TL;DR from Wikipedia: In photon upconversion, two or more incident photons of relatively low energy are absorbed and converted into one emitted photon with higher energy.
Basically photons are combined into a photon that is nearer in wavelength to visible light.
TL;DR from Wikipedia: In photon upconversion, two or more incident photons of relatively low energy are absorbed and converted into one emitted photon with higher energy.
Basically photons are combined into a photon that is nearer in wavelength to visible light.