Colors check during printing
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Customer complaints
“Our products are personally printed with the machine. Each version is the first 'OK' that I confirm the color and signed. The color matching I have confirmed on the shop floor is very high, the color effect is also very good, and I am very Satisfied, but now the color of the product I received and the color of the digital sample are far from each other. What is going on here? How do you manage quality?"
Similar complaints from customers like this, I believe that many printing companies have received it, and customers have never even returned. In fact, this is not the printing master did not match the good color, not the process quality control is not good, "metachromatic" is the culprit.
The problem analysis
When shopping in a mall in real life, you feel like the color of a certain product, hat, etc. under the fluorescent light, but the color seen under the incandescent lamp is not the same when you buy it home. If you are a printing buyer, the color of the printed products confirmed by the printing company at the time of printing will be observed in your own office environment, and the color perception of the printed matter may be biased.
The formation of color is inseparable from the spectrum, and any object has a specific spectral reflectance curve. When the spectral tristimulus values of two objects of different spectral reflectance curves are equal, these two colors are considered to be equal conditions of color. Due to the different energy distribution of each light source, once the light source changes, the generated spectral tristimulus values are no longer equal, and the color results are also different.
The generation of color sensation requires four major elements: light source, color object, eye, and brain. When any one element is indeterminate or changes during observation, the color result of the feedback will change, and the observer cannot accurately determine the final color effect. Therefore, the factors affecting the results of color evaluation in real life are not only the observer itself, but also the light source, the size of the object, the angle of the observer, and the background color of the target observer.
There is a certain relationship between the reproduction of a color and the characteristics of the light source that observes the color. Some two objects appear the same color under one light source, but under another light source, they show different colors. This phenomenon is called "the same color. Abnormal spectrum phenomenon. In fact, the process of color printing and copying is to use the same-color different spectrum phenomenon to achieve the purpose of reproducing color. However, the results of color matching can easily be broken when the conditions of observation change. The color matching of the printing process often only achieves the homochromatic matching under a certain lighting condition.
Color and density measurements are a common method of controlling the quality of an item's reproduction in printing. In the printing process, we usually compare the copied image with the original image to determine the degree of color reduction, so as to obtain the faithfulness of the original image. Although we try to take quantitative indicators to avoid the influence of human factors on the degree of image reduction, the observation of human eyes still plays an important role in image restoration, so it is obvious that the error in color observation in the printing field is controlled. Especially necessary.
Because the light source and the observation conditions are different, there are also differences in the coloration of each material (such as paper and ink). This directly affects people's judgment of the final effect of the printed matter. In order to avoid misunderstanding of the effect of color reproduction, the reproduction of the printed matter and the final observation are ensured. The link has a unified observation condition. The ISO 3664 standard makes strict regulations on the standard light source for color evaluation in the printing industry. The following section gives a brief introduction to some of the indicators in the regulation.
The color of a light source, called the color temperature, is expressed in terms of the absolute temperature Kelvin (K) by the different colored lights emitted by the black body to different temperatures. The color temperature of the light source determines the color characteristics of the light source. The color temperature of the light source depends on the relative energy ratio of the light of different wavelengths in the light emitted by the light source. The higher the K value, the more the color developed tends to white blue; the lower the K value is The color of the appearing tends to be yellowish red.
The illuminance is the intensity of the light. The higher the illuminance, the brighter the light source; the lower the illuminance, the darker the light source. For printing applications, there are two main use cases. One is to use high-illuminance light source illumination, so that you can find every detail of the print, which is used when strictly comparing the prints, but for the highlights of the highlights. The effect is greater and may result in loss; the other case is the use of relatively appropriate illumination as a daily inspection of the printed matter.
The color rendering index Ra is a measure of the ratio between the color that can be seen by a certain light source and the color that can be seen under natural light. The closer Ra is to 100%, the smaller the difference between the color developed and the color developed under natural light illumination. For the printing industry, the higher the color rendering index, the higher the degree of reduction in color observed therein; the lower the color rendering index, the lower the degree of color reduction. Generally, Ra ≥ 90%.
The metamerism index (ultraviolet light region) is less than 1.5, preferably less than 1.0. Paper mills add fluorescent whitening agents to the paper in order to increase the whiteness of the paper. The UV spectrum in the light source will excite the fluorescent components in the paper, causing errors in the resulting color results. The higher the UV spectral content, the greater the difference in color.
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Solve the proposal
First of all, no matter if it is a print production or a visual evaluation of the final color of the printed matter, it must be conducted under a uniform light source that conforms to the ISO 3664 standard, otherwise it will produce metamerism and eventually lead to color differences in visual perception. The D50 standard light source is the first choice for color visual evaluation.
Secondly, when the customer wants to visually evaluate the color and there is no standard light source available, if the weather is good, the customer can be advised to take the sample to the outdoor natural light for color visual evaluation, so that the color error will be reduced by a lot, but it must be kept in mind However, it must be noted that during the period around noon, it is important not to perform visual assessment of colors in the early morning or evening hours, nor to let the sun shine directly on target evaluation samples.
Finally, since there are scientific methods, there are unscientific methods, and sometimes unconventional methods can help us solve or cope with immediate problems. In reality, it is inevitable that you will encounter customers who are smashing, so that you can't reverse their thinking, insist on visual evaluation of colors according to their own non-standard light source conditions, and customers are core customers, and can't afford to offend, what should I do? It is a temporary measure to meet the customer's requirements by temporarily installing the same type of light source as the printing process on the printing machine's sample platform. However, companies still must constantly implement standard methods for customers to train and guide customers to manage and evaluate colors in the right way. It must not make "non-standard" become "standard."
Of course, the best way to color evaluate is to use a standard instrument for data measurements. The instrument is not affected by the mood, and there is no judgment difference caused by human sensory differences. It is the most scientific and objective color evaluation method.
The "half-color spectrum" phenomenon is one of the principles of printing technology. It is a double-edged sword. On the one hand, it enables the reproduction of printing colors, and on the other, it imposes limitations on the color evaluation of reproductions. Light source conditions for color reproduction and evaluation. In the color management process, the standard light source plays a vital role, and it is the most easily overlooked. Whether it's a printing company or a print buyer, it's important to remember that a standard must be based on one or more standards.