Keyword recalling from a word in GP comment: photon downconversion is relatively easy and used widely, used in LEDs already, but upconversion require more exoti

Keyword recalling from a word in GP comment: photon downconversion is relatively easy and used widely, used in LEDs already, but upconversion require more exotic compounds and their efficiency is magnitude worse. Possible but not yet useful. It's certainly not like just using Lithium instead of Neon.