Overview of Printing Color Measurement Instruments

When controlling and detecting colors, densitometers, colorimeters, and spectrophotometers serve as the primary tools for color measurement in printing. Although these three instruments have distinct functions, they all measure color by utilizing reflected (or transmitted) light. The process involves illuminating the sample with a standard light source inside the instrument, where the sample selectively absorbs, reflects, and scatters light. The instrument’s photodetector then detects the reflected light and compares it to the standard light source. When using a single-wavelength filter or spectral beam splitter, the sensor analyzes color and intensity by wavelength, processes the information, and provides required data such as density values or colorimetric parameters.

1. Densitometers

• Typically designed with 3–4 filters (red, green, blue, etc.), each allowing approximately 1/3 of the visible spectrum to reach the photodetector. They measure the entire visible spectrum to obtain density values for yellow, magenta, and cyan inks.

• Built-in functions often include: density, ink trapping rate, gray scale, saturation, dot area, tone error, and printing contrast. Among these, density measurement is the most critical function, as density values directly reflect information about ink thickness and concentration on printed sheets.

2. Colorimeters

• Two types are currently available in the market: tristimulus colorimeters and spectral colorimeters.

• Tristimulus colorimeters are designed for color observation, functioning similarly to the human eye, whereas densitometers are designed with specific sensitivity to inks.

• Tristimulus colorimeters can process and calculate various color data (e.g., color space conversion, color difference calculation) and allow users to plot color coordinates in 3D space—functions not available in densitometers.

• Tristimulus colorimeters: Similar in design to densitometers, they include primary color (red, green, blue) filters that divide visible white light into three primary colors. The key differences are:

• Spectral colorimeters (spectrocolorimeters): Divide the visible spectrum into very narrow intervals, each representing different wavelength parts of white light. By splitting the spectrum into numerous small segments, they collect more data, offering higher precision and better measurement repeatability than densitometers. Like tristimulus colorimeters, spectral colorimeters convert measurements into three displayable numbers. While ideal for accurate color reproduction (though less reliable than spectrophotometers), they have limitations in four-color printing compared to densitometers, which can individually measure metrics like density, dot area rate, and ink trapping rate—functions that colorimeters lack as they only measure color.

3. Spectrophotometers

• Similar to colorimeters, spectrophotometers come in two types: filter-based and spectral dispersion-based, with measurement principles analogous to spectrocolorimeters. The visible spectrum is divided into segments using narrow-band filters (filter-based) or diffraction gratings (dispersion-based). Filter-based instruments operate similarly to densitometers but with more filters, enabling high spectral resolution. Their simple design makes them rugged and capable of withstanding harsh daily environments. In contrast, dispersion-based spectrophotometers are sensitive to impacts, fragile, expensive, and unsuitable for field or production use, making them better suited for laboratory settings.

• All spectrophotometers can output the same data as colorimeters and additionally provide spectral curves. Each curve represents the measured color, allowing identification of ink pigment components like a fingerprint.

In summary, integrating colorimetric and density measurements into a single instrument is ideal for the printing industry. Such instruments, like the Gretag SPM 100 spectrophotometer by Gretag, Switzerland, have been developed. Although currently expensive, they represent the future of color measurement in printing.

Requirements for Using Color Measurement Instruments in Printing

Requirements for Color Densitometers

1. Ease of use and standardization: In experimental production, print quality control relies heavily on densitometers. Cumbersome measurement and calibration processes would affect speed and accuracy. Regular calibration checks using specialized reflective (or transmissive) gray scales are also necessary.

2. Sensitivity suitable for printing color measurement: Typically using CIE A light source and T-status density. T-status density is an objective physical measurement designed for color separation and printing, with blue, green, and red light as complementary colors to yellow, magenta, and cyan inks—enabling effective detection and control of primary color modulation and relative ink layer thicknesses during color separation and printing.

3. Compliance with performance specifications: Densitometers must meet standards for accuracy, repeatability, reproducibility, and internal consistency, allowing performance comparison and measurement beyond specified timeframes.

Requirements for Colorimeters

1. Portable design: Enables flexible positioning on test prints and adapts to large-format sheet measurements.

2. Measurement geometry: 45°/0° or 0°/45°; standard light source: C or D65; CIE 2° small field of view standard observer (suitable for evaluating small-color areas in printing).

3. Measurement aperture ≤ 5mm: Since printing color patches are typically<10mm² (e.g., 6mm² on print quality control strips) and continuous-tone color images require even smaller measurement ranges, the aperture should not exceed 5mm.

4. Output values: Include standard color values (e.g., XYZ) and CIE LAB/CIE LUV color coordinates.

   
   

Key Term Notes

• Densitometer: Measures ink density, critical for evaluating ink thickness and concentration.

• Tristimulus colorimeter: Mimics human eye color perception, suitable for color space conversion.

• Spectrophotometer: Provides spectral curves for precise pigment analysis, ideal for laboratory use.

• CIE Lab/LUV: International color spaces for standardized color representation and comparison.