Noun
English Wikipedia has an article on:photosensitizerWikipedia
photosensitizer (plural photosensitizers)
(chemistry) A molecular or atomic species that initiates a photochemical reaction
Briefly, the principle of photochemiluminescence involves generating defined free radicals—superoxide anion radicals, in this case—by exposing a photosensitizer to a UV light source. Source: Internet
In this case, the photosensitizer is applied as a topical cream to the cancerous lesion and then exposed to the proper light wavelength from the surface. Source: Internet