Popularize the standard lighting conditions of the printing industry

The spectral characteristics of the illumination source have a great influence on the color of the observed print. Through the discussion of the color characteristics of standard light sources, the standard lighting and observation conditions in the printing industry are introduced, and various indicators are briefly described. Suggestions are made on how the printing companies can implement the standardization of lighting conditions.

I. INTRODUCTION In 1991, China's printing industry promulgated standards for lighting and observation conditions used by the printing industry to evaluate colors, standardized and unified the conditions for color evaluation, and laid the foundation for color information exchange between the printing industry and other industries. .

However, due to the long-standing demand for product quality in China's printing companies is still not very high, but also lack of understanding of the impact of lighting conditions on the quality of the product is still insufficient, so only a few companies use standard lighting conditions, and even many companies are fundamental It is not clear what is the lighting conditions in the printing industry. The most common example is that newly purchased printing presses are equipped with a standard viewing stage. The light source used in the sample stage is a high color rendering D65 lamp. The color temperature and color rendering index meet the requirements of the lighting standard. The observed color under illumination is similar to that observed under sunlight. However, when such a lamp is damaged, it is usually replaced with a general fluorescent lamp. However, a general fluorescent lamp cannot satisfy the requirements of the lighting standard. Observing the color under such a light source will cause a large color perception error.

With the continuous progress of printing technology, digital technology has gradually entered all aspects of printing production, and the use of color measuring instruments has become more and more widespread, and it has been widely used in the industry. At present, the color information in the production process is almost completely transmitted in digital form. Especially when performing color management and color control, lighting and observation conditions are an essential parameter in the device's color profile, which plays an important role in color reproduction. The role. Nowadays, more and more companies pay more and more attention to the digitization and standardization of the printing process. The color lighting and observation conditions are a basic standard in the standardization system and should be given sufficient attention.
Second, the color of the lighting directly affects the color of the print. The color is the feeling that the light hits the object, enters the human eye after passing through the object, and is produced in the human visual system. Therefore, this color perception is not only related to the optical properties of the surface of the irradiated object but also to the spectral characteristics of the illumination light. The influence of Mingguang on the color of objects is a very complicated issue, among which there are optical problems, such as differences in spectral composition, and also the physiological causes of the human color vision system, such as the phenomenon of color adaptation. Simply put, the light emitted by different light sources has different colors, or the proportion of red, green and blue primary colors contained in the illumination light is different, and the proportions of the three primary colors of red, green and blue reflected by the object after being irradiated are also Different, so it will cause a different color feel. Many people have had this experience. The color of a piece of clothing is very satisfying in the lighting conditions of the shopping mall, but the colors they see after buying it are not the same, or they wear the same clothes under different lighting conditions. The colors of clothes look different. This phenomenon is mainly caused by the different lighting.

If you use the general formula for calculating the color, the color is more affected by the illumination light. Let the spectral reflectance of the printed sample be ρ(λ), which represents the optical properties of the absorbed and reflected light on the surface of the printed sample. The light emitted by the light source is represented by the relative spectral power distribution S(λ), different S(λ), The color of light does not feel the same. The printed sample color at this time is expressed as CIE tristimulus values ​​as:
X=κ∫S (λ) ρ(λ)χ(λ)dλ
Y=κ∫S(λ) ρ(λ)y(λ)dλ
Z=κ∫S(λ) ρ(λ)z(λ)dλ (1)
In the formula, χ(λ), y(λ) and z(λ) are the CIE1931 standard observer functions, which respectively represent the human eye's spectral response curves for red, green, and blue light, so CIE tristimulus values ​​X, Y, Z Respectively corresponds to the proportion of red, green, and blue primary colors perceived by human eyes. It can be seen from formula (1) that when the object and observer are not changed, the illumination source is changed, that is, S(λ) in formula (1) is changed, then the CIE tristimulus values ​​X, Y, and Z of the object are changed. , that is, the color sense changes.
Third, the standard light source
From the above discussion, it can be seen that the color perception of an object changes with the color of the illumination source (the spectral power distribution of the light source), and the perception of viewing the color under different lighting conditions will be different. So, what kind of light source lighting effect should be used? Or, what is the standard lighting conditions?

In principle, no one light source is better than another light source, and it can only be said that people are more accustomed to seeing the color under the lighting conditions, or the lighting conditions when viewing the colors are unified to the same conditions. Since human beings live in the sunlight environment for a long time and are most familiar with sunlight, they are most adaptable. Most of the observations are made in daylight. Therefore, sunlight should be regarded as the most suitable light source. The standard for daylight as lighting is Very reasonable. In other words, a light source that is close to the spectral power distribution of daylight should be used as the illumination condition for observing colors. The closer it is to sunlight, the better the lighting effect is, and the more accurate the observation color is. However, due to the differences in time, climate, season, and other conditions, the spectral power distribution of natural daylight is not exactly the same. Only one of them can be used as a standard or its average value can be used as a standard. Through a large number of measurements and experiments, the International Commission on Illumination (CIE) recommended that the standard illuminant D represents the spectral power distribution of sunlight at different times and conditions (corresponding to different color temperature, representing different periods and conditions of sunlight), among which the standard illuminant is recommended. D65. (The D illuminant with a correlated color temperature of 6504 K) is the preferred illuminant and should be used as much as possible because it is closest to the conditions of daylight illumination in most cases.

Because of this, it is specified in CY/T3-1999 "Color evaluation illumination and observation conditions" that a standard illumination body D65 is used as a standard illumination condition for observing the color of a print. However, since the spectral power distribution of sunlight is very special, currently no artificial light source can emit light that is exactly the same as the D65 spectral power distribution and can only be approximated. Therefore, the approximation degree of the simulation becomes an important parameter for evaluating the quality of the light source. It is directly related to the effect of observing the color.

The method of strictly checking whether the light source conforms to the D65 spectral power distribution is to measure the spectral power distribution curve of the light source and compare the spectral power distribution curve of the light source with daylight. However, this inspection method is very complicated, requires special equipment and the measurement results do not represent the actual color effects seen by the eye, and are not easy to apply. In practical applications, two parameters are generally used to characterize the color characteristics of a light source.
(1) The color temperature of the light source/correlation color temperature (TC) represents the color of light emitted by the light source and is expressed by the absolute temperature scale K. Below 5000K is a low color temperature light source, 5000K above is a high color temperature light source. The color temperature is low, and the color of the light is reddish yellow. When the color temperature is high, the color is blue. The color temperature of a typical incandescent light bulb is less than 3000K, which is a low color temperature light source, so the color of the light is reddish yellow; and the correlated color temperature of a household fluorescent lamp is generally about 6000-7000K, which is more bluish than the light emitted by an incandescent light bulb.
(2) Color Rendering Index of Light Source
Indicates the degree of sensation of the observed color in a certain light source lighting condition and the similarity of the color sensation when the same sample is observed in sunlight, expressed by calculating the color difference under the two conditions. The color rendering index of the light source ranges from 0 to 100. The higher the value, the better the effect of observing the color under such a light source (the closer it is to sunlight), and vice versa. Due to the requirement to use daylight lighting as the standard, this parameter indicates the accuracy of the observed color, which is a very important parameter for lighting sources in industries where there is a high demand for color. According to the CIE's regulations, according to the color samples used to calculate the color difference, the color rendering index is divided into the general color rendering index Ra and the special color rendering index R i , which respectively represent the color differences generated when illuminating different color samples.

The above two parameters respectively reflect the two aspects of the color characteristics of the light source. Using them together can describe the color characteristics of the light source more comprehensively. The two parameters are generally given in the technical parameters of the light source at the time of leaving the factory. Only when the two parameters of the light source meet the lighting standards, is it suitable for use in the printing industry. For example, the general color rendering index Ra of an incandescent lamp is ≥95, which satisfies the requirements, but its color temperature is only about 2800K, and the light color is reddish, so it is not used in the printing industry. The general use of fluorescent