In the color printing process, it is generally only used yellow, magenta, cyan and black four-color overprinting to obtain a variety of mixed colors, in the batch printing to check whether the printed product is consistent with the proof color. So far, a considerable number of printing companies rely on the visual comparison of the proofs and prints made by the printing workers to complete the print quality evaluation. Because there is a difference in the visual evaluation of each person, this is not conducive to the standardization of print quality and data. Management. Printers and customers also often have disputes due to quality concerns.
In order to objectively perform quality management, control, and detection of color image duplication, in recent years, more and more color photocopying work has used a measuring instrument—a density meter.
Densitometers are classified into transmission and reflection. In the printing industry, transmission densitometers are mainly used to determine the density of negative and positive films after separation, and the percentage of dots, etc. Reflective densitometers are used to measure and evaluate printers. With the rapid development of science and technology, densitometers are constantly being updated. The original visual contrast densitometer can only measure density, while the new density meter expands many functions, such as the X-ray produced by X-Rite, USA. Rite 500 series spectroscopic reflection densitometer can not only measure density, but also measure the chroma value, color difference value, dot area of ​​different color space, calculate contrast, overprint, hue error and gray scale, and it can be connected with computer. Processing and so on.
The working principle of the optoelectronic densitometer is to form an optical path through the light source and the filter, so that the light transmitted or reflected by the light quantity of the test sample is converted into the corresponding intensity of the photocurrent according to the intensity, after the analog-to-digital conversion, and then through the digital The display is displayed to obtain the density value of the transparencies or reflections.
When measuring the density of black and white, the high speed of the filter should meet the Rutherford condition, so that the spectral sensitivity characteristics of the receiver and the human visual sensitivity curve are consistent; when measuring the color density, the instrument selects red and green according to the measured color respectively. , blue filter to get different color density. In addition, the measured ink film layer of the product has a gradually drying process, and the measurement data will produce an error that changes with time. Therefore, there are also reflective densitometers with polarization filters to eliminate this error. There is also a more advanced densitometer with a set of filters that conform to the spectral characteristics of the four-color inks that are spectrophotometrically calibrated, making them the same sensitivity as measuring the other three colors when measuring yellow ink.
Densitometer use characteristics are mainly determined by the spectral energy distribution of the light source, the spectral transmittance of the filter, the spectral characteristics of the sample, the spectral sensitivity of the light receiver, the measurement conditions, and the measurement area. In order to make the use of densitometer accurate and effective, this article gives a brief explanation of several issues that the density meter should pay attention to in the technical measurement.
1 Selection of measuring light source In the process of color image reproduction, from color separation, proofing to printing, color measurement and visual evaluation in each process are performed under certain lighting conditions. Since the color of the surface of the object is related to the spectral distribution of the radiation of the illumination source, in the technical measurement, the calculation of the parameter values ​​of the respective color systems must take into account the type of the light source. The same color sample under different light sources may give the human eye a different color feel. Daylight is the ideal light source. Therefore, daylight can be used as a standard (see light source). For example, when you look at a picture under the sun and then look at a high-pressure mercury lamp, you will notice that the color has changed, pink has become the color of tissue activity, and blue has become blue-violet. If you look at the low-pressure sodium lamp in yellow light, blue turns black again. Comparing incandescent, fluorescent, sodium, and other artificial light sources (measurement light sources) with daylight, the ability to display the same color capability is called the artificial light source's color rendering property. Experiments have shown that the color rendering of light sources will directly affect the results of people's identification of color.
In order to test the degree that the color of the object under the light source to be measured matches the color presented under the reference light source, the “average color rendering index†is used as the quantitative evaluation index. The highest color rendering index is 100. The light sources used in photographic color separation, electronic color separation, and various color measurement instruments require that the color rendering index is usually not less than 90 (Ra≥90); color photography, color movie, television, Dye and other industrial sectors also require the use of a light source with a high color rendering index (Ra ≥ 85). Table 1 shows the color rendering index of lighting sources commonly used in printing and copying work.
Table 1 General color rendering index of commonly used light sources
Normal > light source name
Normal > correlated color temperature ( K )
Normal > General Color Rendering Index ( Ra )
Normal > Incandescent ( 500W )
Normal > 2900
Normal > 95~100
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