Ⅰ. Overview of Printing Color Measurement Instruments:
When it comes to controlling and measuring colors, densitometers, Colorimeters, and Spectrophotometer s are the primary tools for printing color measurement. Although these three instruments have different functions, they all utilize reflected (or transmitted) light to measure colors. By illuminating the sample with a standardized light source inside the instrument, the sample selectively absorbs, reflects, and scatters light, which is detected by the instrument's photoDetectors. The detected light is compared to the standard light source, and when using single-wavelength filters or spectrometers, the sensor analyzes the color and intensity wavelength by wavelength, processes the information, and provides the necessary data such as density values or colorimetric parameters.
Densitometer: It typically has 3-4 filters (e.g., red, green, blue), allowing approximately 1/3 of the visible spectrum to reach the photoDetector. It can measure the entire visible spectrum and provide density values for yellow, magenta, and cyan. The densitometer usually includes functions such as density, ink trapping, dot gain, grayscale, saturation, dot area, color deviation, and print contrast. Among these, density measurement is the most crucial function of a densitometer as it directly reflects information about ink thickness and concentration on the printed sheet.
Colorimeter: There are currently two types of Colorimeters in the market: tristimulus and spectrophotometric.
Tristimulus Colorimeters have a design similar to densitometers, including filters for the primary colors (red, green, blue) to divide visible white light into these primary colors. However, there are two main differences:
Tristimulus Colorimeters are designed for color observation, aiming to replicate human vision, while densitometers consider the unique sensitivity of inks.
Tristimulus Colorimeters can process and calculate different color data (e.g., perform color space transformations, color difference calculations), and allow users to plot color coordinates in three-dimensional space, while densitometers lack this functionality to describe colors.
Spectrophotometric Colorimeters, also known as Spectrophotometer s, divide the visible spectrum into very narrow intervals, where each interval represents a different wavelength in the white light spectrum. Due to the ability to divide the white light spectrum into smaller parts, spectrophotometric Colorimeters collect more data, resulting in higher accuracy compared to densitometers. Thus, these instruments offer better measurement repeatability. Similar to tristimulus Colorimeters, spectrophotometric Colorimeters can transform measurement results into three displayable numerical values. When accurate color reproduction is required, Spectrophotometer s are the preferred choice (although they are less reliable than Spectrophotometer s). However, in four-color printing applications, Spectrophotometer s have limitations compared to densitometers, as densitometers can individually measure metrics specific to four-color printing, such as density, dot area, ink trapping, etc., while Colorimeters only measure color.
Spectrophotometer: Similar to Colorimeters, Spectrophotometer s come in two types: filter-based and spectrally dispersed. Their measurement principles are similar to those of Spectrophotometer s. Visible light spectra are divided into small segments using narrowband filters (filter-based) or diffraction gratings (spectrally dispersed). Filter-based instruments are similar to densitometers but have more filters inside, allowing for high-resolution measurements across the spectrum. Additionally, due to their simple design, filter-based Spectrophotometer s are robust and capable of withstanding harsh environments, making them suitable for daily use. On the other hand, spectrally dispersed Spectrophotometer s are more sensitive to collisions, fragile, and expensive, making them unsuitable for portable or production environments. They are more suitable for laboratory settings.
All Spectrophotometer s can output the same data as Colorimeters, and they can also provide spectral curves. Each curve represents the color measured at a specific time, making it possible to identify ink pigment compositions like fingerprints.
In conclusion, an ideal instrument for the printing industry would combine colorimetric and densitometric measurements. Such instruments have been developed and produced, such as the Gretag SPM 100 Spectrophotometer produced by Swiss company Gretag. Although these instruments are currently expensive, they represent the direction of color measurement development in the printing industry.
Ⅱ. Requirements for the Use of Color Measurement Instruments in the Printing Industry:
For color densitometers:
Easy to use and align with standards: In both laboratory and production settings, the use of densitometers is integral to quality control throughout the printing process. If each measurement and calibration process is time-consuming and cumbersome, it will affect speed and accuracy. Furthermore, periodic calibration checks using dedicated reflection (or transmission) gray scales or rulers are necessary.
Appropriate instrument sensitivity for printing color measurement: Generally, CIE A light source and T-status density are used. T-status density is a specialized objective physical Measurement Instrument designed for color separation and printing. The blue, green, and red lights of the T-status density instrument are complementary colors to the yellow, magenta, and cyan inks, allowing for the detection and control of color separation and the modulation of primary color quantities and relative thickness of yellow, magenta, and cyan ink layers.
Performance should meet specification requirements: Accuracy, repeatability, reproducibility, and instrument internal agreement are performance specifications for densitometers. They should be compared and measured beyond the specified time.
For Colorimeters:
Lightweight and portable design: To enable flexible positioning on tested printed materials and adapt to measurements on large-format sheets.
Measurement geometries of 45°/0° or 0°/45° and the use of C or D65 standard illuminants: The CIE 2° observer is suitable as small color areas are evaluated in printing operations.
Aperture size of the Colorimeter should not exceed 5mm: Typically, color patches for printing are smaller than 10mm², and color blocks on print quality control bars are only 6mm². Particularly for continuous tone color image measurements, the range to be measured is even smaller, so the aperture size of the Colorimeter should not exceed 5mm.
Colorimeter output should provide not only standard color values (e.g., XYZ) but also coordinates for CIE LAB and CIE LUV color spaces.
