Color Measurement Types and Instruments (1)

One or two color measurement methods

Density measurement is also an important form of color measurement. Densitometers have their own unique advantages, which are mainly for the control of the printing process. Densitometers are cheap, read quickly, and in many ways exceed other precision-produced measuring instruments, such as those used to control the thickness of ink layers. They are also used in simple and meaningful measurements. However, the density meter has the following disadvantages:

1 The consistency between the instruments is poor because of differences in the spectral characteristics between the light source, the photomultiplier tube, and the color filter. However, technological advances have also increased the value of densitometers. There are now on-line densitometers with stroboscopic light sources, photodiode sensors, and measurements without contact with the sample surface; microprocessor-equipped densitometers can also perform simple calculations (such as calculating ink overprint rates).

2 Densitometers cannot provide psychophysical measurements related to human eye sensitivity, and their analytical measurement capabilities are limited.

3 Density measurements cannot be associated with the CIE* color system in some way, whereas the CIE* color system is a recognized color language.

New colorimeters and spectrophotometers have enabled the printing industry to realize the potential of colorimetric measurements that are closely related to the spectral sensitivity of the human eye and provide CIE color system parameters.

There are two main colorimetric methods. The first method is a method of measuring color using a photoelectric colorimeter. The photoelectric colorimeter is very similar to the density meter in principle, and its appearance, operation method and price are also similar to the density meter. The electro-optical chromaticity is connected to display the tristimulus values ​​x(λ), y(λ), z(λ), and most of them also convert the tristimulus values ​​into a colorimetric spatial scale, such as a CIELAB scale, but most There is only one or two kinds of lighting, so the color measured with the photoelectric colorimeter does not always show the visual color. In addition, the CIELAB color space is not the best color system for print reproduction, because it cannot be like the CIELUV. Calculate saturation. The accuracy of the photoelectric colorimeter is definitely enough to determine the color difference, and it can be used as a measurement of color difference comparison in the printing shop. Many opto-electronic colorimeters are also accurate enough to measure absolute color and relative color differences. However, in general, people prefer to use spectrophotometers to accomplish these tasks.

The colorimeter can be thought of as a reflectometer, or a densitometer without a logarithmic converter but with a special set of color filters. Of course, this is a way to complete the color measurement. The purpose of attaching a set of color filters is to weight each wavelength of the spectrum in each channel of the colorimeter based on the CIE spectral tristimulus value. However, unlike colorimeters, colorimeters mainly deal with reflectivity rather than logarithmic problems, but reflectivity can easily be converted to density, and vice versa. The spectral composition of the colorimeter is considered to have a good linear relationship with human visual acuity. But in fact this is impossible (involved with Luther's conditional problems), so the photoelectric colorimeter has errors in principle.

The second method is to use a spectrophotometer to measure color. Just as the three-color filter photoelectric colorimeter can be regarded as a special reflectivity measuring instrument, the spectrophotometer can be seen as such, but unlike the photoelectric colorimeter, the spectrophotometer measures the whole of an object. Visible reflectance spectroscopy, spectrophotometer point-by-point measurement in the visible spectral domain, that is measured at some discrete points, usually measured every 10 or 20nm a point, measured in the range of 400 ~ 700mm 16 ~ 31 points. Some spectrophotometers measure the spectrum continuously, while the third filter opto-chromatometer measures only three points, so the Dulux photometer can provide much more information, at least for 16 points. measuring.

Spectrophotometers measure color as a physical phenomenon that is not subject to the observer. In order to obtain a tristimulus value it can integrate the reflection spectrum and can interpret the color as a visual response. It is the most flexible color measurement instrument.

For non-fluorescent materials, the spectrophotometer provides measurement results that are independent of the lighting used and can be evaluated under fluorescent light, incandescent light, and daylight because the spectrophotometer measures the reflectance spectrum. Automatically and objectively evaluate the effect of ink transfer. In fact, if the reflectance spectra of two color samples are matched, then the two objects can be considered to have the same color, and it is irrelevant to observe under which light source.

Fluorescent materials are commonly used in the printing industry. Many papers contain fluorescent materials (such as brighteners). Many yellow inks also produce a certain degree of fluorescence. Fluorescence affects the color of printed materials.

Some phenomena in the printing process, such as paper web spot coverage, ink strength, etc., are essentially physical phenomena that occur within a narrow band. Of course, it is best to use narrow-band measurements for evaluation. However, it should be noted that narrow density measurements (such as A-state density) cannot be used to measure visual color, but spectrophotometry can solve this problem. Because the measurements it makes are narrow-band measurements, the sampling of the spectrum is sufficient, so it is possible to make color measurements that are consistent with the vision. For the intended type of measurement (narrowband or wideband), a calculation program can be pre-programmed for the spectrophotometer. Many new spectrophotometers include a computer, and it is appropriate to perform standard print copy quality control and narrowband measurement according to the program, but it is obviously more expensive than the density meter.

As we all know, the most basic method for measuring the color is the subjective visual method. This method visually matches the unknown color based on the colors in the chromatogram. The color data measured by the spectrophotometer is finer than that of the human eye. This is useful for analyzing the concentration of pigments, and only needs to be based on some formulas. With the calculation, you can analyze and control the amount of raw materials.

According to the spectrophotometer measurement values ​​can be calculated density and chromaticity values ​​(but the reverse calculation is incorrect); metamerism can be analyzed; new spectrophotometer can also be spectrophotometric data can be directly converted to other colors The system parameters and conversion methods are the same as for the colorimeter.