With the arrival of the post-quota era of textile trade, the textile supply chain will change, and textile printing and dyeing companies are facing another reshuffle.
The quality of fabric color control will become an important aspect of fabric supplier competition. The color selection of fabrics (here refers to clothing fabrics) comes from the designer’s creative inspiration. Together with the clothing style, it becomes an element of the work and determines the market prospects. If the color does not meet the designer’s expectations under certain light source environments, the image of the product he designed will be affected. The color of high-quality fabrics will never allow metamerism.
1 The phenomenon of metamerism
1.1 The concept of metamerism
Metamerism (shown as jumping lights), often referred to in the printing and dyeing industry, refers to the color matching of two colors when observed by a standard observer under a specific light source (CWF light source is often used as the main light source in the US market). When changing this specific light source (using D65, A light source as the secondary light source), the colors no longer match.
From the perspective of color theory, the explanation of metamerism should be: two color samples have different spectral reflectance curves and the same tristimulus value, then the two colors are called metameric colors. .
1.2 Causes of Metamerism
1.2.1 Light Source
Light is a kind of electromagnetic wave that can cause the human eye to The part of visual perception is called visible light, and its wavelength range is 400~700nm. Light that has a single wavelength and can produce a specific color feeling is called monochromatic light, but the light emitted by a general light source is mixed with monochromatic light of different wavelengths. This kind of light is called polychromatic light.
Due to the different monochromatic lights involved in composing complex-colored light, the spectra composing these lights can be divided into discontinuous spectrum and continuous spectrum (Table 1).
For example, some light is made up of a mixture of several monochromatic lights, and the spectrum that makes up these lights is called a discontinuous spectrum; while some light is made up of a mixture of individual monochromatic lights in the range of 400~700nm. The spectrum that makes up these light rays is called a continuous spectrum.
Figures 1 to 3 show the energy distribution of light source A, light source ACWF, and light source D65 respectively.
1.2.2 Color
Color is caused by the light source acting on the object. Through the absorption and transmission of the object, it is finally reflected from the object to the observer, and then the observer makes a visual response.
1.2.3 Causes of Metamerism
The color of any object has its specific spectral reflectance curve. When the object is exposed to a specified light source Reflecting the visible spectrum to a standard observer produces spectral tristimulus values.
When the spectral tristimulus values of two objects with different spectral reflectance curves are equal, the two objects are considered to be conditional metamers. Once the light source changes, due to the different energy distribution of each light source, the generated spectral tristimulus values are no longer equal, which results in the phenomenon of light jumping; there are many posts about light jumping in Dyer Without Borders.
For example, color A and color B are metachromatic under CWF light source. Color A is the standard color sample and color B is the reference color sample. The spectral reflectance curves of colors A and B are as follows (Figure 4) ：
The average energy of A light source near 700nm is much higher than that of CWF light source. Near 700nm is a red-orange light source, while the spectrum of color A is near 700nm. The reflectivity is higher than color B, so when switching from a CWF light source to a D65 light source, color A will appear redder than color B.
The average energy of the D65 light source in the 450~550nm wavelength band is much higher than that of the CWF light source. The color of the light source in this wavelength band is blue-green. The spectral reflectance of color A in this band is higher than that of color B, so When converting from CWF light source to D65 light source, color A appears bluer-green than color B.
When customers give proofing requirements to the dyeing factory, they often ask the factory to use 2 to 3 light sources for color matching, and there must be no jumping lights.
The color standard may be the actual color, or it may be a set of spectral reflectance data. However, because the dyes selected by the factory only take into account one of the light sources, the reflectance curve of the dyed sample is different from the standard sample. , this will inevitably cause the light to jump.
1.3 Judgment method of metamerism
1.3.1 Naked visual observation method
This method is more intuitive and customers often use This method is used to judge whether the light jumps, the direction of the light jump, and whether the degree of the light jump is acceptable. The required equipment is relatively simple and only requires a multi-light light box.
1.3.2 Colorimeter evaluation method
This method is more accurate and uses data to determine the direction and extent of the light jump.
Method 1 is based on the spectral reflectance curve of the two colors;
The spectral reflectance curve in Figure 4 shows the color matching of the two khaki colors under the CWF light source. Color A is the color standard sample, and color B is the comparison sample. However, these two curves intersect many times, and the color inversion rate of color A is much higher than that of color B near 700nm, so when the light sourceWhen switching from CWF to A, color A is redder than color B, that is, reference color sample B has a green jump phenomenon under light source A.
Similarly, the average energy of the D65 light source in the 450~550nm range is higher than that of CWF. The reflectivity of color A in this wavelength range is slightly higher than that of color B. Therefore, when the CWF light source is converted to the D65 light source, the color A appears greener than color B, that is, color B jumps to red under the D65 light source.
Method 2 uses the CIE1976 LAB system
Using one color as the standard, measure the △ L* of another color under each light source against this standard color. △ a*, △ b*, △ E*, and then judge the direction and degree of the light jump based on the values under each light source.
Table A shows the color difference values of CIE1976 LAB between reference color sample B and standard color sample A under CWF, A, and D65 light sources.
From the table (as can be seen from the data, the standard color sample) and the reference color sample B have metamerism. The △ a* under the A light source is smaller than that under the CWF light source. , indicating that when the light source is switched from CWF to A, the reference color sample B (compared to the standard color sample) has a green jump phenomenon.
Similarly, when switching from the light source CWF to D65, color B jumps red and blue compared to color A. The degree of light tripping can be judged from the difference between them.
1.4 Color categories prone to metamerism
Represented by the three-dimensional space of Figure 5 For all colors, the vertical axis represents the change in brightness of the black and white series. The upper end is white, the lower end is black, and the middle is a variety of transitional grays.
Hue is represented by a circle on the water plane. Each point on the circle represents various hues in the visible spectrum (red, orange, yellow, green, cyan, blue and violet). The center of the circle is gray. Its brightness is the same as the brightness of the various hues on the circle. The color’s saturation gradually increases from the center of the circle outward.
The saturation of various colors on the circle is the highest. When the direction of the circle changes upward (white) or downward (black), the saturation of the color also decreases.
As a rule of thumb, the more saturated the color, the less likely it is to be metameric.
For example, if you use single-color yellow, red, and blue, there is basically no light jumping phenomenon. There is no metamerism in the colors shown on the cone surface in the upper half of Figure 5. The lighter the color, the smaller the degree of metamerism. If the color is as light as white, there will be no flashing lights.
On the contrary, the darker the color, the greater the possibility of metamerism. The darker the color, the greater the degree of light jump. For example, the possibility and degree of light jump of black, navy blue, brown and other colors are Very big. That is, in the lower half of the cone in the picture, the further downward (black) and toward the center of the circle (gray), the greater the possibility of metamerism.
2 Methods to avoid metamerism
2.1 Computer color matching method
Collect dyes extensively and establish Dye database, based on the customer’s color standards, select a group of dyes that have no metamerism or minimal metamerism and are suitable for production.
2.2 Empirical method
Analyze and summarize each dye to determine whether each dye is prone to light tripping and how to avoid light tripping. If necessary, four-color matching can be used to improve the dye control level to meet customer needs. </p
