Spectrophotometers and colorimeters

Spectrophotometers and colorimeters are instruments for measuring the color of samples with high accuracy, used in the quality control of paints, textiles, plastics, food, cosmetics, packaging, printed matter and ceramics, and in every context where color is a commercial or aesthetic specification. They provide color coordinates according to the standardized CIE systems (Commission Internationale de l’Éclairage): L* (lightness), a* (red-green axis), b* (yellow-blue axis) in the CIE Lab space; or XYZ, xyY, RGB, CMYK. They allow shade control with precise tolerances (ΔE – perceptual color difference) and the generation of color formulations for color matching.

Spectrophotometers vs colorimeters

Colorimeters use 3 photodetectors with tristimulus filters (red, green, blue) – an approximation of the response of the human eye – to calculate the L*a*b* coordinates. They are compact, fast and inexpensive, but with limited accuracy (typical ΔE of 0.5-1) and subject to metamerism (two colors perceived as different that give the same reading). Spectrophotometers measure the entire visible spectrum (400-700 nm in steps of 10 nm or 20 nm) by means of a grating or multiple optical filters with a photodetector array; they calculate the colorimetric coordinates from the full spectrum, with higher accuracy (ΔE of 0.1-0.3) and independence from metamerism. Spectrophotometers are the reference for high-end applications.

Measurement geometries

The main measurement geometries are: 45°/0° (illumination at 45°, collection perpendicular to the surface – it eliminates the reflected specular component and simulates the view of the human eye, and is the most widespread for paints and matte products); d/8° (with an integrating sphere for diffuse illumination and collection at 8° – for more stable measurements on textured surfaces); SCI/SCE (Specular Component Included / Excluded – with or without the specular component of reflection, important for glossy surfaces). The choice of geometry depends on the type of surface (glossy, matte, textured) and on the application (paint control, color control of plastics, and so on).

Technical characteristics

The typical characteristics of portable and benchtop spectrophotometers are: spectral range of 360-740 nm (including UV and NIR portions for advanced applications); resolution of 10 or 20 nm; repeatability below ΔE 0.05; inter-instrument agreement below ΔE 0.3; measurement time of 1-3 seconds; selectable measurement spot (3, 8, 14, 25 mm depending on the surface); a database of standard illuminants (D65, D50, A, F2, F7, F11); standard observers of 2° (for small objects) and 10° (for large surfaces); ΔE76, ΔE94, ΔE2000 and CMC calculation formulas. USB, Bluetooth and WiFi connectivity.

Applications

The applications are wide-ranging: the automotive industry (checking the color accuracy of bodywork and of rivets); the coatings industry (color formulation, quality control, color matching against a sample); textile and clothing production (shade control on yarn and fabric); the plastics industry (color control of pellets and finished parts); cosmetics (color control of products such as lipsticks and foundations); the food industry (color control of food to optimize consumer perception); high-quality printing (checking color correspondence between proofs and the final print run); the ceramic industry (glazes and pigments for ceramics).

Color standards

The main color standardization systems are: CIE Lab (the universal scientific system); the Pantone Matching System (PMS, for industrial printing and packaging); RAL (for industrial and architectural paints); NCS (Natural Color System, Nordic); Munsell (for industrial and research and development applications). Spectrophotometers include libraries of these color scales to allow immediate comparison with standard commercial references. Acceptance tolerances are expressed as a maximum ΔE relative to the target color, with typical values from 1.0 (high quality) to 3.0 (standard quality) for matte surfaces.

Reference standards

The main standards are: ISO 11664 (CIE colorimetry, parts 1-6); ISO 13655 (for printing); ASTM E308 (colorimetric calculation); DIN 6174 (industrial color systems); CIE 15 (the official CIE publication on colorimetry). Periodic calibration is carried out with reference tiles certified to ISO 17025 (generally a set of 3-5 reference color tiles) or with a white and black tile for basic calibration. The typical calibration interval is annual for high-end applications.