Short answer
Opaque materials are measured in reflectance: the instrument lights the surface and reports the fraction reflected at each wavelength. Transparent liquids, films and glass are measured in transmittance: light passes through and the instrument reports the fraction that emerges. Translucent materials sit in between and are the hardest, because what is behind or around them changes the reading — so thickness and backing must be fixed and stated.
A reflectance measurement compares the light returned by the sample with what a perfect reflecting diffuser would return under the same geometry. Instruments are calibrated on a white tile whose own reflectance is known, so the result is a reflectance factor per wavelength, typically from 0 to 1 (or 0–100 %). Glossy samples can exceed 1 at the specular angle, which is one reason geometry is part of every result. From the reflectance curve, any illuminant and observer can be applied to obtain CIELAB. Reflectance is the right mode for paint, printed paper, plastics, textiles and anything light does not pass through.
For transparent samples the reference is the empty light path, or a cuvette of solvent, and the result is the fraction transmitted at each wavelength. Path length matters directly: doubling a liquid's thickness roughly squares its transmittance at each wavelength, so a beer, wine or dye solution is always measured in a cuvette of stated length. Regular (straight-through) transmittance suits clear liquids and filters; total transmittance, collected with an integrating sphere, is needed for hazy films and diffusers that scatter light sideways. Colour grading scales in food and chemicals are mostly transmittance measurements at fixed path length.
Paper, thin plastics, textiles and some coatings let part of the light through. Measured in reflectance, the result depends on what is behind them: a white backing reflects transmitted light back and lightens the reading, a black backing absorbs it and darkens it. Graphic-arts measurement therefore specifies backing — the Fogra offset data are measured on white backing, while WAN-IFRA's newsprint aims give black backing as normative with white backing informative, and the two differ by several L* units for the same paper. ISO 13655 defines a white backing material and uses the ratio of readings over black and white backing as a measure of opacity.
For translucent solids, either measure at a thickness where nothing shows through, or state the backing and keep it constant. For powders, pack a fixed amount into a cell of fixed shape so density and surface are repeatable. For textured or patterned samples, use the largest aperture available or average several readings in different places. For samples whose colour changes with temperature, measure at a stable room temperature after the sample has equilibrated. All of these are about making the measurement describe the material rather than the circumstances of that one reading.
Why: One measured over a white backing and the other over a black backing or a stack of the same paper.
Fix: Agree the backing (and measurement condition) in the specification and record it with results.
Why: Cuvette path length, temperature or cleanliness not controlled.
Fix: Use the specified path length, temper the sample and zero on the solvent each session.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
Measurements of semi-transparent samples are influenced by what lies behind them; recommended remedies are increasing sample thickness until no light is transmitted or placing an opaque white surface behind the sample.
Source: Precise Color Communication
StandardModerate evidence
WAN-IFRA's guide to ISO 12647-3:2013 gives the newsprint aim as L* 82, a* 0, b* 3 over black backing (normative) and L* 85, a* 1, b* 5 over white backing (informative).
Caveat: Trade-body summary of the standard's aims.
Source: ISO 12647-3:2013 — quality standard for newspaper production
StandardStrong evidence
ISO 13655:2017 specifies the optical properties of the white backing material used in graphic-arts measurement and defines an opacity-type ratio from readings over black and white backing.
FactStrong evidence
For textured or patterned samples, readings vary with position when the measured area is small, so the largest aperture or an average of several locations should be used.
Source: Precise Color Communication
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.