Analysis of factors affecting printing quality affected by observation conditions

Observation condition

The appearance color of printed matter and other objects is largely affected by the observation conditions. Therefore, all judgments about color must be made under repeatable conditions. To achieve this, everyone involved in color reproduction must use standard viewing conditions. The term "standard observation conditions" refers to an environment in which the following conditions are carefully controlled:

●Color of the light source ●Intensity of the light source ●The effect of spectral reflection ●Environmental color ●The influence of non-standard ambient light source ●The visual adaptation of the observer

An environment that meets the above conditions can be obtained by equipment designed and supplied in accordance with ISO 3664. The observation frame or observation station requires regular maintenance to ensure that its conditions meet the criteria to be maintained. The maintenance schedule should include the following tasks:

● All neutral surfaces, fluorescent tubes and separators should be cleaned at least once a month, and the cleaning materials used should not cause damage to these surfaces.
● Fluorescent tubes should be replaced after 2000 hours of use.

The color temperature of the standard observation source is 5000K, and some people suggest that it is more appropriate to use the user's observation conditions. There are two important reasons why this is not realistic:

(1) It is impractical to combine every possible light source. (2) The goal of many prints is to coordinate cooperation and simultaneous input. This input comes from people with different divisions of labor, they are in their respective companies and even their respective countries. And thus have different environments. Standard observation conditions ensure that all participants, regardless of where they are evaluated, can be consistent with the starting point environment.

The only non-standard lighting conditions that should be used are the conditions in which the conditions of the primary environment have been determined and controlled. Typical examples of such conditions are at retail locations, such as supermarkets, for packaging or display materials ordered for supermarkets or shopping malls. The final assessment should be carried out under the lighting conditions associated with the mall environment.

For many printers, looking for an observation environment that faithfully displays the color of the original, to identify a very troublesome problem. An original manuscript that is undergoing production reproduction, under what environmental conditions, should the evaluation be sufficient, so that there will be no color difference?

The light source will change with time and environment, and the object has different coloring conditions under different light sources. If it is necessary to produce printed matter suitable for observation under different light sources, cost and manpower will cause unnecessary waste. Therefore, we must develop an objective and standard environment to ensure that uniform observation conditions can be used throughout production. [next]

5000K misunderstanding

Some manufacturers say that the color source light source used by them is already a 5000K color temperature light source. Why is the same 5000K light source environment for the guests, and the color of the manuscript still has errors? In fact, 5000K only describes the color temperature of the light source, and does not represent the whole of the light source. Correctly speaking, the light source emitted by the D50 standard illuminator is the light source index for the approved printing process. Please be sure to know that this light source must have sufficient color rendering in addition to the proper color temperature.

Let's talk about the origin of 5000K color temperature. We will heat a standard black body that can completely absorb and radiate energy. When the temperature is gradually increased, the color of the light source will change accordingly. The color temperature of 5000K is to heat the black body to 5000K. When the light color changes in white, and the spectral energy distribution is measured, the energy of the band in the blue, green and red regions is in an equipotential state, that is, the most ideal white light, and the color cast is the lowest, so the printing process uses such a light source.

In addition. It is the problem of color rendering. Color rendering refers to the degree to which an object conforms to the color of sunlight and artificial light. The color of the object in daylight is the most accurate, because the spectral energy distribution in daylight is the most comprehensive, and it can fully express the color of the object. When the artificial illuminating body responds to different materials, the spectral energy distribution is still very different from that of daylight. However, even if the illuminating body color temperature has reached 5000K artificial white light, the object color is still with sunlight. The following is different. Most of the problems are that some monochromatic light components are often missing from artificial light sources, and their color rendering index is also lower than 100. The color rendering index represents the distortion of the object. The high color rendering index does not mean that there is no color cast. The color temperature and color rendering of the light source should match each other. The color temperature is the index of the light source color, and the color rendering is the quality index of the light. The setting of the CIE D50 standard illuminating body is Explain that the quality hue of the light source should be standardized.

Standard illuminator and standard light source

The CIE International Lighting Standards Association has different degrees of color effects on objects according to different light sources, thus specifying the following standard illuminators and specifying their spectral energy distribution.

However, the above standard illuminators can be realized by different combinations of light sources (simulated spectral energy distribution), but different substances have different stability, so C! E recommends a series of standard light sources (artificial light sources) to match the standard illuminators described above. [next]

ISO3664 standard observation conditions

When talking about so many light sources and illuminating materials, what is the most ideal color drafting environment? The ISO International Standards Accreditation Association has proposed "ISO3664 to observe the lighting conditions of color transparency and reproductions" (new version is ISO3664:2000), which refers to the observation conditions in the printing and copying process, see the following four points: 1. Lighting source Spectral energy distribution, 2. illuminance and uniformity of the light source, 3. observation of environmental conditions, 4. stability of the lighting environment.

In fact, ISO3664:2000 has stricter requirements on lighting sources. It considers the new color imaging technology and drafting system that is currently being proposed, and also sets the standard for the problem of the production of metamerism under different light sources.

Illuminance

Another important factor is that it recommends two illuminance standards, which are used in the following situations:

1. High illumination 20001±500IUX - used to evaluate and compare images, and strictly evaluate prints.
2. Low illumination 5001 ± 25 IUX - used to distinguish image dark detail under similar final viewing conditions.

Observation environment

Observing the environment may be a neglect of the printing industry. I believe you must have seen it. There are many things around the observation environment in the workshop, such as posters, inks, cigarette packets, mobile phones, etc. Some places on the color draft are still on the window sill. Next to each other, some are more dusty, with the following tips in the ISO standard:

1. Minimize ambient interference.
2. After entering the observation environment, the printed products should not be judged immediately, so that the workers have time to adapt to the environment.
3. There should be no extra light entering the viewing range (including reflections).
4. There should be no strong colors around the (worker's overalls).
5. The surrounding area of ​​the observation range should be a neutral gray dull and a color block with a reflectance of less than 60%.

In fact, the above are all issues that the printing factory should pay attention to. These conditions are just a signal that can be implemented, which not only improves the environment of the manuscript, but also improves the quality of production and enhances the professional image in the factory. [next]

How to choose the right light source equipment

The easiest way is to buy a light box that conforms to the ISO standard. In addition to meeting the iSO requirements, there are a variety of light sources to choose from, such as D65, D50, UV, TL84, A, etc., suitable for different industrial needs. Please refer to the following points when purchasing:

1. There are many brands of light pipes on the market that indicate a foot of 5000K (unfortunately, CIE has not yet specified the standard light source application of D50 standard body), but it does not necessarily meet the requirements specified by ISO: color temperature 5000K, color rendering index ≥ 9.
2. As long as the panel material can be close to the ISO limit, such as the chroma is medium gray - the color index of the surface of the material can be measured by a spherical spectrodensitometer, close to Lab-50, 0, 0, and the reflectance is measured by gloss meter. The reflectivity is three 60%.

test method

It is generally sufficient to measure the change in color temperature during production because the stability of the illuminator changes with time and environment. There is a cheaper way to check the color temperature on a daily basis. A sticker called GATF/RHEM (produced by GATF, sold by the Hong Kong Graphic Arts Society). When we put the indicator on the observation environment, when the color temperature changes. It will warn you if the operator needs to replace the light pipe. [next]

Metamerism in color printing

The phenomenon of metamerism is a phenomenon that is ubiquitous in nature. Simply put, it refers to a color whose appearance looks the same but whose spectral composition is not the same. In other words, the two color samples have different spectral reflection curves, but have the same visual effect under the illumination of a certain light source.

Metamerism is ubiquitous in the field of color reproduction, and it often encounters the problem of color matching in the work, that is, it requires the same color as the sample. It is difficult to achieve the same color appearance and the same spectral characteristics, so most of them are metachromatic.

The color materials used in the color printing process cannot be exactly the same as the color materials used in various types of color originals, such as Chinese painting pigments and watercolor paints used in various painting originals. Therefore, the color printing reproduction process utilizes the phenomenon of metachromatic color to achieve the purpose of reproducing colors.

The metachromatic color is conditional and must be the same under the same viewing conditions, mainly the source. Experiments have shown that two meta-colors of the same color no longer maintain the same color due to the change of the illumination source. In most cases, accurate meta-color matching is difficult to achieve, and generally only approximate isomers can be matched. In the color printing, when the three primary color inks are used to match the spot color inks or the three primary color dots are used to overprint the various colors, there will be a certain color difference. In actual production, the color difference should be allowed when the replica and the original are matched by the same color spectrum, but the copyer should control the color difference as much as possible within the allowable range specified by the standard.

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