Metamerism is why two materials can match under one light and look different under another. The most common case is illuminant metamerism: the samples have different spectral reflectance curves, so changing the light changes their relationship.
The problem is metameric failure
Strictly, metamerism is the successful match produced by different spectra under one condition; the visible disagreement after the illuminant or observer changes is metameric failure. In commercial work the shorter term “metamerism” is commonly used for the whole problem.
Observer metameric failure can also occur when two people respond differently to the same spectral pair. Changes in measurement geometry or visual field size can create related conditional mismatches, so difficult approvals should define lighting, observer, geometry and viewing field—not only one ΔE result.
A match at one light is not necessarily a match
Two samples can produce close L*a*b* and ΔE values under one agreed illuminant while reflecting different proportions of light across the visible spectrum. Under another illuminant, the weighting changes and their calculated color can separate. This is different from a single sample’s color inconstancy, and different again from color travel in an effect pigment as the viewing angle changes.
A practical investigation
01
Confirm the reference method
Align geometry, aperture, SCI/SCE, illuminant, observer and sample presentation.
02
Compare spectra and conditions
Where available, inspect the spectral curves and calculate both samples under each specified illuminant.
03
Validate under end-use light
Use a controlled visual assessment and record the light source, observer and acceptance decision.
A spectrophotometer reveals the risk; it does not eliminate it. Reducing risk can require matching pigment systems and validating materials together under the end-use lights. For fluorescent materials, physical UV excitation must also be controlled—changing a calculation illuminant alone is not enough.
Use illuminants, spectral reflectance and the troubleshooting workflow to structure the check.
Last reviewed 21/09/2026 · Level: Fundamental
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