Photoacids are molecules which become more acidic upon absorption of light. Either the light causes a photodissociation to produce a strong acid or the light causes photoassociation (such as a ring forming reaction) that leads to an increased acidity and dissociation of a proton.
There are two main types of molecules that release protons upon illumination: photoacid generators (PAGs) and photoacids (PAHs). PAGs undergo proton photodissociation irreversibly, while PAHs are molecules that undergo proton photodissociation and thermal reassociation. In this latter case, the excited state is strongly acidic, but reversible.