Page "Enzyme assay" Paragraph 22

Fluorescence is when a molecule emits light of one wavelength after absorbing light of a different wavelength.

Fluorometric assays use a difference in the fluorescence of substrate from product to measure the enzyme reaction.

These assays are in general much more sensitive than spectrophotometric assays, but can suffer from interference caused by impurities and the instability of many fluorescent compounds when exposed to light.