Health & Environmental Research Online (HERO)


Print Feedback Export to File
4854779 
Book/Book Chapter 
Encyclopedia 
Optical brighteners. 
Siegrist, AE; Eckhardt, C; Kaschig, J; Schmidt, E 
2012 
Yes 
Wiley-VH Verlag GmbH & Co. 
Hoboken, NJ 
Ullmann's Encyclopedia of Industrial Chemistry 
25 
English 
is a chapter of 3827349 Ullmann's encyclopedia of industrial chemistry
Optical brighteners or, more adequately, fluorescent whitening agents (FWA) are colorless to weakly colored organic compounds that, in solution or applied to a substrate, absorb ultraviolet light (e.g., from daylight at ca. 300 – 430 nm) and reemit most of the absorbed energy as blue fluorescent light between ca. 400 and 500 nm.

In daylight optical brighteners can thus compensate for the aesthetically undesirable yellowish cast found in white industrial substrates, such as textiles, papers, or plastics. Furthermore, since a portion of the daylight spectrum not perceived by the eye is converted to visible light, the brightness of the material is enhanced to give a dazzling white.

Figure 1 illustrates the action of optical brighteners 1-3. White substrates show a yellowish cast because their reflectance for incident visible light is lower at short wavelengths than at long ones (curve a), i.e., they absorb short‐wavelength light.

The first method of compensating for the yellowish cast was the use of a blue dye such as ultramarine or indigo. Bluing lowers the reflectance of the sample in the long‐wavelength part of the visible spectrum (curve b). As a result, the sample takes on a neutral white appearance, but at the same time it loses brightness so that it looks grayer.

Optical brighteners lower the reflectance mainly in the UV and near‐visible by absorption (curve c1); at visible wavelengths (mostly with a maximum at 435 – 440 nm), they greatly increase the reflectance through fluorescence (curve c2). The brightener acts as a supplementary emission source.

Optical brighteners are more effective the cleaner and whiter the substrate is. In no case can they replace cleaning or bleaching. 
7th 
Elvers, B 
9783527329434