While in chemical bleaching the impurities are oxidized or reduced to colorless products, optical brighteners that are used in physical bleaching involve introduction of a complementary colour whereby the undesired colour is made invisible to the eye in an optical manner.

Optical brighteners absorb the invisible uv portion of the daylight spectrum and convert this energy into fluorescent blue to blue-violet light. This phenomenon is used to compensates for the absobtion of the short-wavelength light by the textile fibers, which results in yellow cast to fabrics. The end result is attaining a neutral, or even complete white appearance.

Through the use of excess optical brightener more uv radiation can be converted into visible light so that the whiteness is made more sparkling. Since the fluorescent light of a optical brightener is blue-to-violet, excessive use gives either a blue-to-violet or a bluish green cast.