A process where Light can be emitted with photon energies higher than those of the light generating the excitation.

When a medium (e.g. a Laser Gain medium) emits fluorescence as a consequence of being excited with incident light, the Wavelength of the fluorescence is usually longer than that of the exciting light. This means that the photon Energy is reduced. However, under some circumstances upconversion fluorescence can occur, where the wavelength of the emitted light is shorter. This is possible via excitation mechanisms which involve more than one absorbed photon per emitted photon.