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.

Upconversion occurs through the sequential absorption of photons and is usually considered distinct from two-photon absorption processes where the photons are absorbed simultaneously.
Upconversion has many important medical and technological applications such as improved biological imaging and sensing using upconversion nanoparticles,1 enhancing the light-harvesting of solar cells,2 and deeper tissue penetration in photodynamic therapy for cancer treatment,3 amongst others.

Sooooo many new things I am learing about...Scalar wave...question is will I have enough time to learn all these new things before I die...I hate to go ignorant...I love learning and feel robbed by the Cabal/DS....