Photon upconversion is the sequential absorption of two or more long wavelength photons leading to the emission of a photon with a shorter wavelength. The fact that the emission is at a shorter wavelength than the excitation light makes upconversion distinct from other forms of photoluminescence such as fluorescence and phosphorescence where the emission always occurs at a longer wavelength (see Stokes Shift), and upconversion luminescence is therefore also known as anti-Stokes fluorescence.