Optical Brightener
A white cotton shirt washed in many household detergents gives off visible light of its own in daylight.1, 2
The light comes from a fluorescent whitening agent, a colourless dye that absorbs ultraviolet and re-emits it as blue.1
Untreated white fabric tends to look slightly yellow.1 Optical brighteners correct this by absorbing invisible ultraviolet in the range of roughly 300 to 430 nanometres and emitting blue light between about 400 and 500, peaking at 435 to 440.1 The added blue cancels the yellow, and because the material is now emitting as well as reflecting, it can look brighter than white.1 They work only on clean fabric and are no substitute for washing or bleaching.1
The most important brightener for household detergents is a stilbene compound known in Europe's industry risk assessments as FWA-1.2 More than 90 per cent of its production goes into household detergents, typically at 0.05 to 0.15 per cent of the product.2 It is designed to cling to cotton like a colourless dye, and whiteness builds up over repeated washes.2 The 2004 assessment concluded that this use is safe for consumers.2
The technique began in banknote printing, where invisible fluorescent compounds were used to deter forgery.3 The first recorded use to whiten textiles was by P. Krais, writing in the textile journal Melliand in 1929.3 An older version of the trick existed already: linen bleached on lawns was treated with an extract of horse chestnut bark that contains a natural fluorescent agent.3 Synthetic brighteners became common early in the Second World War and largely replaced blueing agents such as ultramarine.3
The effect depends on the lighting.4 Most white LEDs emit mainly between 420 and 700 nanometres and contain little of the ultraviolet that excites a brightener.4 Under them, treated whites can look duller than they do in daylight.4 The cinematographer Jay Holben found that two of four detergents he tested fluoresced strongly under 395 nanometre light, and two did not.4
The white of a laundered shirt is partly a light source, and it is only as white as the light that reaches it.
References
- Siegrist, A. E., Eckhardt, C., Kaschig, J., & Schmidt, E. (2012). Optical brighteners. In Ullmann's Encyclopedia of Industrial Chemistry (7th ed., Vol. 25). Wiley-VCH. doi:10.1002/14356007.a18_153 Link Peer-reviewed reference work (industrial chemistry) Full chapter paywalled (Wiley). Claims taken from the abstract as indexed by the US EPA HERO database.
- HERA (2004). Targeted Risk Assessment of FWA-1, disodium 4,4'-bis[(4-anilino-6-morpholino-1,3,5-triazin-2-yl)amino]stilbene-2,2'-disulphonate. Draft, October 2004. Human and Environmental Risk Assessment on ingredients of household cleaning products. Link Industry risk assessment (HERA, European household products) Free PDF. Prepared by Ciba Specialty Chemicals with data from Bayer, i.e. by manufacturers; the safety conclusion is reported as HERA's own finding.
- Fluorescent brighteners. (1990). The Alkaline Paper Advocate, 3(6). Conservation OnLine (CoOL). Link Professional publication (paper conservation) Free to read. No author named; information credited to John A. Burt of Mobay Corporation. Krais's 1929 paper was not read.
- Holben, J. (2025, January). Detergents, fluorescence and photography. American Cinematographer, Shot Craft column. Link Trade technical publication (cinematography) Free to read. The detergent test is the author's own informal comparison.
Sources last checked 18 October 2026.
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