Fluorescence is the emission of light by a molecule or an atom that has absorbed light or other electromagnetic radiation. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, than the absorbed radiation. A perceptible example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the electromagnetic spectrum (invisible to the human eye), while the emitted light is in the visible region; this gives the fluorescent substance a distinct color that can only be seen when the substance has been exposed to UV light.
Biofluorescence is fluorescence emitted by a living organism. Biofluorescence requires an external light source and a biomolecular substance that converts absorbed light into a new one. The fluorescent substance absorbs light at one wavelength, often blue or UV, and emits at another, longer wavelength, green, red, or anything in between. In a living organism, the fluorescent agent often is a protein (or several), but it could be other biomolecules as well.
Since biofluorescence was discovered in Aequorea victoria and the green fluorescent protein structure was resolved, many other organisms have been shown to exhibit biofluorescence and many new fluorescent proteins have been discovered.[1][2][3]