UV Control and Optical Brighteners in Color Measurement

For fluorescent or optically brightened materials, the UV condition is part of the measurement itself—not a cosmetic option. The same sample can return different spectral, L*a*b* and whiteness results when it is physically excited by different amounts of ultraviolet energy, even if both software screens say D65.

Why optical brighteners change the measurement

Fluorescent whitening agents (FWAs), also called optical brightening agents, compensate for a material’s yellowish cast by converting invisible ultraviolet energy into visible blue light. Common FWAs absorb most strongly in roughly the 340–380 nm region and re-emit at longer wavelengths, commonly around 420–470 nm. The exact bands depend on the brightener and substrate.

A conventional non-fluorescent sample only redirects part of the incident light. A brightened sample adds emitted blue energy to the reflected blue energy. The instrument therefore measures a total spectral radiance factor—ordinary reflection plus fluorescence. In the blue region, that reported radiance or apparent reflectance factor can legitimately exceed 100%. It does not mean the sample has reflected more of the same blue light than arrived; UV energy has been converted and emitted at a longer wavelength.

UV included, UV excluded and UV calibrated

UV excluded is an optical condition, not a software setting. Selecting a checkbox that merely changes a calculation cannot remove UV that already illuminated the specimen. Confirm that the instrument actually inserts a cut-off filter or provides an equivalent controlled optical path.

Why the instrument’s optical design matters

A practical one-monochromator instrument can include fluorescence only when it illuminates the sample polychromatically—using broadband light—and spectrally analyses the light after it leaves the sample. This is often described as reverse optics: polychromatic illumination with monochromatic viewing, or an array spectrometer on the detection side.

That arrangement lets UV wavelengths excite the brightener while the detector records the visible blue emission at the wavelength where it emerges. A one-monochromator design that illuminates one wavelength at a time and uses broadband detection can assign emitted energy to the excitation wavelength instead of the emission wavelength. It is therefore unsuitable for this form of fluorescent color measurement.

Many integrating-sphere instruments use the required reverse arrangement, which is why UV control is common on sphere benchtops. The sphere itself is not the deciding feature: confirm the source, filter mechanism and location of spectral analysis in the actual model. ASTM E991 defines the one-monochromator approach around continuous broadband polychromatic illumination and a viewing monochromator.

How UV calibration is performed

Normal black and white standardisation corrects the instrument’s photometric response; it does not establish the right UV stimulus for a fluorescent specimen. UV calibration is a separate operation tied to a named method, reference material and assigned value.

The full Ganz–Griesser procedure uses a set of fluorescent white reference specimens with assigned whiteness values to characterise the instrument and establish its UV adjustment. A durable fluorescent tile can then serve as a transfer or working standard: it is assigned a value at the established setting, and routine adjustment positions the filter until the tile returns that value. The tile does not create traceability by itself; its assignment comes from the primary-reference procedure.

Other applications use different primary standards and target quantities—for example CIE whiteness on textiles or paper, or ISO brightness. ISO 11475 specifies UV adjustment for D65/10° paper whiteness because results on papers containing FWAs depend on the UV incident on the sample. Do not substitute one industry’s fluorescent reference simply because its nominal whiteness number looks similar.

Maintaining a UV-adjusted condition

• Use the working standard on a controlled schedule. Follow the instrument maker and your quality plan; shorten the interval after lamp replacement, repair, transport or failed verification. Some instrument procedures use a weekly fluorescent-tile check, but that is an example—not a universal rule.
• Protect the standards. Keep fluorescent references clean, covered and conditioned as instructed. Replace or reassign primary standards at the provider’s interval because contamination, ageing and light exposure can change them.
• Keep the assignment complete. Record the standard ID, assigned value, formula, illuminant, observer, geometry, aperture, specular condition, date and filter position.
• Verify independently. After adjustment, measure one or more control materials that were not used to set the filter. A useful control set spans the fluorescence levels and substrates encountered in production.
• Trend, do not merely pass. Record the working-tile result and required filter movement. A growing correction can reveal lamp ageing, contamination or optical service needs before production comparisons fail.

The reference method is bispectral

A UV-adjustable production spectrophotometer measures total radiance factor for its controlled source and is a practical approximation for routine QC. It does not completely characterise how the material will respond to every possible excitation spectrum.

Rigorous fluorescence measurement uses a two-monochromator bispectrometer. One monochromator selects each excitation wavelength and the other measures the resulting reflection and emission wavelengths. The two-dimensional result is a Donaldson matrix. ASTM E2153 identifies this bispectral method as the definitive route to illuminant-independent radiation-transfer data for fluorescent specimens; color can then be calculated for a chosen illuminant and observer.

Check the UV quality of daylight simulators

A lamp can look like daylight and match D65 reasonably in the visible region while still providing the wrong UV stimulus for an optically brightened sample. CIE 051.2 assesses daylight simulators with a visible-range metamerism index and a separate ultraviolet-range index, MI_uv, using fluorescent/non-fluorescent metameric pairs. Performance is reported in categories A to E. The separate UV grade is the relevant warning that visible agreement alone does not qualify a simulator for fluorescent whites.

When a specification relies on a physical D65 simulator—inside an instrument or a visual assessment cabinet—record the applicable visible and UV-range grade rather than relying only on correlated color temperature or the word “daylight.”

Why two instruments can agree—until the sample is brightened

Two instruments may agree closely on ceramic tiles, BCRA tiles and ordinary non-fluorescent production colors because those checks mainly test wavelength scale, photometric response and conventional reflectance. They can still disagree badly on brightened paper, textile or plastic if their lamps deliver different UV energy, their filters are in different states, or their UV adjustments are tied to different standards.

For defensible inter-instrument agreement, match the optical UV condition as well as geometry, aperture, SCI/SCE, sample backing, calculation illuminant, observer and formula. Then verify with fluorescent transfer standards and representative production materials—not only a non-fluorescent white tile.

References and further reading

• ASTM E991: one-monochromator color measurement of fluorescent specimens.
• ASTM E2153: bispectral photometric data for fluorescent color.
• ISO 11475: CIE whiteness and UV adjustment for paper and board.
• CIE 051.2: daylight-simulator quality, including the UV-range metamerism index.
• Peer-reviewed overview of optical-brightener absorption and emission bands.
• Datacolor: UV calibration, primary standards and fluorescent working tiles.

Continue with whiteness index, standard illuminants, spectral reflectance, spectrophotometer calibration and instrument disagreement.

Last reviewed 21/09/2026 · Level: Intermediate

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