Fluorescent whitening agents absorb ultraviolet energy and emit part of it as visible blue light. A brightened textile therefore returns reflected light plus fluorescent emission, so its measured spectrum and color depend on the UV energy that physically illuminated it. Two repeatable instruments can disagree if their UV conditions differ.
Why fluorescence changes the result
An untreated textile mainly redirects incident visible light. A brightened specimen can add emitted blue energy, making it look whiter or bluer and sometimes producing apparent reflectance-factor values above 100% in part of the visible spectrum. The effect varies with brightener chemistry, fibre, concentration, finishing and source spectrum.
D65 on screen is not physical D65 excitation
Changing the calculation illuminant reweights measured spectral data. It does not change the lamp, insert a UV filter or recreate fluorescence that a different physical source would have produced.
Choose the UV condition for the decision
| Condition | What it does | Use |
|---|---|---|
| UV included | Uses the instrument’s available UV so fluorescence contributes to the result. | Routine assessment when the physical source and its maintenance are defined. |
| UV excluded | Physically suppresses the main UV excitation band with a cut-off condition. | Diagnoses substrate or colorant response with most brightener contribution removed. |
| UV calibrated or adjusted | Adjusts source UV against an assigned fluorescent textile reference. | Improves inter-instrument agreement for a specified textile-brightener workflow. |
UV excluded and UV calibrated are not interchangeable. A paired UV-included/UV-excluded comparison can reveal fluorescent contribution, but acceptance must use the condition named by the specification.
Calibrate for textiles, not another material
AATCC EP11 uses the Textile UV Calibration Standard for inter-instrument calibration of spectrophotometer UV energy on optically brightened textiles. Its scope notes that textile fluorescent whitening agents differ from those used in plastics and other materials, so a generic fluorescent reference should not be substituted merely because it is convenient. Confirm that the instrument can perform the required physical adjustment and that the reference is current and handled as instructed.
Prepare and compare samples consistently
- Condition the textile and use the same face, direction, layers and backing for target and sample.
- Standardise the instrument, then perform any separate UV calibration required by the method.
- Use the same aperture, geometry, SCI/SCE mode, illuminant, observer and whiteness or color-difference calculation.
- Measure representative positions and retain spectra as well as L*, a*, b* or whiteness values.
- Assess critical whites visually in a qualified booth with appropriate visible and UV performance.
Diagnose site-to-site disagreement
If instruments agree on non-fluorescent tiles and ordinary colored textiles but diverge on brightened whites or pale shades, compare source type, UV filter state, textile calibration standard, assigned value, aperture and sample preparation. A viewing-booth mismatch can also arise when two lamps have similar color temperature but different UV spectral power.
References and further reading
- AATCC EP11: UV Energy of Optically Brightened Textiles.
- UV Control and Optical Brighteners in Color Measurement.
- CIE 051.2: daylight-simulator quality, including UV assessment.
Continue with textile illuminants, metamerism and instrument selection.
Last reviewed 22/09/2026 · Level: Intermediate
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