New requirements for the use of color measuring instruments in the printing industry

First, printing color measuring instrument outline

Density meters, colorimeters, and spectrophotometers will be the primary tools for printing color measurements when controlling and detecting colors. Although these three instruments have different functions, they use reflected (or transmitted) light to measure color. Using a standard light source inside the instrument to illuminate the sample, the sample selectively absorbs, reflects, and scatters, and the instrument's photodetector detects the reflected light and compares it with a standard light source. When using a single-wavelength filter or a spectral splitter, the sensor will analyze the color and intensity according to the wavelength, perform information processing, and give the data required for the density or colorimetric parameters.

1. Density meter. There are usually 3 to 4 color filters (red, green, blue, etc.), each of which allows approximately 1/3 of the visible spectrum to reach the photodetector. It can measure the entire visible spectrum, giving yellow, magenta, and blue values. . Density meters are generally built-in features: density, ink overprint rate, grayscale, saturation, dot area, hue error, print contrast. Among them, measuring the density is the most important function of the density meter, and the density value can directly reflect the information of the thickness and concentration of the printing ink.

2. Colorimeter. At present, there are two colorimeters on the market: tristimulus values ​​and spectra.

Tristimulus value meters and densitometers are similar in design. They all include trichromatic (red, green, and blue) filters that divide visible white light into three primary colors, but their main differences are two points: 1 The tristimulus value colorimeter is Designed to observe colors, its function is similar to that of the human eye, but the design of the density meter must consider the special sensitivity of the ink. 2 tristimulus value colorimeter can process and calculate different color data (such as: for color space conversion, color difference calculation, etc.), and allows users to draw the color coordinates in three-dimensional space, and densitometer does not describe the color function .

Spectral colorimeter, also known as spectrophotometer, is to divide the visible spectrum into very narrow and fine intervals, and each interval represents the different wavelengths of white light. Since the white light spectrum can be divided into many small parts, the spectrocolorimeter More information can be collected to make it more accurate than densitometers. Therefore, this kind of spectral instrument has better measurement repeatability. Similar to the tristimulus colorimeter, the spectral colorimeter can change the measurement result to three numbers that can be displayed. When an accurate color reproduction is required, the colorimeter is the ideal choice (but the reliability is worse than the spectrophotometer), but in the four-color printing application, the colorimeter also has many places where the densitometer is not available because the density meter can Individually separated four-color printing metrics, such as density, dot area rate, ink overprint rate, etc., while the colorimeter only measures color.

3. Spectrophotometer. Like the colorimeter, there are two types of spectrophotometers: filter type and spectral dispersion type, and its measurement principle is similar to the spectrophotometer. The visible light spectrum is divided into small segments using a narrowband filter (filter type) or a diffraction grating (dispersion type). Filter-type instruments are similar to densitometers in their use, except that more filters in the spectrophotometer can produce high resolution in the spectrum. In addition, due to the simple design of the instrument, the filter spectrophotometer is very rugged and able to withstand the harsh environment of everyday life. The decentralized spectrophotometer is more sensitive to collisions, is fragile and expensive, and is not suitable for carrying and production, and is suitable for operation in a laboratory environment.

All spectrophotometers can output the same data as the colorimeter, and the spectrophotometer can also output spectral curves. Each curve represents the color of each measurement and it can be used as a fingerprint to identify the pigment composition of the ink.

In summary, combining the colorimetric measurement with the density measurement will be an ideal instrument for the printing industry. At present, such instruments have been developed and produced, such as: Gretag, produced by Gretag, Switzerland

SPM 100 spectrophotometer. Although such colorimetric instruments are currently expensive, they represent the direction of color measurement in the printing industry.

Second, the printing industry requirements for the use of color measuring instruments

For color densitometer requirements:

1 Easy to use and standard, in the experimental production, the print quality control can not be separated from the density meter. If every measurement and calibration is complicated, it will affect the speed and accuracy. Moreover, the calibration of the densitometer is regularly checked with a dedicated reflective (or transmissive) ash scale or ruler.

2 The instrument sensitivity should be suitable for printing color measurement, generally CIE A light source, T state density. Because the T-state density is an objective physical measurement instrument designed for color separation and printing, the blue, green, and red light of the T-state densitometer are complementary colors of the three primary color inks yellow, magenta, and cyan. Detect and control the modulation of the three primary colors in the color separation and printing process and the relative thickness of the yellow, magenta, and cyan ink layers.

3 The performance of the instrument should meet the requirements of the specification. Accuracy, repeatability, reproducibility, and internal fit of the instrument are the performance specifications of the densitometer. They should be able to compare and measure performance beyond a specified time.

For colorimeter requirements:

1 The appearance is light, in order to have the flexibility to be positioned on the printed matter being tested and adapted to the measurement of the large-scale printed sheet.

2 The measurement geometry should be 45°/0° or 0°/45°; the standard light source is a C or D65 light source; a CIE 2° small field of view observer is appropriate (because the area of ​​the evaluation color in the print job is small) .

3 colorimeter measurement aperture should not exceed 5mm. In general, chromatograms for printing are all smaller than 10 mm2, and color patches on the print quality control strip are only 6 mm2. In particular, the continuous color image needs to be measured in a smaller range, so the measurement aperture of the colorimeter should be no more than 5mm.

The colorimeter output value is not only the standard color value (eg: XYZ), but also the coordinate planting of CIE LAB and CIE LUV chromaticity.

Source: Guangzhou Printing Network