Differentiating the same color in the printing industry and its control

In real life, a pair of socks or gloves with the same color under the fluorescent light when shopping in shopping malls and supermarkets is not the same as when buying a home under an incandescent lamp. In the printing and packaging industry, the exquisitely printed packaging box before the factory was of the same color and accuracy, but in the hands of major shops or users, it was found that the colors were quite different. These will make users doubt the authenticity of the goods, require returns or even complaints, the credibility of the natural business is also damaged. In fact, this phenomenon is called metamerism. The following discusses the generation, detection, and control of homochromism:
First, the occurrence of metamerism phenomenon Each kind of object has its fixed optical characteristics, that is, which color light it absorbs, and the nature of the reflected light is fixed. In the full spectrum of sunlight, the color of each object is fixed, and the intrinsic color does not change, meaning that the optical properties of the object and the colors present in daylight do not change. It is not difficult to think that when the spectral composition of the light source changes, both the monochromatic light of a certain wavelength range is missing in the light source, and the color light of this wavelength range is the color light reflected by the optical characteristics of an object. Under this light source does not show its inherent color. In this variation, the spectral data of the object itself is unchanged. In other words, the spectral data describes the surface properties of the colored object. Unlike the observer, the copy method has no effect on the reproduction method. While in the printing color (CIE system) characterization, three-dimensional coordinates are used to represent the colors in the color space, color reproduction (copying) such as RGB or CMYK to generate three colors of mixed colors, which depends on the viewing conditions to reproduce Color device type, observe the type of lighting used by the color. That is, the spectral data is the only true description of the actual color.
Based on this, when a pair of colors is under the same light source, the same color appears, but under another light source, the colors present are different. This phenomenon is called metamerism. Actually two color stimulus values ​​(three data representing the color), although their spectral distributions are different, have the same color visual effect, ie have equal tristimulus values. When the intersection of the spectral reflection factor curves of two samples is more, the number of light sources that can keep the colors of the two matches is more. If the sample is matched under any light source, it is called homochromatic spectrum phenomenon, and its spectral reflection curve shape is also the same.
Second, the detection of metamerism The definition of lighting properties is part of the description of the color, CIE provides a series of A, B, C, D and other lighting, A lighting which represents the color temperature of 2856 ° K simulation of the northern sunny morning 10:00 to The sunshine at the northern window between 3 pm, because the light of the northern window is relatively stable, the sun rises east and west descends, the color temperature changes little, and the spectral composition is complete. In ancient times, humans depended on daylight during the day, and nights depended on fire (fire) for color discrimination. Therefore, it is most ideal for us to observe homochromatic phenomena under these two types of illumination.
The International Commission on Illumination (CIE) has recommended a set of metamerism indices to evaluate the degree of metamerism of colors. The method is for a specific reference illuminator (CIE D65, CIE A) and an observer, two different spectrally identical samples with the same tristimulus value, illuminated with test illuminators with different spectral power distributions, resulting in The color difference (ΔE) is called the metamerism index Mt.
At present, there are only D65 standard light sources for color light boxes in the domestic standard, which do not have the effect of testing homochromatic spectrum. As a leading supplier of testing equipment for the packaging and printing industry, Jinan Languang is currently working on the development of such testing equipment. According to an authoritative source from the company, the equipment will be unveiled at the South China International Packaging and Printing Exhibition in April 2002.
Third, the control of the same-color different spectrum phenomenon Modern printing is developing in the direction of multi-color, anti-counterfeit, etc., and more and more spot colors have been prevented in addition to yellow, color, blue and black base colors. The same batch of spot color inks are prone to metamerism after printing. Common four-color printing uses different batches of ink, and even though the conditions such as the color sequence of printing are the same, the same-color heterochromatic phenomenon will occur. Therefore, when metamerism is found, the ink supplier should be required to provide the same ink. Printing houses should also use the same batch of ink as much as possible to reduce or avoid metamerism.