Metamerism Color

Metamerism is a central concept in colour science, describing situations in which two colours appear identical to the human eye despite having different spectral power distributions. Colours that match in this way are known as metamers. This phenomenon arises because human colour vision is trichromatic: the eye relies on three types of cone cells, each sensitive to broad bands of wavelengths. As a result, many different spectral combinations can stimulate the cones in the same proportions, leading to identical colour perceptions. Metamerism underpins much of modern colour reproduction technology, yet it also presents challenges in industries that demand strict colour tolerances.

Sensory Basis of Metamerism

A spectral power distribution specifies the proportion of light emitted, reflected or transmitted at each wavelength. While this describes the physical composition of a colour stimulus, the visual system reduces this incoming light to three values—the tristimulus values—based on cone responses. Because each cone integrates a wide band of wavelengths, numerous distinct spectra can produce identical triplets of cone signals. Colour matching functions are used in colour science to numerically describe cone sensitivities and predict metameric matches.
Metamerism is common in near-neutral colours such as whites, greys and blacks, where numerous spectral combinations can evoke similar sensations. As colours become more saturated or vivid, the tolerable range of discrepant spectra narrows, making metamerism less likely.

Types and Sources of Metamerism

Metameric failure occurs when two samples that match under one set of viewing conditions fail to match under another. Several forms of metameric failure are recognised in applied colourimetry.
Illuminant metamerism arises when a pair of materials matches under one light source but not another. Fluorescent lamps with irregular, peaky emission spectra commonly cause mismatches, even when two samples match under smoother incandescent lighting. The interaction between a material’s spectral reflectance curve and the illuminant’s spectral output determines the final perceived colour.
Geometric metamerism occurs when samples match from one viewing angle but not from another. This is often observed in pearlescent, metallic or interference-based coatings, such as automotive finishes or speciality photographic papers, where angular-dependent reflection alters colour appearance.
Observer metamerism reflects variations among human observers. Differences in cone ratios, spectral sensitivities and age-related factors such as lens yellowing can cause two samples to match in one person’s vision but not another’s. Even individuals with typical colour vision show subtle variations that may produce observer-dependent mismatches.
Field-size metamerism results from differences in cone distribution between the fovea and peripheral retina. Colours that match when presented as small, centrally fixated fields may diverge when displayed as large surfaces. Industrial colour standards therefore often rely on large-field viewing conditions.
Device metamerism arises from inconsistencies among colour measuring instruments. Colourimeters incorporate filters and sensor arrays that differ between devices or manufacturers, leading to variations in measured values. These differences may cause two stimuli to be classified as matching by one instrument but not by another.
Metamerism is sometimes described in terms of a degree of metamerism, indicating the sensitivity of a match to changes in illuminant, observer or geometry. High degrees of metamerism imply unstable matches that are easily disrupted.

Measurement of Metamerism

Several quantitative metrics are used to assess metamerism, particularly in lighting and colour reproduction industries.
The Colour Rendering Index (CRI), traditionally the most widely known metric, compares a test light source to a reference by calculating the mean distance between colour samples in a CIE 1964 colour space. Although still used, its reliance on an outdated and non-uniform colour space limits accuracy.
The modern IES TM-30 metric supersedes CRI for many applications. It provides a more accurate evaluation of colour fidelity and includes assessments of saturation and hue shift, enabling a nuanced analysis of how a test light alters colour appearance.
The CIE Metamerism Index (MI) is used primarily for daylight simulators. It quantifies metamerism by calculating mean colour differences for eight standard metamers in CIELAB or CIELUV space, which are perceptually more uniform. MI can be separated into MIvis_{vis}vis​ and MIUV_{UV}UV​ components when only parts of the spectrum are relevant. The results are categorised into letter grades for practical interpretation.
These metrics allow manufacturers to evaluate the stability of colour matches under different lighting conditions and to design illuminants with improved colour rendering properties.

Industrial Impact and Applications

Metamerism has significant implications in industries where precise colour matching is essential.
Automobile industry: Interior fabrics, plastics and paints may be matched under a specific illuminant such as cool white fluorescent light. However, such matches often fail under daylight or incandescent light sources due to large spectral differences in the materials used. Because true spectral matches are rare, automotive manufacturers must carefully control colourants and inspect components under multiple lighting conditions.
Textile industry: Dye formulations for fibres, yarns and fabrics frequently show illuminant, observer and field-size metamerism. While large textile samples can be compared under several lights, small fibres viewed through microscopes—typically illuminated by a single source—pose challenges. Microspectroscopy, which combines microscopic imaging with spectroscopic analysis, is used to detect metameric fibres and ensure colour consistency.
Paint industry: Paint manufacturers aim for spectral rather than strictly metameric matches. A spectral match minimises metamerism by aligning the reflectance curves of two samples, reducing sensitivity to illuminant or observer variations. When the original pigment formulation is unknown, spectrophotometric tools are required to minimise metamerism. Using identical pigments and binders in reproductions remains the most reliable strategy.
Printing industry: Inkjet printing relies on colour mixing under specific illuminants, often leading to metameric mismatches when viewed under different light sources. Minimising metamerism involves measuring the spectral reflectance of the original sample, selecting suitable ink combinations and iterating the printing process until the reflectance characteristics align closely. Nevertheless, gamut limitations and material constraints may prevent achieving an ideal match.
Across industries, managing metamerism is essential to maintaining colour standards, ensuring consumer satisfaction and achieving accurate reproductions.