Several problems should be paid attention to when using density meter
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In the color printing process, usually only four colors of yellow, magenta, cyan and black are used to obtain various mixed colors, and in the batch printing, it is checked whether the printed product is consistent with the color of the proofing sample. So far, a considerable number of printing houses rely on the visual comparison of proofing proofs and printed materials by the printing workers to complete the printing quality evaluation. Because of the differences in visual evaluation of each person, this is not conducive to the standardization and data of printing quality. Management. Printing houses and customers often have disputes over quality glimpses.
In order to objectively perform quality management, control, and detection of color image reproduction, in recent years, measurement instruments, density meters, have been increasingly used in color copying work.
Densitometers are divided into transmission and reflection. In the printing industry, the transmission densitometer is mainly used to measure the density of the negative and negative negatives after separation and the percentage of dots, and the reflection densitometer is used to measure and evaluate the printer. With the rapid development of science and technology, the density meter is constantly being updated. The original visual contrast density meter can only measure density, while the new density meter expands many functions, such as X- produced by X-Rite. Rite500 series spectroscopic reflection densitometer can not only measure density, but also measure chromaticity value, color difference value, dot area, calculation contrast, overprint, hue error and gray scale in different color spaces. It can also be connected with computer and measurement data. Processing and so on.
The working principle of the photoelectric densitometer is to form an optical path through the light source and the filter, so that the light transmitted or reflected by the light amount of the tested sample is converted into a photocurrent of a corresponding intensity 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 reflected images.
When measuring the density of black and white, the high speed of the filter should meet the Russell condition, so that the spectral sensitivity of the receiver is consistent with the human visual sensitivity curve; when measuring the color density, the instrument will select red and green according to the measured color. , blue three kinds of filters to get the density of different colors. In addition, the measured ink film layer of the product has a gradual drying process, and the measurement data produces an error that varies with time. Therefore, there is also a reflection densitometer with a polarizing filter to eliminate this error. There are also more advanced densitometers with a set of filters that match the spectral characteristics of the four-color inks calibrated by the spectrophotometer, making it the same sensitivity when measuring yellow inks as when measuring the other three colors.
The use characteristics of the densitometer are mainly determined by factors such as 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 the density meter accurate and effective, this paper gives a brief explanation about several problems that should be paid attention to in the technical measurement of the density meter.
1 Measurement source selection
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 radiation spectral distribution of the illumination source, in the technical measurement, the calculation of the parameter values of each color system must take into account the type of the light source. The same color sample may cause different color perceptions to the human eye under different light sources. Daylight is the ideal source of light, so daylight can be used as a standard (reference source). For example, when you observe a picture in daylight and then take a high-pressure mercury lamp and observe it, you will notice a change in color. The pink color becomes a tissue activity color, and the blue color turns blue-violet. If you look down at the low-pressure sodium lamp in yellow light, the blue color turns black again. Comparing artificial light sources (measuring light sources) such as incandescent lamps, fluorescent lamps, and sodium lamps with daylight, the intensity of the same color is called the color rendering of the artificial light source. Experiments have shown that the quality of the light source will directly affect the results of people's color discrimination.
In order to check the extent to which the color exhibited by the object under the light source to be measured coincides with the color presented under the reference light source, the "average color rendering index" is used as a quantitative evaluation index. The color rendering index is up to 100. The color source used in photographic color separation, electronic color separation and various color measuring instruments requires that the color rendering index should not be lower than 90 (Ra ≥ 90); color photography, color film, television, Industrial sectors such as dyes also require the use of light sources with high color rendering index (Ra ≥ 85). Table 1 shows the color rendering index of the illumination source commonly used in printing and copying work.
Table 1 General color rendering index of common light source for printing
Light source name Correlation color temperature (K) General color rendering index (Ra)
Incandescent lamp (500W) 2900 95~100
Iodine tungsten lamp (500W) 2700 95~100
Xenon lamp (1000W) 4300 85~95
Fluorescent light (daylight 40W) 6600 70~80
It can be seen from Table 1 that the color temperature and the average color rendering index of the light source are parameters indicating the color characteristics of the light source. From the relationship between them, it can be seen that the height of the correlated color temperature does not mean the color rendering index, such as The color temperature of an incandescent lamp is low, but its color rendering index is very high. On the contrary, the color temperature of a fluorescent lamp is as high as 6600K, but its color rendering index is only between 70 and 80.
In technical measurements, the calculation of the values of the various color system parameters depends on the type of light source. In order to unify the color measurement standards, CIE (National Display Lighting Committee) stipulates four standard light sources A, B, C, and D that are commonly used internationally. The standards promulgated by the National Bureau of Standards of China also stipulate the lighting and observation conditions for color evaluation. The CIE standard light source A is a tungsten light with a color temperature of 2855.6K. The CIE standard light source B is the color light obtained by filtering the tungsten light through a specific liquid filter, which approximates the average direct sunlight at a noon temperature of 4874K, but the CIE has issued an advice to stop using the B light source. The CIE standard light source C is light obtained by filtering a standard liquid source A with a specific liquid filter, and is close to the sunlight of the northern sky with a correlated color temperature of 6744K. The CIE standard light source D50 has a color temperature of 5003K, indicating a typical dawn. The CIE standard light source D65 has a co-correlated color temperature of 6504K, indicating typical daylight (average daylight). The D50 and D65 standard light sources are not currently manufactured from physical objects, so they can only be called illuminants, and the actual artificial source measurements are D50 and D65 simulations.
China's national standards stipulate that the artificial light source used for observing the transmission sample should be D50 simulation body. The artificial light source for observing the reflection sample should be the simulation body of D65. The general color rendering index Ra of the light source should be not less than 90.
The new density meter is powerful, and the instrument is equipped with a variety of light sources to meet different measurement requirements. For example, the X-Rite500 series of spectroscopic reflection densitometers are equipped with A, C, D50, D65, D75, F2, F9, F11, F12 light sources. From the theoretical analysis and experimental results, the measurement results are different when the light source used in the measurement is different. Table 2 shows the results of several ink products of Sino-Japanese joint venture Hanghua Ink Chemical Co., Ltd. under different light source conditions.
Table 2 Measurement results of Hanghua ink products under different light sources
B0—333 B0—232 B0—131
D65 light source (10°)
L*=50.07 L*=83.67 L*=39.15
a*=34.75 a*=0.87 a*=73.00
b*=-44.08 b*=114.65 b*=-6.78
C light source (10°)
L*=49.73 L*=82.88 L*=39.42
a*=-30.31 a*=-0.21 a*=71.66
b*=-45.15 b*=115.74 b*=-6.44
Relative color difference △E*ab=4.58 △E*ab=1.50 △E*ab=1.41
It can be seen from Table 2 that when the measurement light source is different, the instrument will produce different readings and chromatic aberration. The reason is that there is a metamerism between the two light sources.
2 Densitometer response
In color density measurement, the measured value displayed by the densitometer is related to the spectral product of the light source, the color filter, and the receiver. After the measurement source is selected, the choice of color filter also has an effect on the measurement. In general, a densitometer uses red, green, and blue color filters for color density measurement. The density measured with the red, green, and blue beams is unique to a particular densitometer. For the same color sample, the readings it provides cannot be directly compared to the readings taken by another densitometer. If this density meter is used to detect and monitor the uniformity of ink volume in the printing shop, this test is effective as long as the same ink is used, because the red, green and blue color filters of the same density meter are used. The spectra of the slices are independent of each other and reflect the differences between the different colors. To determine the amount of yellow, magenta, and blue printing inks, the density values measured by almost any red, green, and blue light can be compared to determine the change in the amount of ink. However, in order to carry out international and inter-plant exchanges and standardization, such density meters must be appropriately restricted and unified as much as possible. Therefore, the International Organization for Standardization and China National Standard GB11501-89 have correspondingly made the density of the three color filters. The regulations are referred to as "ISO standard visual density", "ISO standard A state density", "ISO standard M state density", and "ISO standard T state density", respectively.
The ISO standard visual density is also known as the brightness density. Because the light of different wavelengths of the equal energy of the human eye has different degrees of psychological light and dark feeling, this is the spectral light efficiency V(λ) of the human eye. Visual density is often used to observe the image blackness directly or by projection. In addition to being used for black and white images, other types of images are also applicable. Even if the colors of the two images are not the same, the brightness of the images is often compared with their brightness. In the measurement, the spectral sensitivity of the receiver is high enough to coincide with the human eye's visual sensitivity during the day. Therefore, the density of the black and white image or color image measured by the standard visual density is consistent with the human visual sensitivity.
The ISO standard A state response method is mostly used for the determination of the gray balance in the printing job, the color correction, and the transmission of the color positive film, the color slide, and the like.
The ISO standard M-state response method is mostly used for the measurement of color photographic materials for photographic and print reproduction, and thus this density response method is often used to evaluate various films for printing.
In the ISO standard T-state response mode, the spectral sensitivity S(λ) of the receiver is not limited by the CISE spectral tristimulus value, and does not include the conversion values of human visual color physiological factors and psychological factors. Since the spectral product in the standard T state is a known quantity, and the unknown quantity has only the reflectance R(λ) of the surface color of the special body, the color sample having the same reflectance, the standard T state measured by the different type density meter The density should be equal.