In nature, light and color are inseparable. Without light, there is no color. Different light sources have different radiant energy and appear different colors when irradiated on different objects. Even if the same color under different lighting conditions can make the human eye feel different colors. Therefore, in the printing and copying, technical measurement, control, and visual evaluation of color need to be performed under uniform standard light illumination conditions.

1 Standard Illuminators and Standard Light Sources

In order to unify color measurement and evaluation standards, CIE (International Commission on Illumination) specifies four standard illumination bodies A, B, C, D and three standard light sources A, B, and C. The standard illuminant specified by CIE refers to a specific spectral energy distribution and is a prescribed light source color standard. This particular spectral energy distribution does not have to be directly provided by a light source, nor is it necessarily implemented by a certain light source, but can be implemented by using multiple light sources and auxiliary bodies of the same performance. The standard light source is a light source used to realize the spectral power distribution of the standard illuminator.

Among them, the CIE standard illumination body D65 represents a typical neon light with a correlated color temperature of 6504K, which is close to the daylight illumination conditions in most cases. The CIE standard illuminant D50 represents a typical calorimeter with a correlated color temperature of 5003 K. The energy distributions of the blue, green, and red bands of the spectrum are close to the isothermal state.

The CIE standard illuminants A, B, C are implemented by the standard illuminants A, B, C, but for the standard illuminants D65 and D50 that simulate typical daylight, the CIE does not currently recommend a corresponding standard illuminant. Because its spectral energy distribution can not be accurately realized by a real light source at present. The three types of artificial D65 light sources currently being developed in the world are: high-pressure xenon arc lamps with filters, incandescent lamps with filters, and fluorescent lamps.

2 Color Sample Lighting and Observation Conditions

The data and standardization in the production process of the modern printing industry are increasingly valued. Digitalized color information is being transmitted in various process steps of printing production. Especially in the process of managing and controlling colors, the standardization of color lighting and observation conditions deserves more attention. In actual production, China's press and publication industry standard CY/T3-1999 and the International Organization for Standardization's "ISO 3664:2000 observation of color transmission film and photocopying lighting conditions" standard, should be used as a printing and copying industry color technology measurement and color evaluation The main criteria.

1) Lighting conditions

For observation color samples (reflective originals and replicas), the CIE Standard Illuminant D65 shall be used. The color coordinates of the parameter indicators on the IEl931 chromaticity diagram are x=0.3127, y=0.3291; on the CIEl 960 UCS chromaticity diagram The chromaticity coordinates are u=0.1978, v=0.3122. The artificial light source used is the simulated body of the standard illuminant D65. The color product deviation ΔC between the light source and the standard illuminant should be less than 0.008, and the general color rendering index Ra of the light source should be greater than or equal to 90, special color rendering index Ri (test color samples 9 to 15) should be greater than or equal to 80. (The method for the calculation of the color deviation value ΔC and the light source color rendering index can be found in CY/T3-1999 and GB/T 5702). Also, the light source used to observe the reflected color sample should produce uniform diffuse light illumination at the viewing surface, with illuminance ranging from 500 lux to 1500 lux, and depending on the brightness of the observed sample. In addition, the illumination of the observation surface should be as uniform as possible, there can be no sudden change in illuminance, and the uniformity of illuminance should be greater than 80%.

For the observation of transmitted color samples, CIE standard illumination body D50 shall be used. Its parameter index shall be on the CIEl 931 chromaticity diagram, and the chromaticity coordinates of the illuminator shall be X=0.3457, y=0.3586; the color products on the CIE 1960 UCS chromaticity diagram The coordinates are u=0.2091 and v=0.3254. The artificial light source used is the simulated body of D50. The chromatic aberration deviation ΔC between the light source and the standard illuminant should be less than 0.008.

In addition, it should be noted that the standard for the observation of reflective samples using D65 light source and for the observation of transmission samples D50 light source is limited to China's press and publication industry standards, for the implementation of "ISO3664: 2000 observation of color transmission film and copy lighting conditions" standard, reflection Identification of printed products, actual evaluation of reflected prints, direct observation of transmission samples, and other ISO specified observation conditions using D50 standard light source.

2) Observation conditions

When observing the reflection color sample, as shown in Fig. 1, the light source should be incident from the vertical direction of the color sample surface, and the observation direction should observe the diffuse reflection light of the color sample at an angle of 45° from the normal direction of the sample surface, which corresponds to 0/ 45 Lighting observation conditions. Under the premise of ensuring the uniform illumination of the observation surface, the observation conditions shown in FIG. 2 can also be adopted. The light source is incident at an angle of 45° from the normal to the color sample surface, and the observation direction is observed from the vertical direction of the sample surface. Reflected light, which corresponds to a 45/0 illumination viewing condition. In addition, the background when observing the reflected color sample should be a dull Munsell color N5/~N6/, and the chroma value is generally less than 0.3. For occasions requiring high color matching, the chroma value should be less than 0.2. When observing a sample with a large surface gloss, such as a laser jam, the specular reflection light cannot be directly observed. The observation angle can be adjusted by adjusting the angle within a certain range to find out the best viewing angle.

When viewing transmitted color samples, apply uniform diffuse light to illuminate the back of the sample and observe it perpendicular to the surface of the sample. Observe that the sample should be placed in the middle of the lighting surface as much as possible so that there are 50mm wide illuminated borders at least three sides away. When the area of ​​the observed transmission sample is less than 70mm x 70mm, the width of the illuminated boundary should be appropriately reduced so that the boundary area does not exceed 4 times the area of ​​the sample, and the excess part is covered with a gray opaque light blocking material.

2-color evaluation field of view

The resolution of the human eye to observe the details of an object is related to the size of the field of view during observation. Similarly, the resolution of the human eye to the 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 discriminate color differences increase, but when the field of view further increases, the accuracy of color matching increases. Not much. This is because the 10° standard field of view is more sensitive to the shortwave spectrum in the 400-500 nm region. Therefore, in the printing industry, using color measuring instruments for color data determination, China's national standards GB7705-87, B7706-87, and GB7707-87 provide regulations for lithographic decorative prints, letterpress decorative prints, and gravure decorating prints, respectively, to measure the same batch of products. When the color difference of the color, the light source uses D65, and the measurement field of view uses 10°.

3 Environmental Factors and Control

In actual production, the surrounding environment is the factor that has the most influence on the standard lighting and observation conditions. For example, the objects around the observation environment and their own colors and brightness may greatly affect the observer's color perception of the observation object. Workers should try to eliminate the influence of the surrounding environment. Note: 1) Avoid the surrounding environment with additional light sources or flares, which will affect the correct color discrimination under standard light sources. 2) Avoid strong contrast in the observation field of view or strong color reflections on the surface of the environment, such as surface reflections from white walls, floors, etc. The reflectance of the surrounding environment is preferably less than 20%. Perform observations in a stable environment. 3) As the subjective impression of the person plays an important role in observing and judging the sample, after entering the observation environment, the eyes should be allowed to adapt to the environment for a period of time before the observation and judgment.

In short, in the process of printing and decentralization, to ensure effective color transfer, measurement, observation and evaluation must be carried out under standard lighting conditions and observation conditions. Adopting and strictly implementing standard lighting and observation standards in printing production is helpful. The key to solving the problem of color quality in the enterprise.

Source: "Printing World" Author: Wang Yi Zhou Shisheng