Vigorously popularize the standard lighting conditions in the printing industry

In 1991, China's printing industry promulgated the standards for lighting and observation conditions used in the printing industry to evaluate color, standardized and unified the conditions for color evaluation, and laid the foundation for the 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, there is still insufficient understanding of the impact of lighting conditions on product quality. Therefore, only a few companies use standard lighting conditions, and even many companies still It is simply not clear what is the lighting conditions in the printing industry. The most common example is that the newly purchased presses have a standard look-up table. The light source used in the sample stand 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 daylight. However, when such a lamp is worn out, it is usually replaced by a general fluorescent tube, and a general fluorescent tube cannot meet the requirements of the illumination standard, and observing the color under such a source causes a large color sensation error.

With the continuous advancement 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 has been widely used in the industry. At present, the color information in the production process is almost completely transmitted in the form of numbers. Especially in color management and color control, lighting and observation conditions are an essential parameter in the color characterization file of the device, which is very important for color reproduction. The role. Nowadays, more and more enterprises are paying more and more attention to the dataization and standardization of the printing process. The lighting and observation conditions of color are a basic standard in the standardization system and should be paid enough attention.

2 The color of the illumination light directly affects the color of the printed matter. It is the kind of sensation produced by the light in the human visual system after it is irradiated onto the object and then enters the human eye through the action of the object. Due to the receipt period: 2003-04-22, this color perception is not only related to the optical properties of the surface of the object being illuminated, but also related to the spectral characteristics of the illumination light. The effect of illumination on the color of objects is a very complex problem, including optical problems, such as differences in spectral composition, as well as physiological causes of human color vision systems, such as color adaptation. Simply put, the colors of the light emitted by different light sources are different, or the proportions of the three primary colors of red, green and blue contained in the illumination light are different, and the proportions of the three primary colors of red, green and blue reflected by the object are also Different, so it will cause different color feelings. Many people have had this experience. A garment brand ink cleaning agent will not burn the iron can packaging. It is safe and convenient. It can quickly clean the ink on the ink roller to remove the dirt, ink and paper powder on the blanket. Add water to the ratio of 1:1 to 1:3, shake it and use the company's product catalog-F wetting powder quick-drying type moisturizing powder water bucket dampening solution (water bucket liquid) alcohol-free quick-drying water bucket liquid level Printing glue PS version protective glue) F-50 type powder spraying agent F-30 type powder spraying agent F-20 type powder spraying agent Shanghai Lide Fine Chemical Co., Ltd. Address: Room 701, Zhenyuan Building, No. 2052 Zhongshan North Road, Shanghai, China) The color is very satisfactory in the lighting conditions of the mall, but the color seen after buying it is different, or wearing the same clothes in different lighting environments, the clothes look different in color. This phenomenon is mainly caused by the difference in illumination light.

If you use the general formula for calculating color, the color is more clearly affected by the illumination. Let the spectral reflectance of the printed sample be pA), which represents the optical properties of the absorbed and reflected light on the surface of the printed sample, and the relative spectral power distribution of the light emitted by the source! iU said that different! A), the color of light feels different. The color of the printed sample at this time is expressed as the 1931 standard observer function by the CIE tristimulus value, which represents the spectral response curve of the human eye to red, green, and blue light, respectively, so the CIE tristimulus value ", ',) corresponds to the human eye. The ratio of the three primary colors of red, green, and blue is changed. It changes from the public, that is, the color feel changes.

3 Standard light source As can be seen from the above discussion, the color perception of an object changes with the spectral power distribution of the color source of the illumination source, and the perception of color under different illumination conditions will be different.

So, what kind of light source lighting effect should I use? Or what is the standard lighting condition? In principle, no light source is better than another light source, only people can say It is accustomed to observe the color under any lighting conditions, or to unify the lighting conditions when observing the color to the same conditions. Since human beings live in a daylight environment for a long time, the most familiar and most suitable for daylight, most people observe the color in daylight, so daylight should be considered as the most suitable light source, and the standard of daylight is Very reasonable. In other words, a light source close to the spectral power distribution of daylight should be used as the illumination condition for observing the color. The closer to the daylight, the better the illumination effect is, and the more accurate the observed color is. However, due to differences in time, climate, seasons, etc., the spectral power distribution of natural daylight is not exactly the same, and only one of them can be used as a standard, or the average value thereof can be used as a standard. Through a large number of measurements and experiments, the International Commission on Illumination (CIE) recommended the standard illuminant D to represent the spectral power distribution of daylight under different time periods and conditions, and the color temperature is different, representing daylight in different time periods and conditions, and it is recommended to use standard illuminant D+ The D illuminator with a correlated color temperature of 6504/) is the preferred illuminator and should be used as much as possible because it is the most consistent with 14 print quality and standardization for most cases of daylighting conditions.

-1999 Color Evaluation Lighting and Observation Conditions stipulates that the standard illuminant D+5 is used as the standard lighting condition for observing the color of the printed matter. However, since the spectral power distribution of daylight is very special, no artificial light source can emit the same light with the D+5 spectral power distribution, which can only be approximated. Therefore, the degree of approximation of the simulation becomes an important parameter for evaluating the quality of the light source, which is directly related to the effect of observing the color.

The method of strictly checking whether the light source conforms to the D+5 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 the sunlight. However, this method of inspection is very complicated, requires special equipment, and the measurement results are not representative of the color effect actually seen by the eye, which is not convenient for application. In practical applications, two parameters are generally used to characterize the color characteristics of the light source.

1) The color temperature/correlation color temperature of the light source +4) indicates the color of the light source, which is indicated by the absolute temperature scale.

5000/min is a low color temperature light source, and 5000/above is a high color temperature light source. The color temperature is low, the color of the light is reddish yellow; when the color temperature is high, the color is blue.

Generally, the color temperature of an incandescent light bulb is lower 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 the household fluorescent lamp is roughly bluer than the light emitted by the incandescent light bulb at 60007000 IK.

7) The color rendering index of the light source indicates the degree of proximity of the perceived color to the color perception when the same sample is observed under daylight under certain light source illumination conditions, and is expressed by calculating the color difference under the two conditions. The color rendering index of the light source ranges from 0100. The higher the value, the better the effect of observing the color under such a light source is closer to sunlight, and vice versa.

Since the illumination is specified as daylight, this parameter indicates the accuracy of the observed color. This is a very important parameter for lighting sources in industries where color is very high. According to the CIE regulations, according to the color samples used to calculate the color difference, the color rendering index is further divided into a general color rendering index, a and a special color rendering index, 9, respectively, indicating the color difference produced when lighting different color samples.

The above two parameters respectively reflect the two aspects of the color characteristics of the light source. When they are used together, the color characteristics of the light source can be described more comprehensively. These two parameters are generally given in the technical parameters when the light source is shipped. It is only suitable for the printing industry when these two parameters of the light source meet the lighting standards. For example, the general color rendering index of incandescent lamps, a" (5, meets the requirements, but its color temperature is only about 2800 / about, the light color is reddish, so it is not used in the printing industry. The color temperature of the commonly used fluorescent lamps can reach 6500K. Twilight light color, but its general color rendering index, a is around 70, lower than the standard, the color observed under this light source will have a large deviation from the observation under the light, not suitable for accurate color observation. It is not recommended to use. Fluorescent lamps that meet the lighting standards of the printing industry are a special type of fluorescent lamps, called high color rendering fluorescent lamps. This is the lamp installed on many imported printing machine supporting samples. Fluorescent lamps, but such fluorescent lamps are special lamps, which are generally difficult to buy and expensive. However, because their sizes are the same as those of ordinary household lamps, many original printers use ordinary fluorescent tubes instead when the original lamps are worn out. Therefore, the accuracy of observing the color is lost, which should cause the printer to pay attention to this issue.

4 Printing industry standard lighting and observation conditions The lighting factors that have a great influence on the observed color are: the color temperature of the light source or the correlated color temperature, the color rendering index of the light source, the illumination of the observation surface, the uniformity of the illumination on the observation surface, the color of the ambient light, and observation. Ways, etc. These factors are specified in the CY/T 3-1999 Color Evaluation Lighting and Observation Conditions. The following only describes the conditions for observing print samples.

1) Observing the reflected sample print) Light source used: A light source simulating the spectral power distribution of the standard illuminant D65.

The color temperature deviation of the illuminating body D*+ is described by the chromaticity deviation C, and the chromaticity deviation is required! 0.008. Color rendering index of light source: general color rendering index "a should not be less than 90, special color rendering index" 3 test color sample 915) should not be less than 80, for the calculation of color rendering index, please refer to 5B/T 5702 or related Colorimetric books.

Illumination of the observation surface: The light source produces uniform diffuse illumination on the observation surface, and the illumination range is 5001500lx, depending on the brightness of the color of the observed sample.

Illumination uniformity: The illumination of the observation surface should be as uniform as possible, and there must be no sudden change in illumination, and the difference between the illumination center and the ambient illumination should not be greater than 20:. Standard methods for measuring illuminance uniformity are specified in the standard.

Ambient light color: The surrounding area should be neutral gray, and the chroma value should be as small as possible. The background and substrate of the proof sheet should also be gray to avoid the effects of color contrast.

0 = illumination and observation mode, that is, the vertical illumination of the surface of the light source is observed in a direction of 45. from the vertical direction; or the light source is illuminated at a direction of 45. to the surface of the sample, and is observed in the direction of the surface of the vertical sample.

The most important factor affecting the color effect is the correlated color temperature and color rendering index of the light source. In general, the printing factory has no conditions to measure the relative spectral power distribution of the light source, the correlated color temperature and the color rendering index, and only the technical specifications provided by the manufacturer. Therefore, industry standardization bodies and technical supervision agencies should provide technical services to enterprises, guide enterprises to purchase what kind of lamps, and how to design lighting conditions.

The color rendering index of the light source provided by the general manufacturer is the general color rendering index "a, which can explain the effect of the light source on the color as a whole. However, any artificial light source cannot be completely the same as the spectral distribution of sunlight, and may be some The special color observation effect is not good, resulting in a large observation color difference. Especially for packaging and printing enterprises, often use spot color printing, it is possible to produce illumination color difference for a certain spot color, so it should also have a special color rendering index. The way is also one of the important factors that affect the color perception.

The main point of observing the color mode is that you can only view the diffuse light of the sample, but not the specular light directly, because there will be glare when there is specular reflection, and the true color will not be seen. For glossy paper, especially gold and silver cardboard, this phenomenon is very obvious. Both of the above observations are to avoid this situation.

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Address: 1st Floor, Yubei Guest House, No. 51, Liuhua Road, Guangzhou) Printing quality and standardization printing industry lighting standards have been promulgated for more than ten years, but the implementation in the enterprise is not ideal, many companies do not even know this standard. The main reason for this is the lack of awareness of the importance of the standard, and the promotion and implementation of the standard is not enough. From a technical point of view, the design of lighting conditions is a relatively professional technology, and it is not easy to buy professional lamps that are used in the printing industry. In addition, high costs are also one of the factors hindering the implementation of standards.

In fact, as long as the basic requirements of standard lighting conditions are mastered, it is not difficult to implement standard lighting. Achieving standard lighting conditions does not require the D65 source to be used throughout the lighting of the printing house, but only the department or location used to view the color using D65 source illumination. Specifically, it is required to equip the pre-press design and layout workshop, the printing machine sample table, the quality inspection department and the like with a standard light source for the place where the observation color is high, and other general illuminations can still use the ordinary illumination source. In the prepress design and layout workshop, due to the need to scan the original, page color design and creativity, image color adjustment and other operations, the requirements for illumination light are high, and the illumination light has a great influence on the color of the display, conditional Enterprises should use high color rendering fluorescent lighting for this workshop. If you do not have this condition, you should at least use high color rendering fluorescent lighting on the part of the workshop, or use a standard light box to facilitate color inspection.

In the printing workshop, general lighting can use ordinary fluorescent lamps, and high color rendering fluorescent lamps should be used in the sample and delivery platforms.

However, it should be noted that the overall illumination of the workshop and the illumination of the sample booth cannot be very different, avoiding the contrast and adaptation of the eyes of the operator due to the difference in illumination, resulting in a wrong judgment of the color.

In addition to the use of high color rendering fluorescent lighting, in practical applications, you should also pay attention to the selection of suitable lamps, so that the lamps produce uniform diffuse light. In general, diffuse reflection light can be scattered using frosted glass or high reflectivity grid plates.

Since the frosted glass has a certain absorption of the illumination light and also changes the spectral distribution of the light source, the grid plate is often used for scattering.

If four 36W high color rendering fluorescent lamps are used, the grid plate is used for scattering. The lamp tube is 1.2 * from the observation surface, and 10001 or higher illumination can be formed on the observation surface, which can basically meet the requirements of observing the reflection sample. In the standard, the illuminance value is not explicitly specified, and only one illuminance range is given because the contrast requirements are different when observing colors of different brightness levels. In general, a higher illuminance is advantageous for observing the color and image level, especially for observing the lower brightness and darker levels. In the case of strict product color requirements, such as cigarette printing, it is not enough to rely on the printing machine to see the sample platform. It must also be equipped with a higher standard light source observation box for more rigorous visual observation. This kind of observation box is usually equipped with a variety of light sources, and the color of the sample can be observed under different light source illumination, such as A light source and F light source, to check its metamerism.

Due to the limited use of high color rendering fluorescent lamps, they are only used for professional purposes, so they can only be ordered by professional manufacturers at present, and the price is relatively expensive. From the situation of high color rendering fluorescent tubes purchased by the Color Printing Laboratory of Beijing Printing Institute, the current printing quality of imported lamps 16 and the standardized price of 2003.3 tubes are about 100 yuan/root, and the life of lamps used is about 10000h. Electronic ballasts also extend life. According to the calculation of such data, a sample station with 4 lamps can be used for 10,000 hours, calculated for 12 hours per day, and can be used for 23 years. The cost per lamp is 0.43 yuan per day. This cost will not increase the number of enterprises. cost. On the contrary, if it can reduce waste and waste, it will bring considerable economic benefits. If market consumption increases in the future, it is estimated that the price of high color rendering fluorescent tubes will also decrease. In fact, high color rendering fluorescent lamps are also divided into several files, with different correlation color temperature and color rendering index. The higher the color rendering index, the more expensive the price.

For less demanding requirements, you can use a fluorescent tube with a general color rendering index of 85, the price ratio! a "90 fluorescent tube is much cheaper, only a few tens of dollars. This kind of tube is more suitable for general lighting in the prepress workshop, the effect will be much better than the ordinary fluorescent lamp! a"70.

With the digitization of the printing process, the color control of printed matter will become more and more strict in the future, and digitalization will be gradually realized. This puts higher requirements on the infrastructure of enterprises, and the standardization of lighting in enterprises is one of the important infrastructures. . Nowadays, many companies have begun to realize its importance, and some users have also tested color chromatic aberration as one of the conditions for product acceptance. Lighting standards are also an important part of implementing color management in an enterprise. Among the various standards related to printing colors, the CIE unified color standard is being gradually described. Therefore, accelerating the standardization of lighting in printing enterprises is an urgent task for printing companies and an inevitable way for the development of printing enterprises.

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