Application of color measurement in wood science

Color is the basic attribute of wood. The color of wood and its products after dyeing or painting is more complex and changeable. Color measurement is very important in wood science research. The development and classification of color measurement methods are summarized, and the principles and characteristics of photoelectric integration, spectrophotometry and digital color measurement are summarized; On this basis, the application status of these three kinds of analyzers in wood science is expounded; Finally, the three types of colorimeters are compared and analyzed from four evaluation indexes of portability, speed, price and accuracy by using radar chart method, and their main application areas in the field of wood science are proposed. The purpose of the study is to determine the characteristics of several color measurement methods in order to play a greater role in wood science research, such as wood dyeing quality detection, wood visual property evaluation, wood color matching, etc.

With the rapid development and wide application of color measurement instruments, color measurement has become one of the evaluation methods for many industrial products in the paper industry, national defense, textile printing and dyeing industry, coating industry, etc. However, in the field of furniture and wood, there are few reports on the application of color measurement methods. Color contains a lot of important information about wood. On the one hand, it reflects the aesthetic characteristics of wood and can directly transfer the relevant properties of wood; On the other hand, the change and difference of wood color is an important index to evaluate wood quality and measure wood dyeing quality. The diversity and variability of wood color make its color presentation characteristics different from metals, paper, plastics, fabrics, etc., so the research on color measurement methods is particularly important for wood science. This paper briefly introduces the principles and characteristics of different color measurement methods, and combs the application of these methods in wood detection and evaluation, providing some reference for the application research of surface color of wood products and furniture materials.

With the rapid development and wide application of color measurement instruments, color measurement has become one of the evaluation methods for many industrial products in the paper industry, national defense, textile printing and dyeing industry, coating industry, etc. However, in the field of furniture and wood, there are few reports on the application of color measurement methods. Color contains a lot of important information about wood. On the one hand, it reflects the aesthetic characteristics of wood and can directly transfer the relevant properties of wood; On the other hand, the change and difference of wood color is an important index to evaluate wood quality and measure wood dyeing quality. The diversity and variability of wood color make its color presentation characteristics different from metals, paper, plastics, fabrics, etc., so the research on color measurement methods is particularly important for wood science. This paper briefly introduces the principles and characteristics of different color measurement methods, and combs the application of these methods in wood detection and evaluation, providing some reference for the application research of surface color of wood products and furniture materials.

Overview of wood color measurement

Development of wood color measurement

Wood has many properties, including: visual properties, olfactory properties, tactile properties and decorative properties. Color is an important embodiment of wood visual characteristics and decorative characteristics. The color tone of wood often gives people a comfortable, quiet and calm aesthetic feeling, and is widely used in the field of architecture and furniture production [1]. The interval distribution of growth rings and early and late wood makes the wood present a texture pattern. The wood cells contain various pigments, resins, tannins, gums, oils and other substances, so that the original wood with no obvious color has a variety of colors. The texture pattern and color form the natural decoration of the wood [2].
People like the color of wood of different species, but natural wood is scarce, especially precious wood. In response to this market demand, scientists and scholars from all walks of life in forestry engineering have studied wood dyeing, refined various dyes, tested and innovated wood dyeing agents, basic principles of dyeing, dyeing processes, color stability, etc., effectively reducing the consumption of natural wood and alleviating the shortage of precious wood resources.
However, with the progress of science and technology, wood processing is gradually industrialized and intensive, and people are increasingly demanding the consistency of wood surface color and texture. The color, texture and glossiness of wood surface are closely related to the quality evaluation of wood products. The color characteristics of wood surface determine people’s evaluation and perception of wood color, especially in furniture manufacturing, floor processing and other industries. The color of wood surface is complex, and different paints are painted, which makes the color of wood product surface more complex and changeable. The evaluation and measurement of wood color have an important impact on wood color characterization and innovation research. Therefore, this paper studies the wood color testing methods and their applications.

Classification of color measurement methods

Since 1960, scientists at home and abroad have explored the quantitative measurement of wood color, and have made certain achievements. At present, color measurement methods are mainly divided into two categories: subjective evaluation method and objective evaluation method [3].

Subjective evaluation method

People have been using color terms such as yellowish brown, turquoise, purplish red to describe the color of wood, which is still used to describe the basic characteristics of wood. However, the industrial production standard requires quantitative expression of color, which is obviously not satisfied. Before the 1960s, the subjective evaluation method was mainly used: visual method. This method is a relatively traditional and simple color measurement method, but it is easily affected by light source, sample size, physiological and psychological conditions of the observer, etc., so it is gradually replaced by the objective evaluation method.

Objective evaluation method

The objective evaluation method is to use objective tools instead of subjective evaluation to measure the color of wood to meet the requirements of color quality control [4]. It is a method based on optical principles and colorimetric theory, using measuring instruments that can quantify color information.
Objective instrument measurement methods are mainly divided into contact measurement and non-contact measurement. There are two types of contact color measuring instruments. One is based on the principle of photoelectric integration, which appeared from 1960 to 1970; After 1970, a new kind of colorimeter based on spectrophotometry appeared; After 2003, non-contact color measuring instrument based on digital color measuring method appeared.

Principle and characteristics of color measurement method

The human eye is one of the oldest color measuring instruments, but due to different spectral responses of different human eyes, poor repeatability and vulnerability to other factors, it is not suitable for quantitative analysis of wood color. The colorimeter simulates the spectral response characteristics of normal human eyes, maintains the illuminance observation conditions similar to human eyes when observing color samples, receives the tristimulus value of human eye radiation spectrum, gives the color value, and realizes the quantitative description of color.

Photoelectric integration method

Photoelectric integration method uses photoelectric converter to simulate the “integration” process of color vision and obtain the integration value of color stimulus. It is a measurement method based on chromaticity. The main measuring instruments are colorimeter and colorimeter. Because the colorimeter is not accurate enough, it can only be used for simple basic colorimetry, so most of the instruments are portable and the price is relatively low.
When measuring and evaluating the color difference of wood, CIELAB color space is the most commonly used color model, as shown in Figure 1. It is based on the theory that a color cannot be both green and red, nor blue and yellow at the same time. When a color is tested with CIE for L *, a * and b *, the L * axis represents the value of the lightness coordinate, with black at the bottom being 0 and white at the top being 100+ A * is red and – a * is green+ B * is yellow and – b * is blue; Color changes are displayed by a *, b *, and the brightness changes of colors are reflected by L * values. All colors in nature can be described by L *, a *, b * [5].
The specific calculation formula of CIEL * a * b * surface color system is as follows:
C*=[(a*)2+(b*)2]1/2
20221203015938 33917 - Application of color measurement in wood science
Figure.1 CIELAB spatial color model

  • ∆L*=L*-Ls*
  • ∆a*=a*-as*
  • ∆b*=b*-bs*
  • ∆C*=C*-Cs*
  • ∆E*=[(∆L*)2+(∆a*)2+(∆b*)2]1/2
  • ∆H*=[(∆E*)2-(∆L*)2-(∆C*)2]1/2

In the formula: as *, bs * and Ls * represent the data of untreated test pieces; A *, b * and L * represent the data of the processed test piece. Generally, 6 points will be selected equidistant on the test piece for measurement; Δ L * stands for brightness difference; Δ A * and Δ B * represents chromaticity index difference; Δ C * represents color saturation difference; Δ H * represents color difference; Δ E * represents the overall color difference, Δ The larger the E *, the greater the degree of discoloration of the object [6].

Spectrophotometry

Spectrophotometry is a measurement method based on Lambert ratio law. The chromaticity value of a sample is calculated by its spectral reflection or transmission characteristics. Spectrophotometry is mainly used for quantitative and qualitative analysis of the tested substance to determine the absorbance of the substance within a certain wavelength range or at a specific wavelength. The application scope of spectrophotometry includes ultraviolet (200-400nm), visible (400-760nm) and infrared (2.5-25 μm). Commonly used instruments include color spectrophotometer, spectrometer, optical radiometer, etc. Spectrophotometer is the most basic instrument in color measurement, which has the advantages of high accuracy and wide applicability. However, it requires high optical accuracy and complex data processing.

Digital color measurement

In the field of instrument color measurement, with the development of the Internet and the popularity of digital imaging equipment, digital color measurement based on digital photography has gradually become a high-tech in the industrial field. Its working principle is that under the stable standard light source environment, first correct the color and white balance through the whiteboard and standard color card, then use the fixed shooting parameters to take high-definition digital photos, then calculate the RGB pixel color value of the sample, and obtain the comprehensive chromaticity data of the sample after correction.
As a non-contact color measurement method, the advantage of digital color measurement method is that it can measure tiny, rough, curved or irregular objects in high-resolution images. It not only retains the natural state of the surface of the object to be measured and the light source environment, but also restores the true color measurement results of the sample. It is very suitable for color testing of uneven terry fabrics, even including powder, frost and liquid. Although there may be some limitations of color gamut, the particularity and applicability of digital color measurement function have a trend of wide application in various fields.

Application of different colorimeters in wood science

Color measurement with colorimeter

The principle of colorimeter is mainly based on photoelectric integration method. This method is fast and cheap, but the accuracy of its chromaticity coordinate value is low due to the poor integral value of color stimulus, which is more suitable for color measurement in industrial mass production [7]. For example, Xu Li [8] and others used CR-5 colorimeter to study the surface color parameters of 12 wood samples in 2020, and made statistical analysis on the three elements of color and aesthetic evaluation data, and established a positive correlation model between the aesthetic evaluation of wood and color brightness, which has certain reference value for finding substitutes for interior decoration materials. Jia Tianyu [9] and others are using CS-260 portable colorimeter to measure the color physical parameters of 22 pieces of Ming style furniture commonly used wood, and study their aesthetic characteristics, so as to explore the basic law of their color emotional trend.
In the above research, the colorimeter can eliminate human subjective factors and measure the parameters of different wood species with different wood colors, so as to analyze the relationship between their surface color and aesthetic characteristics, further establish a color evaluation model, further understand the wood color, and guide dyeing and color matching.

Color measurement with spectrophotometer

In the spectrophotometer, when light of different wavelengths continuously irradiates the sample solution of a certain concentration, the absorption intensity corresponding to different wavelengths can be obtained. The method of determining colorless substances with ultraviolet light source is called ultraviolet spectrophotometry; The method of measuring colored substances with visible light source is called visible light photometry [10]. Spectrophotometry is widely used in the research on the production of new substances that have an impact on color in material color synthesis and mechanism reaction. For example, Zhang Aoran [11] and others designed a new type of online detection device in 2019 to achieve high-precision online detection of wood dyeing production. The standard data set was obtained through the spectrophotometer, and then the spectral reflectance was reconstructed to establish a model. Finally, the color difference was analyzed to evaluate the color quality. Zhang Wentao [12] et al. used phenol reagent spectrophotometry to determine formaldehyde emission in 2016. As the main detection method for formaldehyde content of indoor furniture at room temperature, it is simple, sensitive, fast and accurate. In 2011, Gong Bao [13] et al. determined the content of total flavonoids in the remaining leftovers of Huangli wood furniture manufacturing by ultraviolet spectrophotometry, and finally determined that the content was 3.1%. This result is of great significance for the development and utilization of total flavonoids in the leftovers of Huangli wood furniture.
In the actual wood dyeing experiment, the content of a substance is judged or determined by the height of the special absorption peak in the absorption spectrum. The spectrophotometer can be used to detect the dyeing quality of wood or determine the content of certain substances in wood, which is helpful to determine the appropriate dyes and dyeing methods, and also can be used to judge the green environmental performance of wood.

Non contact image two-dimensional color texture analyzer

The image type two-dimensional color texture analysis instrument uses the special filter of the full color gamut visual camera body developed by Shizuoka University Shimada Research Laboratory, which has completely different characteristics from traditional RGB cameras. By implementing a function that is very close to the sensitivity of the human eye, namely the CIEXYZ color matching function developed by Shizuoka University, the filter can truly reproduce the sensitivity of the human eye and accurately capture colors [14].
It has made many breakthroughs in color measurement: (1) The time for photogrammetric display results has been greatly accelerated, and the non-contact type makes the measured objects compatible; (2) Complex and detailed colors, patterns and subtle textures can be compared. For example, color differences due to different metal luminosity can be compared through the consistency of color distribution, which is difficult to achieve by traditional measurement methods [15]; (3) It can be used to establish a database of accumulated colors or achieve traceability of detection. Therefore, even if the inspector has little experience, quantitative numerical data and functional image data can be used for remote inspection and color matching.
For example, Zhu Yayuan [16] and others measured the color of 15 kinds of natural solid wood and transparent paint boards with a digital colorimeter in 2018, obtained digital wood grain, and conducted color difference adjustment test and analysis on them, which to some extent helps to improve the utilization rate of natural wood and better meet the market’s decorative needs for the color and texture of natural wood.
Because wood is a natural product, it is impossible to match the color and grain exactly, and the wood has been selected by experienced carpenters so far. The traditional photometer measurement is to select multiple points on the wood for measurement and analysis. In this case, the distribution of early and late wood is not considered, which has limitations. The two-dimensional colorimeter reflects the difference between color and wood grain through the “consistency of color distribution”, so the two-dimensional colorimeter can be used in the texture color measurement of wood materials.

Comparison and analysis of three colorimeters

By analyzing and comparing the colorimeter, spectrophotometer and the latest non-contact image type two-dimensional color texture analyzer, the comparative analysis radar chart of three colorimeters (Figure 2) is drawn, and the following conclusions are drawn:

  • (1) Colorimeter has the advantages of fast, cheap and portable. However, due to the inaccuracy of colorimeter, it is often used for simple and basic color comparison to study the parameters of wood surface color and explore its aesthetic evaluation and emotional trend of color.
  • (2) The spectrophotometer is very accurate in color measurement, but lacks in price, speed and portability. As a professional inspection equipment in factories and laboratories, it is mainly used for wood dyeing quality detection and formaldehyde emission measurement in furniture.
  • (3) The non-contact image type two-dimensional color texture analyzer has fast measurement speed, high accuracy, relatively high price and poor portability. It uses a special filter of the full color gamut camera body, which is no longer limited to the individual points of the measured object, and has a consistent analysis of the consistency of color distribution on one surface of the measured object. Therefore, compared with colorimeter and spectrophotometer, it is more suitable for wood and some more complex and detailed patterns and textures to assist computer color matching and meet more consumers’ pursuit of the beauty of natural wood color and texture.

20221203020553 70868 - Application of color measurement in wood science

Figure.2 Comparison and Analysis of Three Colorimeters

Conclusion

In the wood dyeing experiment, the evaluation of wood visual characteristics of wood dyeing is an important step. In the process of color matching, accurate measurement of wood color is the basis for establishing color matching model. In the quality inspection link, the measurement and analysis of wood color and color fastness of dyed wood is an effective basis for product qualification verification and quality grading. Therefore, as one of the physical properties of wood, color can improve the grade of wood, increase the added value and make better use of wood. As a professional color inspection tool, understanding its principle and characteristics will help the development of wood dyeing experiment, wood surface decoration and furniture product manufacturing. Color measurement will play a greater role in wood science research.

Author: Sun Yue, Qi Yiqing

Source: China Color Measurement Solutions Provider – www.spectrumgfa.com

Reference:

  • [1] Mao Yiqun, Xu Wei. Research on the modification of fast-growing wood for household use [J] Furniture and Interior Decoration, 2019, (6): 13-15
  • [2] Liu Yechun, Guan Xuesong, Sun Qi. Decorative Art of Wood – Taking Black Walnut as an Example [J]. Furniture and Interior Decoration, 2019, (2): 82-83
  • [3] Li Junling. Study on Factors Affecting the Sunshine Fastness of Poplar Dyeing and Preliminary Study on Its Dyeing Mechanism [D]. Zhejiang University of Technology, 2010
  • [4] Xie Wanzi, Yang Zhihui, Zhang Jingjing, Hui Zhikui, Zhang Jiayao. Comparison and analysis of accuracy and stability of different spectroscopic color measuring instruments [J] Journal of Textile Science and Engineering, 2018, 35, (4): 50-54
  • [5] A.Conesa,F.C.Manera,J.M. Brotons,J.C.Fernandez-Zapata,l. Simon, S. Simon-Grao ,M. Alfosea-Simon,J .J.Martinez Nicolas,J.  M.Valverde,F.  Garcia-Sanchez .Changes in the content of chlorophylls and carotenoids in the rind of Fino 49 lemons during mat uration and their relationship with parameters from the CIELAB color space[J].  Scientia Horticulturae,2019,243.
  • [6] Kong Fanxu, Shao Hailong, Wang Yanwei, Ye Jiahao, Jin Wenjie, Yu Huilai, He Xuehong. Color changes of three kinds of solid wood floor heating flooring after heat treatment [J] Anhui Agricultural Science, 2019, 47 (18): 191-193+203
  • [7] Xie Wanzi, Yang Zhihui, Zhang Jingjing, Hui Zhikui, Zhang Jiayao. Comparison and analysis of accuracy and stability of different spectroscopic color measuring instruments [J] Journal of Textile Science and Engineering, 2018, 35 (4): 50-54
  • [8] Xu Li, Wang Xinyi, Guan Xuesong, Zhu Nanfeng, Xu Changyan, Li Ju Ju. Colorimetric study on macro structural patterns of wood chord sections [J]. Packaging Engineering, 2020, 41 (5): 242-249
  • [9] Jia Tianyu, Niu Xiaoting. Evaluation of color and visual characteristics of 22 kinds of mahogany [J]. Northwest Forestry College News, 2017, 32 (6): 250-258+269
  • [10] Tang Chenglan, Wang Zhaocun, Liu Jinghua. Rapid Determination of Molybdenum in Ferromolybdenum by Thiocyanate Spectrophotometry [J]. Metallurgical Analysis, 2007, (11): 78-79
  • [11] Zhang Aoran, Wang Shuai, Li Xinyu, Bai Wenchao. Design of online testing device for wood dyeing quality [J]. China New Technology and New Products, 2019, (12): 15-16
  • [12] Zhang Wentao, Zhang Peng, Jiao Yuanqi, Fan Hongbo. Determination of formaldehyde emission from furniture based on phenol reagent spectrophotometry [J]. Journal of Dongguan Institute of Technology, 2016,23 (3): 62-64
  • [13] Gong Bao, Feng Jindong, Huang Libiao, Ou Shuling. Spectrophotometric determination of total flavonoids in rosewood furniture leftovers [J] Shi Zhen, Traditional Chinese Medicine, 2011, 22 (4): 894-895
  • [14] Atomic manipulation and lithography using scanning tunneling microscopy:Hosaka,S. Journal of the Institute of Electronics,Information and Communication Engineers[J].  Precision Engineering,1994,16(1):66.
  • [15] Kano Yukinori,Nakajima Masatoshi,Aida Asami,Seki Naoko,Foxton Richard M,Tagami Junji. Influence of enamel prism orientations on color shifting at the border of resin composi te res tora tions.[J]. Den tal ma terials journal,2018,37(2).
  • [16] Zhu Yayuan, Wu Zhizhi. Experimental study on scanning acquisition and chromatic aberration adjustment of digital wood grain [J]. Furniture, 2018, 39 (3): 8-10+21

Related News

  • * No Related Articles
en_USEnglish