Color Measurement Instrument Requirements in the Printing Industry
Printing Color Measurement Instruments and Their Requirements
Color Measurement Instruments Used in Printing
Densitometers, colorimeters, and spectrophotometers are the main instruments used to measure printing color for control and inspection. Although each has different functions, all measure color using reflected or transmitted light. An instrument's internal standard illuminant illuminates the sample; the sample selectively absorbs, reflects, and scatters the light. A photodetector measures the reflected light and compares it with the reference illumination. With single-wavelength filters or a spectrometer, sensors analyze color and intensity by wavelength, process the information, and provide density values or colorimetric parameters.
## 1. Densitometers
Densitometers are generally designed with three or four filters, such as red, green, and blue. Each filter allows approximately one third of the visible spectrum to reach the photodetector. The instrument measures across the visible range and obtains density values for yellow, magenta, and cyan.
Common built-in functions include density, ink trapping, grayness, saturation, halftone dot area, hue error, and print contrast. Density measurement is the most important function: the density value can directly indicate the ink film thickness and strength on a printed sheet.
## 2. Colorimeters
Two types of colorimeters are commonly available: tristimulus and spectral.
A tristimulus colorimeter is designed similarly to a densitometer and uses red, green, and blue filters to divide visible white light into its three primary components. The key differences are:
1. A tristimulus colorimeter is designed to observe color and function more like human vision, while a densitometer is designed around the specific spectral sensitivity needed for ink measurement.
2. A tristimulus colorimeter can process and calculate different color data, such as color-space conversions and color differences, and can represent color coordinates in three-dimensional space. A densitometer does not provide the same color-description functions.
A spectral colorimeter, also called a spectrocolorimeter, divides the visible spectrum into very narrow intervals, each representing a wavelength range in white light. Because the spectrum is divided into many small parts, the instrument collects more data. The source article describes spectral colorimeters as more accurate than densitometers and notes that these instruments generally offer better measurement repeatability. Like a tristimulus colorimeter, a spectral colorimeter can display the measurement as three values.
When accurate color reproduction is required, a colorimeter may be a suitable choice (the source article notes that its reliability is lower than that of a spectrophotometer). However, for four-color printing, a colorimeter may not replace a densitometer: a densitometer can separately measure printing metrics such as density, dot area, and ink trapping, while a colorimeter measures color.
## 3. Spectrophotometers
Like colorimeters, spectrophotometers can be filter-based or spectrally dispersive, and their measurement principle is similar to that of a spectral colorimeter. The visible spectrum is divided into small bands using narrow-band filters or a diffraction grating.
Filter-based instruments are used much like densitometers, but they contain more filters and can provide high spectral resolution. Their simple design makes them robust enough for demanding day-to-day environments. Dispersive spectrophotometers are more sensitive to impact, fragile, and expensive; they are less suitable for portable or production-floor use and are better suited to a laboratory environment.
All spectrophotometers can output the same types of data as colorimeters and can also provide a spectral curve. Each curve represents the color measured in a particular reading and can act like a fingerprint for identifying ink pigment characteristics.
An instrument that combines color-difference measurement with density measurement would be ideal for the printing industry. The source article cites the Gretag SPM 100 spectrophotometer from Switzerland as an example of this type of instrument. Although such instruments may be expensive, the article describes them as representing a direction for the future of color measurement in printing.
## Requirements for color measurement instruments in printing
### Requirements for color densitometers
1. Easy operation and standards: Densitometers are used throughout print-quality control. Complicated or time-consuming measurements and calibration can affect production speed and accuracy. Check calibration regularly using a dedicated reflective (or transmissive) gray scale or scale.
2. Sensitivity suitable for print-color measurement: A common setup is CIE illuminant A and Status T density. Status T density is an objective physical measurement condition designed for color separation and printing. Its blue, green, and red responses correspond to the complementary colors of the primary yellow, magenta, and cyan inks, helping inspect and control their amounts and relative ink-film thicknesses during separation and printing.
3. Performance that meets specifications: Accuracy, repeatability, reproducibility, and instrument agreement are key performance specifications. These properties should be comparable and verifiable over time.
### Requirements for colorimeters
1. Lightweight construction: The instrument should be easy to position flexibly on printed samples and suitable for large-format sheets.
2. Measurement geometry and viewing conditions: Use 45°/0° or 0°/45° geometry. A standard illuminant such as C or D65 is specified, and the CIE 2° standard observer is recommended because the color areas evaluated in printing are often small.
3. Small measurement aperture: The aperture should be no larger than 5 mm. Printing color-chart patches are often smaller than 10 mm², and patches on print-quality control strips may be only 6 mm². Continuous-tone images may require measuring even smaller areas, so an aperture of 5 mm or less is recommended.
4. Useful colorimetric outputs: In addition to standard color values such as XYZ, the instrument should output coordinates for CIE L*a*b* and CIE L*u*v* color differences.