The photon upconversion is the higher energy photon generation with lower energy photon absorption. Apparently, upconverted photon generation through several complex photophysical pathways is not quite simple. In this dissertation, photophysics of upconversion with two perspectives will be presented. The first perspective is elucidation the mechanism of upconversion luminescence, that is, the photophysical pathways. In addition, the second one is the effect of crystal site-symmetry on upconversion. In particular, this dissertation focuses on a specific lanthanide, erbium. Erbium has various 4f energy levels and can emit green and red photons in the visible region. With focused scope of visible emission from erbium, evaluation of local site-symmetry effect and suggestions of mechanisms in upconversion are presented.