Paint Industrial Association (P edition in 1989). The Munsell values were computed by correction-conversion of the observed spectral reflectance of the colored sample in a similar manner to that applied in Example 1. The result is shown in Table 5. Table 5 indicates that the color difference (ΔE) caused by temperature change is very small and independent of the difference in hue. Example No. Standard colored sample (color sample) Observation temperature (°C) Converted Munsell value Color difference (ΔE) Example 2 P14 - 204 10 2.15Y 7.65/12.68 0.09 20 2.17Y 7.65/12.66 Standard 30 2.15Y 7.65/12.68 0.09 Example 3 P31 - 308 10 2.33Y 8.67/4.06 0.03 20 2.35Y 8.67/ 4.06 Standard 30 2.33Y 8.67/ 4.06 0.03 Example 4 P 6 - 342 10 9.67GY6.02/ 6.12 0.02 20 9.68GY6.02/ 6.12 Standard 30 9.67GY6.02/ 6.12 0.02 Example 5 P16 - 413 10 1.07PB 4.87/10.64 0.11 20 1.09PB 4.87/10.66 Standard 30 1.07PB 4.87/10.64 0.11 Comparative Examples 1 - 4 Four colors were selected as the standard colored samples from the Standard Color Sample Book for Paints (Wide Edition) issued by the Japan Paint Industrial Association (P edition in 1989). Their spectral reflectance at the sample temperature levels of 10°C, 20°C, and 30°C was measured. The Munsell values were computed directly from the observed values. Table 6 shows the obtained Munsell values and the color difference (ΔE). Table 6 indicates that the color difference (ΔE) is considerably dispersed compared with the corresponding examples (refer to Table 5). Example No. Standard colored , sample (color sample) Observation temperature (°C) Converted Munsell value Color difference (ΔE) Comparative Example 1 P14 - 204 10 2.38Y 7.67/12.68 0.78 20 2.17Y 7.65/12.66 Standard 30 1.92Y 7.62/12.71 0.89 Comparative Example 2 P31- 308 10 2.49Y 8.67/ 4.07 0.28 . 20 2.35Y 8.67/ 4.06 Standard 30 2.16Y 8.66/ 4.06 0.35 Comparative Example 3 P 6 - 342 10 9.67GY6.02/ 6.12 0.38 20 9.68GY6.02/ 6.12 Standard 30 9.58GY6.02/ 6.06 0.41 Comparative Example 4 P16 - 413 10 1.12PB 4.86/10.67 0.15 20 1.09PB 4.87/10.66 Standard 30 1.01PB 4.86/10.62 0.36 Example 6 Formed colored plastic plates were prepared by blending pigment of the same hue as that in Example 1 into a polyethylene resin. The colorimetric value was determined in a similar manner to Example 1. The obtained result is given in Table 7. Standard colored sample Observation temperature (°C) Converted Munsell value Color difference (ΔE) Resin plate having the same pigment blend with Example 1 10 2.34Y 7.71/12.25 0.07 20 2.36Y 7.71/12.24 Standard 30 2.35Y 7.71/12.25 0.08 Table 7 indicates that a formed plastic plate colored with dye or pigment also provides industrial accuracy as in the case of coated plate in Example 1 and that the present invention is effectively applicable independent of the kind of colored material. Examples 7 12, Comparative Examples 5 - 10 For samples of colored polyvinyl chloride plate (PVC:100, stabilizer: 3, lubricant: 1) having 1 mm of thickness, the spectral reflectance was measured under the temperature conditions of 25°C, 40°C, and 50°C. From the observed reflectance at 25°C, the values of L*, a*, and b* were computed. Then, the change per 1°C was computed from the observed reflectance values at 40°C and at 50°C, and the estimated reflectance at 25°C was determined from the observed reflectance at 40°C and at 50°C, and the values of L, a*, and b* were computed. Table 8 shows the result. The top row gives the values of L, a*, and b* determined from the observed reflectance at 25°C, and the lower rows give the corrected and converted values of L, a*, and b* along with the color difference (ΔE*) against the standard value in the top row. For comparison, Table 9 shows the L, a*, and b* values computed from the spectral reflectance observed at 25°C, 40°C, and 50°C, and the color difference (ΔE*) at 40°C and at 50°C against the standard value of 25°C. Comparison of Table 8 with Table 9 clearly shows that the corrected color difference (ΔE*) in Examples are reduced to an approximate range of from one third to one fifteenth as compared with the color difference before correction. Example No. Observation temperature (°C) L* a* b* Color difference (ΔE) Example 7 25 63.61 42.74 47.83 Standard 40 63.52 42.61 47.80 0.16 50 63.55 42.62 47.82 0.13 Example 8 25 63.35 34.63 32.16 Standard 40 63.32 34.53 32.14 0.11 50 63.29 34.52 32.12 0.13 Example 9 25 61.57 22.98 23.41 Standard 40 61.45 23.04 23.32 0.16 50 61.49 23.01 23.35 0.10 Example 10 25 69.37 32.11 47.57 Standard 40 69.43 32.15 47.48 0.12 50 69.39 32.11 47.49 0.08 Example 11 25 62.76 45.13 42.97 Standard 40 62.65 44.98 42.92 0.19 50 62.67 45.11 42.94 0.10 Example 12 25 64.04 36.07 12.53 Standard 40 63.95 35.95 12.54 0.15 50 63.93 35.96 11.51 0.16 Example No. Observation temperature (°C) L* a* b* Color difference (ΔE) Comparative Example 5 25 63.61 42.74 47.83 Standard 40 63.22 42.78 47.37 0.60 Observed value at 40°C 50 62.96 42.71 46.94 1.10 Observed value at 40°C Comparative Example 6 25 63.35 34.63 32.16 Standard 40 63.02 34.42 31.65 0.64 50 62.86 34.26 31.31 1.05 Comparative Example 7 25 61.57 22.98 23.41 Standard 40 61.39 23.74 23.47 0.78 50 61.30 24.14 23.28 1.20 Comparative Example 8 25 69.37 32.11 47.57 Standard 40 69.42 31.75 47.30 0.45 50 69.43 31.14 46.97 1.14 Comparative Example 9 25 62.76 45.13 42.97 Standard 40 62.42 44.83 42.23 0.87 50 62.16 44.66 41.72 1.46 Comparative Example 10 25 64.04 36.07 12.53 Standard 40 63.85 35.41 12.10 0.81 50 63.73 35.00 11.77 1.35 Comparative Examples 11-18 Sample sheets of polyvinyl chloride (PVC:100, stabilizer: 3, lubricant: 1) having 1 mm of thickness and colored by various compositions of pigments listed in Table 10 were prepared. The spectral reflectance at 25°C, 40°C, and 50°C was measured using a spectrophotometer (column C; made by Dainippon Seika Kogyo Co. Ltd.). The colorimetric value in the CIE (L*, a*, b*) color system was computed for each of the observed spectral emittance. The result is shown in Table 11 in relation to the corresponding pigment No. The color difference (ΔE) at 40°C and 50°C was determined on the basis of the observed value at 25°C, which is also given in Table 11. Example No. Temperature (°C) L* a* b* ΔE* Comparative Example 11 25 63.60 42.89 48.00 ----- 40 63.22 42.78 47.37 0.75 50 62.96 42.71 46.94 1.25 Comparative Example 12 25 63.26 34.65 32.16 ----- 40 63.02 34.42 31.65 0.61 50 62.86 34.26 31.31 1.01 Comparative Example 13 25 61.54 24.63 23.77 ----- 40 61.39 23.74 23.47 0.95 50 61.30 23.14 23.28 1.58 Comparative Example 14 25 69.37 32.00 47.35 ----- 40 69.42 32.00 47.30 0.07 50 69.43 32.00 41.27 1.11 Comparative Example 15 25 62.80 45.08 42.98 ----- 40 62.42 44.83 42.23 0.88 50 62.16 44.66 41.72 1.47 Comparative Example 16 25 64.04 36.01 12.59 ----- 40 63.85 35.41 12.10 0.80 50 63.73 35.00 11.77 1.34 Comparative Example 17 25 62.91 38.11 0.83 ----- 40 62.99 38.12 0.86 0.09 50 63.04 38.12 0.89 0.14 Comparative Example 18 25 69.71 34.08 34.48 ----- 40 69.79 33.93 34.56 0.19 50 69.85 33.83 34.62 0.32 Examples 13 - 20 The observed value of spectral reflectance at each temperature level obtained in each of the Comparative Examples 11 - 18 was used to estimate the values at 40°C and 50°C on the basis of observed value at 25°C using the correction factor of the base pigment per 1°C, which correction factor had been entered the computer as the database for each wavelength. The estimated value and the computed value of ΔE* at each temperature are listed in Table 12. Comparison of the color difference (ΔE) in Table 12 with the corresponding color difference (ΔE) given in Table 11 indicates that the error in the Examples decreases for every value at each temperature level and that both values agree in Examples with little error. Example No. Temperature (°C) L* a* b* ΔE* Example 13 Estimated value at 40°C 63.22 42.66 47.23 0.18 Estimated value at 50°C 62.96 42.60 46.81 0.18 Example 14 Estimated value at 40°C 63.12 34.40 31.66 0.10 Estimated value at 50°C 62.96 34.24 21.33 0.10 Example 15 Estimated value at 40°C 61.41 23.02 23.10 0.47 Estimated value at 50°C 61.31 23.42 22.89 0.48 Example 16 Estimated value at 40°C 69.41 32.10 47.50 0.23 Estimated value at 50°C 69.43 32.10 47.46 0.22 Example 17 Estimated value at 40°C 62.35 44.88 42.22 0.08 Estimated value at 50°C 62.07 44.71 41.71 0.09 Example 18 Estimated value at 40°C 63.85 35.46 12.04 0.08 Estimated value at 50°C 63.73 35.05 11.72 0.07 Example 19 Estimated value at 40°C 63.05 38.05 0.87 0.09 Estimated value at 50°C 63.10 38.05 0.90 0.09 Example 20 Estimated value at 40°C 69.87 33.96 34.57 0.09 Estimated value at 50°C 69.93 33.76 34.62 0.10 Example 21 Sample sheets of polystyrene colored by 0.5% addition of perylene dye (plasto-red 8315, made by Arimoto Kagaku Kogyo Co., Ltd.) were prepared by injection molding process. The spectral reflectance was measured at 58°C and 25°C. The color difference (ΔE) computed from the observed values was 2.3. Following the same procedure as for Examples 13 - 20, the spectral reflectance was measured at 58°C at each wavelength, and the observed value was corrected by computing the spectral transmittance at 25°C using the predetermined change (ΔD) per 1°C. Comparison of the corrected value and the color difference (ΔE) observed at 25°C showed the difference was 0.06, which is negligibly small. As described above, the present invention suppresses the dispersion of observed values caused by temperature change during color measurement, which dispersion was a disadvantage of prior art, using a simple operation for correcting spectral reflectance, and allows the digitizing of observed values in applying CCM and in the color control of colored material even if a temperature change occurs in each measurement cycle. Consequently, for determining the mixing rate of base dye or pigment, for example, optimum mixing rate is selectable at an arbitrary temperature level. For the numerical control of color, a measurement at an arbitrary temperature can readily be corrected to the value at a specific temperature if only the specific temperature is allotted. As a result, there is no need to remeasure the standard sample for every measurement and no need to store and monitor the standard sample. Thus the method of quality control of hue is significantly improved.

Claims: A method for determining the colorimetric value of a colored material comprising the steps of: measuring the spectral reflectance or transmittance of a colored material sample at at least two different temperatures with a spectrophotometer; calculating a correction factor for the spectral reflectance or transmittance from the change in spectral reflectance or transmittance in the two different temperatures; measuring the spectral reflectance or transmittance of the colored material at an arbitrary temperature using a spectrophotometer; calculating the change of the spectral reflectance or transmittance of the colored material sample corresponding to the temperature difference between the arbitrary temperature and a predetermined temperature, using the correction factor; calculating the spectral reflectance or transmittance at the predetermined temperature from the calculated change and measured spectral reflectance or transmittance of the colored material at the arbitrary temperature; and calculating the colorimetric value of the colored material at the predetermined temperature utilizing the calculated spectral reflectance or transmittance. The method for determining the colorimetric value of a colored material of claim 1, wherein the correction of the measured spectral reflectance of the colored material comprises the step of: measuring the spectral reflectance of a colored material sample containing the coloring agent of a colored material at at least two different temperatures with a spectrophotometer; calculating the absorption and scattering coefficients of the coloring agent for different wavelengths in the at least two different temperatures from the measured spectral reflectance; calculating a correction factor for the absorption and scattering coefficients from the change in the calculated absorption and scattering coefficients of the coloring agent for the different wavelengths in the at least two different temperatures; measuring the spectral reflectance of the colored material at an arbitrary temperature using a spectrophotometer and calculating the ratio of the absorption coefficient to the scattering coefficient of the colored material from the measured spectral reflectance; calculating the change of the absorption and scattering coefficients of the coloring agent corresponding to the temperature difference between the arbitrary temperature and a predetermined temperature, using the correction factor; calculating the change of the ratio of the colored material from the calculated change of the absorption and scattering coefficients of the coloring agent considering the containing ratio of the coloring agent in the colored material; calculating the ratio of the colored material at the predetermined temperature from the calculated change of the ratio and the ratio calculated from the measured spectral reflectance of the colored material at the arbitrary temperature; and calculating the spectral reflectance of the colored material at the predetermined temperature from the ratio at the predetermined temperature.

Inventors: SAITO FUMIYOSHI, SAITO TOMIO, SANO KAZUO, SHIGEMORI YOSHIHIRO, YOSHIDA TOMIKO, SAITO, FUMIYOSHI, SAITO, TOMIO, SANO, KAZUO, SHIGEMORI, YOSHIHIRO, YOSHIDA, TOMIKO
Applicants: DAINICHISEIKA COLOR CHEM, DAINICHISEIKA COLOR& CHEMICALS MFG. CO. LTD.
Application year: 1996
Main classification: G01J3
IPC codes: G01J3