Biofluorescence is fluorescence exhibited by a living organism: part of the organism absorbs light or other radiation at one wavelength and emits visible light at another, usually longer. The absorbed radiation is often blue or ultraviolet, while the light emitted is typically green, red, or anything in between. Biofluorescence requires an external light source and a fluorescent biomolecular substance, which is often one or more proteins, but can consist of other biomolecules.