Density meter consists of light source, lens group, polarizer (optional), color filter, sensor and electronic system, display, etc. The measurement principle of reflection density meter (with color filter) is shown in Figure 1:

 

The density meter uses the built-in red, green, blue filter to measure the light reflection or transmission of yellow, magenta and blue color, and calculates the density value. This is based on the principle of three-color filter, which makes its structure very simple and widely used, but due to the defects of the filter itself, it also constitutes an insurmountable limitation of the density meter: It can only measure the density value of the print, and does not include the expression of the hue, so it can not truly reflect the actual visual effect of the color, which is said to be a "color blind" measuring equipment.

 

In addition, densitometer measurement also has some other limitations, such as: the field of application is limited to the four-color printing process, although often used to assist in monitoring the thickness of the ink layer, but there is no direct link between the density and the thickness of the ink layer. Therefore, the purpose of the densitometer is that the user can compensate and correct the film or plate before press according to the maximum/small density, dot expansion and printing contrast provided by it, guide the production management personnel to correctly screen, determine the amount of ink, exposure, ink balance and other control parameters, and is not good at the measurement and control of the accuracy of color reproduction in color management.

 

Photoelectric colorimeter can be seen as a reflectance meter, it has a special set of three filters, which is different from the density meter's red, green and blue filter, this filter according to the CIE spectral tristimulus value in each channel of the colorimeter to the spectrum of each wavelength weighted, it is mainly involved in the reflectivity problem, rather than the logarithm problem. The measuring principle of photoelectric colorimeter is shown in Figure 2:

 

Compared with the density measurement, the photoelectric colorimeter can specifically describe the color information through the three stimulus values, rather than just limited to the brightness information, because it still uses the principle of three filters, the spectral range of sampling is limited, so the accuracy is not high, not suitable for high-precision color management color measurement and control.

 

 

There are three basic spectrophotometric principles: rotating filter spectrophotometry, scattering prism spectrophotometry, and diffraction grating spectrophotometry, as shown in Figure 3. The first method is to install 20 to 30 narrow-band color filters on the disc, and to achieve light splitting by rotating the disc. The latter two methods use the dispersion of light to decompose the composite radiation of the light source into monochromatic radiation of different wavelengths, and arrange it in a certain order, and the dispersion element used is a prism or diffraction grating. For example, the scanning spectrophotometer on the Heidelberg CPC2 is based on the diffraction grating principle.

 

Commonly used spectrophotometers have wavelength intervals of 10nm or 20nm, and the visible spectrum being recorded is divided into about 30 segments. In some high-precision systems, the measurement interval can also be smaller (up to 1nm). Then, under the specified illumination body and observation field, the light quantity of each wave length is measured by the photodetector one by one, and the chromaticity value of the product to be measured is calculated according to the reflection spectrum or transmission spectrum.

 

 

 

 

In theory, all inks, whether four-color inks or spot color inks, can be measured using a spectrophotometer. The system automatically compares the measured data with the target color value and displays the result of the comparison on the screen. If the density value is selected, the quality can be determined by the traditional method; If the Lab value is selected, the color deviation can be intuitively determined by the △E value, and the amount of deviation will be displayed in a chart. According to the data on the chart, the operator can determine which area of the color is correct, which area of the ink is larger or smaller. If the operator decides to adjust the color, the spectrophotometer is also controlled online and the recommended adjustment data is sent to the ink zone Settings at the press of a button.

 

If the density value cannot represent a spot color at all, the same is true for four-color inks. This is because the density measurement has no relationship with the content of ink pigments, and the density value does not contain the expression of hue, which is a problem that all printing companies are familiar with. If the type of ink changes, although there is the same printing density, it does not necessarily ensure that the same printing color can be obtained.

 

 

 

4. Actual use

At present, the most popular in the market is the X-Rite 500 series spectral density meter, which covers density measurement, dot increase, overprint rate, print contrast, gray balance value, tone error, color comparison and chroma functions, of which 530 can also be used with color quality control software; X-Rite's DTP41 automatic scanning spectrophotometer is also used by many users and has a high degree of automation, making it easy and fast to create ICC Profile feature descriptions.

 

GretagMacbeth's SpectroEye portable spectrophotometer provides all the chromaticity measurement capabilities needed to accurately measure and control spot colors, and has all the chromaticity density measurement capabilities for monitoring colors during the printing process. SpectroEye offers a wide range of color space and color difference formula arrangements, including CMC, FMC II or ΔE*94 color difference formulas for the CIELAB color system, to ensure perfect compatibility with standard procedures for instrumental color quality control, and can even be used as part of the ink color matching system for ink matching.

 

In addition, EFI's ES-1000 spectrophotometer is easy to measure both the display screen and the printed matter, and the general color management software can support it, and the user can choose to measure in a single color block or a single color bar according to the need. Some models can also support offline battery powered mode, this mode EFI spectrophotometer can measure up to 512 color block readings, convenient to upload to the computer, suitable for offline operation.

 

 

However, no matter which kind of measuring equipment is used, it is very important to correct the equipment calibration and parameter setting, only in this way can ensure the normal use of the equipment and the reference value of the measured value. Generally, the measurement parameters that should be considered include standard light source, measurement aperture, field Angle, response mode, calculation formula and coefficient, etc. For density measurement, the selection of three-color filter and polarized light filter should be considered. In the color management process, it is best to use the same measuring instrument and the same color management software, such as digital proofing linearization and the production of ICC Profile, both processes use the same ES-1000 spectrophotometer, which can ensure that the color description is comparable and thus reduce the error.