Optically active additive

Optically active additive (OAA) is an organic or inorganic material which, when added to a coating, makes that coating react to ultraviolet light. This effect enables quick, non-invasive inspection of very large coated areas during the application process allowing the coating inspector to identify and concentrate on defective areas, thus reducing inspection time while assuring the probability of good application and coverage.

Source: Wikipedia — Optically active additive (CC BY-SA 4.0)