Re: Orphek PR-156 LED Units - The Salty Box Reviews
Bascially;
PAR = Photosynthetically Available Radiation
PUR = Photosynthetically Usable Radiation
PUR is a reading of light that a particular species of coral can use to grow. PAR is light that could be used by one of many species that photosynthesise from light (not necessarily limited to corals here, could include plants e.t.c. also) (disclaimer, if I've understood correctly, I'm sure Nick will correct me if I've got the wrong end of the stick)
All the best
Rabbut
"When the facts change, I change my mind" - John Maynard Keynes