Spectrophotometers vs. Colorimeters: Choosing the Right Instrument for Colour Measurement

At a Glance: Spectrophotometers vs Colorimeters

Colorimeters measure colour with three filters aligned to human vision and can use large apertures, useful for averaging non-uniform samples. Spectrophotometers measure data for spectral reflectance, providing colour data that correlates with human vision but with more information, allowing them to detect metamerism, and simulate multiple illuminants.

Portable spectrophotometers have smaller apertures restricted by their sphere size, while benchtop spectrophotometers offer greater versatility, often including the ability to measure in transmission mode at the expense of portability.

What is a Colorimeter and How Does It Work?

A colorimeter is a tristimulus instrument. It measures colour in a similar way to the cells in the human eye. Light reflected from a sample is filtered into three bands: red, green, and blue, corresponding to the sensitivity of the cones in the human eye. These signals are then converted into tristimulus values (X, Y, Z) and reported in widely used colour spaces such as CIE L*a*b* or XYZ.

Key Features of Colorimeters

• Direct Human Vision Correlation: colorimeters measure colour through three filters aligned to the eye’s sensitivity curves, reporting relatively simple to use colour coordinates such as CIE L*a*b*.
• Large Aperture Options: Portable colorimeters, such as the Konica Minolta CR-410 (50 mm aperture), can feature comparatively large apertures. This makes them particularly useful for averaging non-uniform samples like biscuits, stone or wood, where a small aperture would require multiple measurements to characterise the visual effect of the sample.
• Portable and User-Friendly: Their simpler optical design makes them robust, straightforward to operate, and easy to deploy in production environments.

Limitations of Colorimeters

• Single Illuminant: Most colorimeters use a single illuminant for each measurement (the CR-400/410 uses either D65 or C). This restricts a colorimeter's ability to predict how a colour will look under different lighting.

• Less Precision: Compared with spectrophotometers, repeatability and inter-instrument agreement are lower, which can cause problems when comparing results across devices.
• No Spectral Data: Without full spectral reflectance curves, colorimeters cannot detect metamerism (when two colours look identical in one light but different in another).
• Inadequate for Colour Formulation: Recipes for pigments, dyes, or coatings require full spectral information.

What is a Spectrophotometer and How Does It Work?

A spectrophotometer measures colour in more detail. Instead of three filters, it uses a diffraction grating to separate reflected light into its component wavelengths throughout the visible spectrum (typically 400–700 nm). Multiple sensors then measure reflectance at each interval, often every 10 nm.

From this spectral data, the instrument can calculate tristimulus values and any colour space coordinates. Crucially, it can also simulate how the colour will appear under different standard illuminants such as D65 (daylight), A (incandescent), or modern LED spectra.

Key Features of Spectrophotometers

• Detailed Data: In addition to CIELAB values, spectrophotometers provide spectral reflectance curves that reveal the composition of colourants.
• Versatility: Available in portable (handheld) or benchtop models, with benchtop models often providing the option for measuring both reflectance and transmittance (transparent liquids, films, or glass).
• Multiple Illuminants: Stored spectral power distribution data allows results to be recalculated under different lighting conditions, identifying issues such as metamerism.
• Essential for Colour Formulation: Software can use spectral data to predict recipes for dyes, coatings, or plastics.

Considerations and Challenges

• Complexity: More detailed colour data may mean that operators may require training to interpret spectral data correctly.
• Sample Preparation: Spectrophotometers provide more control when measuring with consideration for gloss or surface. Using the correct geometry and consistent handling is essential.

Certain special-effect colours, such as metallic or pearlescent coatings and fluorescent materials, require additional measurement control. Multi-angle spectrophotometers are used for metallics to capture face and flop differences, while UV-controlled instruments ensure fluorescent samples are measured as perceived.