A standard illuminant is a defined spectral power distribution used with a standard observer to calculate and communicate color. It is a calculation condition—not automatically the same thing as the lamp above a product.
Under the current ISO/CIE 11664-2:2022 terminology, A, D65 and D50 are CIE standard illuminants. D55, D75 and the FL series remain useful CIE illuminants but are not part of that formal standard-illuminant set. This naming distinction matters when a specification cites the standard precisely.
Illuminant, lamp and instrument setting: the difference
A physical light source has its own spectrum. A CIE illuminant is a defined reference spectrum. Instrument software uses the selected illuminant with the measured spectral data and an observer to calculate XYZ and L*a*b*. Two lamps with a similar color temperature can still have different spectra and make materials look different.
| Condition | What it represents | Use it when |
|---|---|---|
| D65 | A daylight reference, approximately 6500 K. | The specification calls for D65 or outdoor/daylight evaluation. |
| D50 | A daylight reference, approximately 5000 K. | The method or industry workflow specifies D50. |
| A | A tungsten/incandescent reference, approximately 2856 K. | The agreed method requires illuminant A; it is not a proxy for every warm LED. |
| TL84 / F11 | TL84 is a commercial Philips lamp designation; F11/FL11 is the CIE relative spectral power distribution commonly used for the calculation. | The specified retail or fluorescent evaluation condition is named. |
Use one recorded condition
Record illuminant and observer with geometry, aperture and ΔE formula. Changing D65 to D50 recalculates the color values; it does not change the physical sample. For fluorescent or optically brightened materials, UV excitation in the illumination can also matter, so a calculation setting alone cannot reproduce a different physical light source.
Continue with 2° vs 10° observer, metamerism and spectral reflectance. See the CIE standard illuminants reference for definitions.
Last reviewed 21/09/2026 · Level: Fundamental
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