Photosynthetic photon flux density (PPFD) The number of photons per unit time and per unit area in the wavelength band 400 to 700 nm Wikipedia https://en.wikipedia.org/wiki/Photosynthetically_active_radiation Added to support Critical Zone Observatory (CZO) data use cases. ODM2 Variable Name Controlled Vocabulary ODM2 Working Group A vocabulary for describing the name of Variables.