What are PAR, PPF, PPFD and CRI?

PAR, short for Photosynthetic Active Radiation, refers to the spectrum between 400-700nm that plants use for photosynthesis. PPF, a.k.a. Photosynthetic Photon Flux, is the total micromoles of photons (400-700nm) an artificial light generates per second. PPFD, short for Photosynthetic Photon Flux Density, refers to the total amount of photons (400-700nm) an artificial light generates that hit an area every second. CRI, or Color Rendering Index, is the ability of a light source that reveal the colors of various objects faithfully in comparison with an ideal or natural light source.