I. Overview of printing color measuring instrument:
In color control and measurement, densitometers, colorimeters and spectrophotometers are the main tools for printing color measurement. Although these three instruments have different functions, they all use reflected (or transmitted) light to measure color. By illuminating the sample with a standard light source inside the instrument, the sample selectively absorbs, reflects, and scatters light, and the instrument’s photodetector detects the reflected light and compares it with the standard light source. When using a single-wavelength filter or spectrometer, the sensor will analyze the color and intensity according to the wavelength respectively, carry out information processing, and provide the required density value or colorimetry parameters and other data.

Densitometer: Usually designed with 3 ~ 4 filters (such as red, green, blue), each filter allows about 1/3 of the visible spectrum to reach the photodetector, it can measure the whole visible spectrum range, get the density value of yellow, magenta, green. Density meters generally have built-in functions including: density, ink overprinting rate, gray level, saturation, dot area, tone error, printing contrast, etc. Among them, measuring the density is an important function of the densitometer, the density value can directly reflect the thickness and concentration information of the ink on the printing sheet.

Demand for color measuring instruments in printing

Colorimeters: There are currently two types of colorimeters on the market: tristimulus value type and spectrometric type.

Tristimulus value colorimeters and densitometers are very similar in design, including filters for red, green and blue primary colors that break visible white light into the three primary colors, but there are two main differences:

1) The tristimulus colorimeter is designed to observe color and functions like the human eye, while the densitometer is designed to take into account the special sensitivity of the ink.

2) The three-stimulus colorimeter can process and calculate different color data (such as color space conversion, calculation of color difference, etc.), and can allow users to draw color coordinates in three-dimensional space, while the densitometer does not describe the color of this function.

Spectrophotometers, also known as spectrophotometers, divide the visible spectrum into very narrow intervals, with each interval representing a different wavelength part of the white light. Because the white spectrum can be divided into many small parts, spectrophotometers can collect more data and are therefore more accurate than densitometers. Therefore, this kind of spectrophotometer has better measurement repeatability. Similar to the tristimulus value colorimeter, the spectrophotometer can convert the measurement results into three disportable numbers. When accurate color reproduction is required, colorimeters are the more appropriate choice (although not as reliable as spectrophotometers), but in four-color printing applications, colorimeters have some limitations compared to densitometers, because densitometers can separately measure metrics related to four-color printing, such as density, dot area, ink overprinting, etc., while colorimeters can only measure color.

Demand for color measuring instruments in printing

Spectrophotometers: Similar to colorimeters, spectrophotometers are divided into two types: filter type and dispersive type, and their measurement principle is similar to that of spectrophotometers. The visible spectrum is divided into small segments using narrowband filters (filter type) or diffraction gratings (dispersive type). The filter type instrument is similar to the densitometer except that the spectrophotometer has more filters inside and can achieve higher resolution in the spectrum. In addition, due to the simple design of the instrument, the filter type spectrophotometer is very rugged and can withstand the harsh environment of daily life. However, dispersive spectrophotometers are very sensitive to collisions, fragile and expensive, which are not suitable for carrying and production use, and are suitable for operation in a laboratory environment.

All spectrophotometers are able to output the same data as colorimeters and also provide spectral curves. Each curve represents the color of each measurement, which can identify ink pigment components like fingerprints.

In conclusion, for the printing industry, instruments that combine chromaticity measurement and density measurement are good choices. At present, such instruments have been developed and produced, such as Gretag SPM 100 spectrophotometer produced by Gretag company in Switzerland. Although these instruments are currently expensive, they represent the direction of color measurement development in the printing industry.

Demand for color measuring instruments in printing

Two, the printing industry for the use of color measuring instrument requirements:
For colorimeters:

Light shape: in order to be able to flexibly locate on the measured print, and adapt to the measurement of large format printing.

The measurement geometry condition should be 45°/0° or 0°/45°; Standard light source is C or D65 light source; Apply CIE 2° small field of view standard observer (because in printing operation, the area of color evaluation is small).

The measuring aperture of the colorimeter should not be greater than 5mm. In general, the color blocks used for printing are less than 10mm², and the color blocks on the printing quality control strip are only 6mm². Especially for the case where continuous color matching images need to be measured, the measurement aperture should not be greater than 5mm.

The output value of the colorimeter should include standard color values (such as XYZ) and should also be able to output CIE LAB and CIE LUV chromaticity space coordinates.

Demand for color measuring instruments in printing

For density meters:

Easy to use and standard: in the experiment and production, printing quality control is always inseparable from the use of densitometer, if each measurement and calibration is tedious and time-consuming, will inevitably affect the speed and accuracy. In addition, the calibration of the densitometer needs to be checked periodically using a dedicated reflection (or transmission) gray scale or scale.

The sensitivity of the instrument should be suitable for printing color measurement, generally CIE A light source, T state density. Because the T-state density is an objective physical measuring instrument designed for color separation and printing, the blue, green and red light of the T-state density meter are complementary colors of the three primary colors (yellow, magenta and green), which can well detect and control the allocation of the three primary colors and the relative thickness of the yellow, magenta and green ink layers in the process of color separation and printing.

The performance of the instrument should meet the requirements of the specification, including accuracy, repeatability, reproducibility and internal consistency of the instrument. They should be capable of performance comparison and measurement outside the prescribed time.