EA007190B1 - Non-destructive control method of ceramics structure - Google Patents

Abstract

The invention relates to methods of determining material's properties using optical methods, in particular, to non-destructive control methods and can be used for quality analysis of siliceous ceramics.The method of nondestructive control of ceramics structure comprises using optical radiation with wavelength λ= 300-850 nm, pre-testing measurements on samples with known characteristics and comparing control measurements with tested results. The control and tested measurements are carried out using a method of reflection factor measurement at different production steps and evaluation of ceramics functional features, wherein at each step the most influencing the value and color parameter ceramics structure is selected. At step of ceramic mass preparing there controlled dispersion of solid fraction particles, determined by whiteness and yellowness rate. At drying step the residual loosely coupled moisture is controlled evaluated by integral reflection factors. The integral reflection factors are calculated as a sum of reflection factors in all visible range. At step of ceramics sintering the structure quality, including a phase composition, pore space, degree of homogeneity is controlled at a selected section. The structure quality is evaluated be color reflection factor. At step of determining the functional characteristics there controlled the intensity of structural changes under mechanical load and thermal shock. In this case the intensity of structural changes is evaluated by color change – integral color characteristics. The reflection factors are determined using a reflection photometer, the average sample illuminated surface is taken being 0.5-2 cm.

Description

The invention relates to methods for determining the properties of materials by optical methods, and in particular to methods of non-destructive testing and can be used to analyze the quality of the structure of silica ceramics.

The quality of the structure of ceramics from the same composition of ceramic mass is determined by the number and nature of the distribution of phases in the material, the number of pores, their size, shape and distribution in the material, depending in turn on the heating rate and holding time at high temperature.

The production of silica ceramics begins with the preparation of the charge - ceramic mass. The main parameter, which determines the quality of the mass and, further, the quality of ceramics at this stage is the size of the particles in the solid phase. The second important technological link is the drying of the charge. At this stage, constant monitoring of the process of removing so-called weakly bound water is required, i.e. water that is not part of the structure of minerals. The third stage of production, the most important, is the process of sintering ceramics. Even with the correctly chosen sintering mode, the products of different thickness can have a structure different in cross section, because due to the low thermal conductivity inside the product, the actual temperature remains for a long time lower than on the surface. Control of the structure of ceramics is also necessary to assess the functional characteristics of the latter.

There is a method of non-destructive quality control of ceramic products from beryllium oxide, including irradiation of a controlled product with light with a wavelength of λ = 180-350 nm, registration of its luminescence, recording of the power of the luminous flux transmitted through the product (Russian patent №1655198, IPC Ν01Ν21 / 64, priority of 08/16/1989, published on 08/20/1999, country of publ. 8i).

The disadvantages of this method are that it allows you to control only thin, transparent products with low density, is applicable for substances with a simple single-phase structure, without pores and is used only for finished products. The known method does not allow for the control of sintered ceramics, especially at various stages of its production. Does not allow to evaluate the functional characteristics.

A known method for analyzing ceramics involves irradiating a controlled product with light with a wavelength of λ = 300-850 nm, excitation of luminescence, recording the latter with a microscope, comparing the results with the results of test measurements.

The disadvantage of this method is that it is applicable to a narrow class of materials, mainly to crystals of the ionic type, for which it is known how luminescence is associated with structural defects. The known method does not allow for the control of porous materials, and also does not allow for the control of ceramics at various stages of its production and evaluation of functional characteristics.

The authors were tasked to develop a method of non-destructive quality control of ceramics, providing control of the structure at each stage of production and evaluation of the functional characteristics of ceramics.

The task is solved by the fact that in the method of non-destructive testing of the structure of ceramics, which consists in using radiation with a wavelength of λ = 300-850 nm, preliminary test measurements on samples with known characteristics, comparing control measurements with test measurements, control and test measurements are carried out by measuring coefficients reflections at various stages of production and evaluation of the functional characteristics of ceramics, and at each stage the most influencing on the structure of ceramics is selected Inu and color parameter which characterizes the latter. At the stage of production, including the preparation of ceramic mass, control the dispersion of particles of solid fractions, which is determined by the degree of whiteness and yellowness. The latter are calculated by the formulas

B = (p457 + P520 ⁺ Rb2o) / 3

W = (pb20P457) / p520 where p ₄ 57, P520 and p ₆ 2o are the reflection coefficients at given wavelengths.

At the stage of production, including the drying of ceramics, the residual weakly bound moisture is monitored, which is estimated from the integral reflection coefficients. The integral reflection coefficients are calculated as the sum of the reflection coefficients over the entire visible range. At the production stage, including ceramics sintering, the quality of the structure, including the phase composition, pore space, the degree of homogeneity, is controlled on a selected surface area. The quality of the structure is estimated by the color reflection coefficients, and the dominant wavelength λ and the purity of the tone P are determined by the color CIE triangle by the formulas

Χι = 0.957 ^ 457 + 36.547 ^ 520 + 68.78OR ₆ 2о + 2.1 7 ^ 2.606- ^ 457 + 73.590 ^ 520 + 25.710 ^ 620 + 2.6 Ζι = 36.585 'К457-2.660' К520 0.085 'Пб2о + О.8 where Κ (λ) - spectral reflection coefficient.

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At the stage of determining the functional characteristics, the intensity of structural changes under mechanical loading and thermal shock is controlled. At the same time, the intensity of structural changes is estimated by color difference - the integral color characteristic, which is calculated by the formula

DE * = [(L | _ *) ² + (Yes *) ² + (AB *) ² ] ^1/2

The coordinates of the color X, Υ, Ζ are converted to the system LА * Ь *, where L * - lightness, and *, L * - chromaticity

Ι _ * = 116 (Υι / Υ ₀ ) ^1/3 -16, a * = 500 [X1 / Ho) ^1/3 - (ι / Υ0) ^1/3 1 * = 200 [(Υ1 / Υο) ^{1 / 3} - (ι / Ζο) ^1/3 ] here X _o = 98.04, _ο = 100, Ζ _ο = 118.10 are the color coordinates of the ideal white surface for a standard light source. In this case, the reflection coefficients are determined using a reflection photometer, and the average illuminated surface area of the sample is chosen to be 0.5-2 cm ² .

The technical effect of the claimed invention consists in the possibility of express analysis of the structure of ceramics at various stages of production, making adjustments to the technological process, as well as determining the optimal operating conditions for sintered ceramics. In addition, the inventive method can significantly reduce energy consumption.

The production of ceramics begins with the preparation of the charge-ceramic mass. The main parameter on which the quality of the mass depends and, further, the quality of ceramics is the size of the particles in the solid phase. Therefore, at this stage, the dispersion of particles of solid fractions is controlled, which is determined by the degree of whiteness and yellowness. The latter are calculated by the formulas

B = (P457 + P520 ⁺ Ρ62θ) / 3

W = (pb20-p457) / p520 where p ₄ 57, P52o and p ₆₂ o are the reflection coefficients at given wavelengths.

Choose a certified powder with known particle sizes. Then for different fractions determine the reflection coefficients in the entire optical range of wavelengths, for which pour sample - powder in a cuvette, which is placed in the ball of the photometer. Build a comparison table. Depending on the composition and color of the material, the radiation wavelength corresponding to the maximum reflection is chosen and the sensitivity of the method is determined. Then carry out control measurements. The comparative tables determine the size of the particles. Measure the reflection coefficients in the most sensitive spectral region.

The essential sign that the control and test measurements are carried out by measuring the reflection coefficients, allows to achieve the maximum sensitivity of the method by choosing the appropriate wavelength. The essential sign that the control and test measurements are carried out at the production stage, including the preparation of ceramic mass, which controls the dispersion of particles of solid fractions, makes it possible to correct the technology at the first stage - to break the powder, if it is not possible - to increase the sintering temperature. Both the first and second options allow you to significantly improve the quality of ceramics or eliminate marriage.

The second important production stage requiring control is the drying of ceramics — the removal of so-called weakly bound water, i.e. not included in the structure of minerals. The residual weakly bound moisture is estimated from the integral reflection coefficients, which are calculated as the sum of the reflection coefficients over the entire visible range. At this stage, first take a batch of reference samples in which the amount of residual water is determined by other methods (for example, by mass or by electrical resistance). Measurements of integral (over the entire visible spectrum) reflection coefficients are carried out. After that, for the same series of samples, the reflection coefficients are determined, for which no preliminary processing of the samples is necessary. Build a comparison table. Conduct control measurements and determine the structure of ceramics according to the comparison table. The method allows for the reflection coefficients to quickly assess the quality of drying, and, therefore, allows you to make adjustments, such as drying. Here the integral reflection coefficients are most effective (the sum of measurements of the reflection coefficients for the wavelengths of 400, 457, 490, 520, 570, 620 and 750 nm).

Therefore, the method allows you to control the quality of manufactured ceramics in the process, and quickly, which allows you to correct the technological mode on the go. It takes about less than 10 minutes to measure the parameters of one sample. Water has a reflection coefficient of 1.5-2% in the visible spectrum, while the average reflection coefficient of a siliceous mass is about 40%. The remaining water after drying is determined by reducing the reflection coefficients by comparing the integral coefficients of the mass under study with the reflection coefficients of the reference samples.

The essential sign that the control and test measurements are carried out at the production stage, which includes drying of ceramics, allows to correct the shortcomings of the drying process - to increase the time, thereby eliminating defects during sintering (breaking of products) and reducing energy consumption during sintering (for final drying of samples due to slow heating) .

-2007190

The control of the structure of sintered ceramics is the most important. At this stage, the quality of the structure, including the phase composition, the pore space, the degree of homogeneity, is monitored on a selected surface area. The structure of ceramics at this stage of production is estimated by more subtle characteristics - color parameters. Any color can be determined by three parameters: color tone λ (μm), intensity P (%) and reflection coefficient p. The color tone λ and the saturation of the tone P are determined using the CIE color triangle using the formulas

X ₁ = O.957K457 + 36.547 K52o + 68.78OKb2o + 2.1 ^ = 2.606- ^ 457 + 73.590 ^ 520 + 25.710- ^ 620 + 2.6 Ζ] = 36.585 'К457-2.660' Р520 0.085 'Rbgo + 0.8 where Κ (λ ) - spectral reflection coefficient.

The color coordinates and the color triangle of the MKO system determine the “quality” of color, the dominant wavelength or color tone λ and the saturation of tone R. The value of Χχ is plotted on the abscissa, the point with coordinates λ, Ρ is obtained on the ordinate axis. The purity or intensity of the P tone characterizes the degree, level, strength of the expression of the color tone λ, nm. FIG. 1 shows the areas of change in the color tones λ and their intensities with an increase in the firing temperature (870-1370) K with exposures (0-2) h. The basic color tone (λ = 580 nm) corresponding to the yellow color observed after low-temperature firing is 870- 1070K, (λ = 58 nm, P ~ 75%; green circles) eroded to λ = 585 nm - yellow-orange 1070 K, 2 h - red circles), the purity of the tone increases to 90%, the samples brighten and become beige (firing at 1270-1370K, 0 and 2 h; yellow circles).

The production stage, which includes the sintering of ceramics, is the most energy-intensive. Optimization of sintering modes, ensuring maximum values of specified properties, is the most effective means of resource saving. Test measurements are carried out on batches of sintered samples for which strength is determined, in particular, the modulus of elasticity. Compile a table of compliance of the elastic modulus and the values of λ and P. The measurement results of control samples are evaluated by reflection coefficients. Labor costs are the same - up to 10 minutes.

Optimization of the modes — temperature and sintering time — consists in minimizing the energy consumption when the same elastic modulus is reached. To do this, it is necessary to determine the effectiveness of raising the temperature or increasing the sintering time for structural changes. For silica ceramics containing, as a rule, an amorphous glass phase, the most reliable method for evaluating the structure — X-ray diffraction phase analysis — turns out to be ineffective. An important indicator of the quality of sintering is the degree of homogeneity of the structure, especially for large products. The degree of homogeneity is assessed by the differential reflection coefficients, determined using masks that highlight a limited surface area.

At the stage of determining the functional characteristics, the intensity of structural changes under mechanical loading (bending) and thermal shock is controlled. The intensity of structural changes is estimated by color difference - the integral color characteristic, which is calculated by the formula

DE * = [(A1_ *) ² + (Aa *) ² + (DH *) ² ] ^1/2

The color coordinates X, Υ, Ζ are converted into the system BA * Ь *, where B * is lightness, and *, Ь * is chromaticity

B * = 116 (Υι / Υ ₀ ) ^1/3 -16 3 * = 500 [Χι / Χο) ¹ ' ³ - (Υι / Υ0) ^1/3 ], (4.4) 6 * = 200 [(Υ1 / Υο ) ^1/3 - (Ζ1 / Ζ _ο ) ^1/3 ]

Here Χ _ο = 98.04, Υ _ο = 100, Ζ _ο = 118.10 are the color coordinates of the ideal white surface for a standard light source.

For a series of reference samples, mechanical tests are carried out - they determine the bending strength and compile tables of structural changes and strength. Experimentally established complete symbity of these parameters. According to these tables, the strength of the obtained products is predicted without testing for the color difference ΔΕ.

At all stages of the production of ceramics, the reflection coefficients are determined using a reflection photometer, and the average illuminated surface area of the sample is chosen to be 0.5-2 cm ² . As a photometer they use a photometer of reflection FD-1, which is based on the Taylor principle: a beam of light of a certain wavelength is directed through the hole to the wall of the integrating ball and after repeated reflection creates illumination Eo /. The same beam of light is directed to the surface of the sample, is reflected from it and creates illumination inside the ball E ^. The absolute reflection coefficient is ρ = Ε ₁ χ / Ε _ο χ. The measurement error in determining the absolute reflection coefficients does not exceed 3%. The advantage of the proposed method is as follows:

allows you to control all the technological operations of obtaining ceramics and in this sense is universal;

-3 007190 does not require special equipment, large labor and time costs, special surface preparation of samples;

is non-destructive, allows to obtain a large statistical sample of results;

allows you to assess the quality and homogeneity of the structure, makes it possible to adjust the technology itself;

the use of one method for controlling the quality of all technological operations makes it possible to trace the dynamics of structural transformations and associate it with functional characteristics;

extends the quality management of ceramics, contributes to labor and energy saving;

useful for developing ceramics with desired properties.

Example 1

Conduct quality control of the structure of ceramics at the stage of production, including the preparation of ceramic mass. The dispersion of particles of solid fractions of a given composition for silica ceramics is determined by the values of whiteness and yellowness. For siliceous ceramics based on Kara-Quiche clay with chemical composition, wt.%) 8ίΟ ₂ - 72.86; A1 ₂ O ₃ - 22.86; Ge ₂ O ₃ ; ΤίΟ ₂ - 1.22; CaO-0.48; M ^ O + K ₂ O + Ka ₂ O - 1.53. The sensitivity of the method is 0.1% μm ′ ¹ for particles in the range of 20-500 μm. This is the most "running" particle size. The reflection coefficients decrease with an increase in the particle size according to a power law, the exponent n = 1.3 (Table 1).

Table 1

Diameter of particles, micron Whiteness B, ref. state, % The degree of yellowness W, ex. state,% Change in whiteness after firing ha,% Yellowness change after firing,% 20 thirty 35 -five 25 60 25 thirty 7 15 160 20 25 ten ten 240 20 25 ten ten 360 17 20 eight five 500 15 20 five 2

Traditional sieve analysis (screening through a sieve) limits the size of the particles from above, i.e. all particles are smaller than a cell in a sieve. The average size of them must be determined additionally and this is a long process - at least a working day. The method allows you to quickly determine the average particle size of the existing reference tables of comparison. The most time consuming process is the determination of particles smaller than 100 microns. A set of such sieves is, firstly, expensive (1 sieve costs more than 100 dollars) and it is not rational to spend them on regular production, since they are quickly erased. Using a set of sieves, electron microscopy, X-ray analysis, optical microscopy, with a rather high error — the average particle size is estimated to 20%, the reflection coefficients are found for them, and then for this composition only the reflection coefficients or degrees of whiteness and yellowness are used. The inventive method of controlling the structure reduces the energy consumption for heat treatment, since the larger the fraction of particles, the higher the temperature and duration of the sintering process. In tab. 1 shows the changes in B and F after firing at 1270 K, 1 h. It is seen that the greatest changes are observed for the smallest particles. Yellowness grows in proportion to the amount of glass phase formed from the melt during sintering. With an increase in particle size, the intensity of phase transformations decreases significantly, which must be taken into account when optimizing the technology for producing ceramics. The amount of water that is added to form the ceramic mass with the desired ductility or viscosity necessary for the molding of products depends on the size of the particles.

Example 2

Conduct quality control of the structure of ceramics at the stage of production, including the drying of ceramics. Drying is ensured by breaking the bonds of water with particles of the clay material, for which considerable energy is required. Therefore, the need to control the drying process and optimization of the regime is understandable. The method is as follows: for the reference batch of samples build curves of residual moisture (mass loss) from the drying time. The drying process is considered complete when the mass of the sample remains constant. In accordance with the humidity in the comparison table contribute integral reflection coefficients. In tab. 2 shows the values of humidity and integral reflection coefficients for the ceramic mass obtained on the basis of clay from the Kara-Quiche deposit, measured every 30, and at the end of the drying - after 15 minutes.

table 2

Drying time, min 0 thirty 60 90 120 150 165 180 Humidity,% 23 14 9 four 2 one 0 0 Integral coefficients, reflections 34 57 68 73 80 85 90 90

Samples are removed from the drying cabinet, cooled in air, placed in a photometer and the integral reflection coefficient is determined by the formula K = K ₄ oo ⁺ K457 ⁺ K49o ⁺ K-52o + K62o ⁺ K-75o. For this ceramic, the drying process is considered complete if the integral reflection coefficient is 90%.

Example 3

Conduct quality control of the structure of ceramics at the stage of production, including sintering ceramics.

Test measurements are carried out on a batch of sintered specimens of a given composition, for which the compressive or bending strengths and then the modulus of elasticity are determined. Compile a table of compliance of the elastic modulus and the values of λ and P (Table 3), which determine the firing mode, which allows to obtain the maximum values of the elastic modulus. The remaining products are sintered at this temperature and their strength is controlled by the parameters λ and P and the correspondence table, without destruction.

Table 3

T spec, K λ, nm R,% P total,% E, GPa 1170 590 60 29 thirty 1220 585 48 23 59 1270 580 55 21 58 1340 580 70 14 78 1390 580 85 9 100 1420 577 80 2 65

The parameters λ and P are determined by the color triangles of the ICL. For siliceous ceramics based on Kara-Quiche clay, the optimum sintering temperature is 1370 K, at which a structure with a maximum modulus of elasticity, E = 100 MPa, is formed, with color parameters λ = 580 nm, P = 85%.

Example 4

Conduct quality control of the structure of ceramics at the stage of determining the functional characteristics. Choose a sample of ceramics and carry out quality control sintering, uniformity of the structure.

For massive products (wall thickness more than 1 cm), the degree of structure homogeneity in the surface layers and in the center of the product is evaluated. For this purpose, a mask is used (black paper is placed on the sample surface, in which a 2 mm thick ring is cut), as well as a conversion factor k, which connects the true reflectance p with the measured value p _from : p = cr _from . The values of k are determined by reference samples with an accuracy of 5%. The results of the quality control of the structure of the sintered products are used both for baking, if possible, and for determining the optimal operating conditions for the products. In both cases, the result will be a significant economic effect. FIG. 2 shows the reflection coefficients from a heterogeneous structure in a product made of silica-based ceramics based on Kara-Quiche clay, sintered for 2 hours at different temperatures. The diameter of the samples is 1 cm. The reflection coefficients from the surface to the center are reduced by 10%, they are used to estimate the actual temperature distribution inside the sample at a known temperature in the furnace. So, for example, for the considered points it will be as follows (Table 4).

Table 4

K, mm 0 1 2 3 4 5 mm

T, K 1370 1300 1250 1100 1000 980 K which is in good agreement with the calculations of the temperature distribution in the sample for a given heating rate and with the results of x-ray phase analysis.

Example 5

Conduct quality control of the structure of ceramics at the stage of determining the functional characteristics. A sample of sintered ceramics is selected, the intensity of structural changes during thermal shock is monitored. The degree of change of the color parameters λ and P is determined by the MKO color triangle (Fig. 3), the structure that most effectively dissipates the excess energy received by the material under thermal loading is selected by them. Bright circles marked

- 5 007190 averaged color coordinates of baked samples. With an increase in the firing temperature, the circles are shifted to the region of short waves and large P values. The gray circles show the color coordinates obtained after the maximum impact temperature sustained by the sample. The arrows indicate the relationship between the sintered and thermally loaded sample. For all samples, the values of λ after thermal shock are shifted to the red region, the purity of the tone P decreases.

The smallest structural changes during thermal shock occur in samples calcined at 1340 K. The structure is not able to dissipate the energy obtained during thermal shock. Rapid crack propagation corresponds to the minimum heat resistance of these samples. The structure of the samples sintered at 1220 and 1270 K undergoes significant changes, which contributes to an increase in heat resistance.

Structural changes in the silica-based ceramics based on Kara-Quiche clay are also analyzed using the color difference ΔΕ, which is the most sensitive characteristic for this composition. Compile tables of color difference and heat resistance. The intensity of structural changes is proportional to the value of ΔΕ. The color difference increases in proportion to the sintering temperature (Table 5), however, the most heat-resistant samples with ΔΕ = 1.34.

Table 5

T sintering, K 1220 1270 1340 1390 Color difference ΔΕ, o.e. 0.43 1.34 3.23 5.58 The number of cycles sustained by the sample eight ten 6 7

This is in good agreement with the results obtained by other research methods - X-ray phase analysis, scanning electron microscopy, strength measurements. The method allows you to quickly determine the optimal structure for the development of ceramics with a given heat resistance.

Claims (13)

1. The method of non-destructive testing of the structure of ceramics, which consists in the use of radiation with a wavelength of λ = 300-850 nm, preliminary test measurements are carried out on samples with known characteristics, control measurements are compared with test ones, characterized in that the control and test measurements are carried out by the method measuring reflection coefficients at various stages of production and evaluating the functional characteristics of ceramics, and at each stage the value and color pair that most affects the structure of the ceramic are selected ameter characterizing the latter. 2. The method according to claim 1, characterized in that at the production stage, including the preparation of ceramic mass, the dispersion of the particles of solid fractions is controlled. 3. The method according to claim 2, characterized in that the dispersion of particles of solid fractions is determined by the coefficients of whiteness and yellowness. 4. The method according to p. 3, characterized in that the coefficients of whiteness and yellowness are calculated by the formulas B = (p457 + P520 ⁺ Pb20) / 3 W = (Pb20 ~ P457) / p520 where p ₄ 57, p 52 ° and p ₆₂ ° are the reflection coefficients at given wavelengths. 5. The method according to claim 1, characterized in that at the production stage, including drying the ceramics, the residual weakly bound moisture is controlled. 6. The method according to claim 5, characterized in that the residual weakly bound moisture is estimated by the integrated reflection coefficients. 7. The method according to claim 5, characterized in that the integrated reflection coefficients are calculated as the sum of the reflection coefficients in the entire visible range. 8. The method according to claim 1, characterized in that at the stage of production, including sintering of ceramics, the quality of the structure, including the phase composition, pore space, degree of uniformity, is controlled on a selected surface area. 9. The method according to claim 8, characterized in that the quality of the structure is estimated by the color reflection coefficients, and the dominant wavelength λ and the purity of the tone P are determined by the color triangle of the MCO according to the formulas Χι = Ο.957 · Κΐ57 + 36.547Ή ₅ 2θ + 68.78ΟΉ62θ + 2.1 ^ = 2.606 ^ 457 + 73.590 ^ 520 + 25.710 ^ 620 + 2.6 Ζι = 36.585 'К457-2.660 ’К520 0.085' Κβ2θ + θ · 8 where Κ (λ) is the spectral reflection coefficient. 10. The method according to claim 8, characterized in that the homogeneity of the structure is estimated by differential reflection coefficients, determined using masks that highlight a limited surface area. 11. The method according to claim 1, characterized in that at the stage of determining the functional characteristics control the intensity of structural changes during mechanical loading and thermal shock. 12. The method according to claim 10, characterized in that the intensity of structural changes is estimated by the color difference — the integral color characteristic, which is calculated by the formula DE * = [(A1_ *) ² + (Yes *) ² + (D * *) ² ] ^1/2 the color coordinates of X, Ζ, Ζ are converted to the system b * b *, where b * is lightness, and *, b * - chromaticity 1 _ * = 116 (^ / Yo) ^1/3 -16 3 * = 500 [Χι / Χ _ο ) ^1/3 - (Υι / Υο) ¹ ' ³ ], (4.4) ό * = 200 [(Υι / Υο) ^1/3 - (Ζ ₁ / Ζ _ο ) ^{1 / 3} ] here X _o = 98.04, Υ _ο = 100, Ζ _ο = 118.10 - color coordinates of a perfectly white surface for a standard light source. 13. The method according to claim 1, characterized in that the reflection coefficients are determined using a reflection photometer, and the average illuminated surface area of the sample is chosen equal to 0.5-2