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.
Took a quick look at the free tier,
That seems pretty reasonable to me. Main account and two accounts to share. With just friends and family, I doubt most people will reach the 100 device limit.