Fluorescent brighteners

One other feature of detergents which can affect colour is worth a mention. Everyone today is familiar with the 'whiter than white' claim made for these products. This effect is produced by the addition to the detergent of a small amount of a substance known as a fluorescent brightener. This is a colourless compound, usually with some affinity for the substrate, whose molecules are fluorescent; that is to say, they absorb energy at a high level from ultraviolet radiation and immediately re-emit it at a lower energy level, usually in the form of blue light. This light, when contained in the eye with the trace of yellowness normally present in a white material, produces a sensation of complete whiteness – that is absence of colour sensation, because, as will be remembered, a sensation of yellow is caused when blue light is removed from the spectrum. Previously, the yellowish tinge of whites was neutralised to some extent by the use of the old-fashioned ‘blue bag’, which contained a reddish blue dye. This dye, combined with a trace of yellow, will give a very pale grey, which is less noticeable than yellow.