Standard lighting and observation conditions for c

Standard lighting and observation conditions for color reproduction in the printing industry

in nature, light and color are inseparable, and there is no color without light. Different light sources have different radiant energy and show different colors when shining on different objects. Even the same color can make human eyes have different color feelings under different lighting conditions. Therefore, in the printing reproduction, the technical measurement, control and visual evaluation of color need to be carried out under the unified standard lighting conditions

1 standard illuminants and standard light sources

in order to unify the color measurement and evaluation standards, CIE (International Commission on illumination) stipulates four standard illuminants a, B, C, D and three standard light sources a, B, C. The standard illuminant specified by CIE refers to the specific spectral energy distribution, which is the specified color standard of light source. This specific spectral energy distribution does not have to be directly provided by one light source, nor does it have to be realized by one light source, but can be implemented by multiple light sources and auxiliaries with the same performance. The standard light source is used to realize the spectral power distribution of the standard illuminant

among them, CIE standard illuminant D65 represents typical daylight with a relevant color temperature of 6504k, which is close to the conditions of daylight lighting in most cases. CIE standard illuminant D50 represents a typical daytime light with a relevant color temperature of 5003k, and the energy distribution in the blue, green and red bands of its spectrum is close to the isoenergetic state

cie standard illuminants a, B and C are realized by standard light sources a, B and C, but for standard illuminants D65 and D50 simulating typical sunlight, CIE has not recommended corresponding standard light sources at present. Because its spectral energy distribution cannot be accurately realized by real light sources at present. At present, there are three kinds of analog D65 artificial light sources being developed internationally: high-pressure xenon arc lamp with filter, incandescent lamp with filter and fluorescent lamp

2 lighting and observation conditions of color samples

data and standardization in the production process of modern printing industry have been paid more and more attention. Digital color information is being transmitted in all process links of printing production, especially in the process of color management and control, the standardization of color lighting and observation with physical modification conditions should be paid more attention. In actual production, China's publishing industry standard cy/t and the standard "iso3664:2000 observation of the lighting conditions of color transparencies and copies" recommended by the international organization for standardization should be the main standards for color technical measurement and color evaluation in the printing and reproduction industry

1) lighting conditions

CIE standard illuminant D65 should be used to observe the reflected color samples (reflected originals and replicas), and its parameter index is x=0.3127, y=0.3291 on the iel931 chromaticity diagram; The chromaticity coordinates on ciel960 UCS chromaticity diagram are u=0.1978, v=0.3122, and the artificial light source used is the simulator of standard illuminant D65. The chromaticity deviation value △ C between the light source and standard illuminant should be less than 0.008, the general color rendering index Ra of the light source should be greater than or equal to 90, and the special color rendering index RI (test color samples 9 ~ 15) should be greater than or equal to 80. (see cy/t3-1999 and gb/t 5702 for the calculation method of chromaticity deviation value △ C and color rendering index of light source). And the light source used to observe the reflected color sample should produce uniform diffuse light illumination on the observation surface, with the illumination range of 500lux ~ 1500lux, depending on the brightness of the observed sample. In addition, the illumination of the observation surface should be as uniform as possible, without sudden illumination changes, and the uniformity of illumination should be greater than 80%

for the observation of transmission color samples, CIE standard illuminant D50 should be used, and its parameter index is on ciel931 chromaticity diagram. The chromaticity coordinate of illuminant is x=0.3457, y=0.3586; The chromaticity coordinates on CIE 1960 UCS chromaticity diagram are u=0.2091, v=0.3254, and the artificial light source used is a D50 simulator. The chromaticity deviation value △ C between the light source and the standard illuminant should be less than 0.008

in addition, it should be noted that the standards of D65 light source for observation reflection samples and D50 light source for observation transmission samples are limited to China's publishing industry standards. For the implementation of the standard "iso3664:2000 observation of lighting conditions for color transmission films and copies", D50 standard light source is used for the identification of reflection prints, the actual evaluation of reflection prints, the direct observation of transmission samples and other ISO specified observation conditions

2) observation conditions

when observing the reflected color sample, as shown in Figure 1, the light source should be incident in the direction perpendicular to the surface of the color sample, and the observation direction should be 45 ° from the normal direction of the sample surface to observe the diffuse reflected light of the color sample, which corresponds to the 0/45 illumination observation condition. On the premise of ensuring uniform illumination of the observation surface, the observation conditions as shown in Figure 2 can also be adopted. The light source is incident at an angle of 45 ° with the normal of the surface of the color sample, and the observation direction is to observe the diffuse light of the color sample from the direction perpendicular to the surface of the sample, that is, the illumination observation conditions corresponding to 45/0. In addition, when observing the reflection color samples, the background should be the matte Munsell color n5/~ n6/, and the chromaticity value is generally less than 0.3. For occasions with high requirements such as color matching, the chromaticity value should be less than 0.2. When observing samples with large surface gloss, such as laser paperboard, you cannot directly watch the specular reflected light. You can adjust the observation angle within a certain range to find the best observation angle

when observing the transmitted color sample, apply uniform diffuse light to illuminate the back of the sample with a capacity of 1million tons, and observe it on the surface perpendicular to the sample. During observation, the sample should be placed in the middle of the lighting surface as far as possible, so that it has a 50mm wide illuminated boundary outside at least three sides. When the area of the observed transmission sample is less than 70mm × When 70mm, the width of the illuminated boundary should be appropriately reduced so that the boundary area does not exceed 4 times the sample area, and the excess part should be covered with gray opaque light blocking material

② color evaluation field of view

the resolution of the human eye when observing object details is related to the size of the field of view during observation. Similarly, the resolution of the human eye to color is also affected by the size of the field of view. Experiments show that when the human eye increases from a small field of view (2 °) to a large field of view (10 °), the accuracy of color matching and the ability to distinguish color differences are improved, but when the field of view is further increased, the accuracy of color matching is not improved. This is because the 10 ° standard field of view is more sensitive to the short wave spectrum in the 400 ~ 500nm region. Therefore, when using color measuring instruments to measure color data in the printing industry, China's national standards GB, B and GB stipulate lithographic upholstery prints, relief upholstery prints and intaglio upholstery prints respectively. When measuring the color difference of the same batch of products, the light source adopts D65 and the measuring field of view adopts 10 °

3 influence and control of environmental factors

in actual production, the surrounding environment is the factor that has the greatest impact on the standard lighting and observation conditions. For example, the objects around the observation environment and their own color and brightness may affect the color perception of the observer to the observation object to a great extent. The staff should try to eliminate the influence of the surrounding environment. Note: 1) avoid the surrounding environment at the same time having additional light sources or spots, which will affect the correct color discrimination under the standard light source. 2) Avoid strong color contrast in the observation field or strong color reflection on the surface of the environment, such as the surface reflection of white walls, floors, etc. The reflectivity of the surrounding environment should preferably be less than 20%. Observe in a stable surrounding environment. 3) Since people's subjective impression plays an important role in observing and judging samples, when entering the observation environment, the production cost of aluminum lithium alloy should be observed and judged after the eyes adapt to the environment for a period of time, which is only 10% of that of carbon fiber composites

in a word, in each process link of printing dispersion, ensuring effective color transmission, measurement, observation and evaluation must be carried out under standard lighting and observation conditions. Adopting and strictly implementing standard lighting and observation standards in printing production is the key to helping enterprises solve color quality problems

information source: PRINT world

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