Spectral reflectance is a sample’s wavelength-by-wavelength response to light under a defined measurement condition. It gives a spectrophotometer richer information than one set of L*a*b* coordinates—but it is not a complete record of visual appearance or chemical identity.
From spectrum to L*a*b*
The instrument measures a reflectance factor across wavelengths. Software combines that spectrum with a selected illuminant and standard observer to calculate XYZ; using the relevant reference white, it can then calculate L*a*b* and color difference. Change the illuminant or observer and the calculated values can change even when the measured sample is the same.
Method still matters
Reported reflectance depends on geometry, specular condition, aperture and sample presentation. Compare spectra only when those conditions are compatible.
What a curve can and cannot tell you
A curve can help diagnose why two samples differ, investigate illuminant metamerism and retain data for color calculations. It cannot uniquely identify a pigment or guarantee how a product will look in every light, angle or surface condition. Reflectance also differs from transmittance: transparent plastics and films may require a defined transmission method rather than backed reflectance.
Fluorescent or optically brightened samples need particular care because UV excitation and emission can affect the measurement. A different calculation illuminant does not recreate a different physical UV condition.
Continue with illuminants, observers and metamerism.
Last reviewed 21/09/2026 · Level: Fundamental
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