The basic idea is that PAR is all the light energy between 400nm and 700nm. PUR (Photosynthetically Usable Radiation) looks at light energy over the same wavelengths, but only the spectrum where chlorophyll utilizes the energy. The best example is the green spectrum. Green is not very well utilized by chlorophyll - so if you had a light that only produced green light you could have very high PAR values, but very little PUR because the light can't be used for making energy by the chlorophyll very well.
However while something like a green wavelength may not do much for the coral, it can do wonders for our viewing pleasure. For this reason I think lime LEDs are great. You can turn them off to measure intensity for light and coral placement, then turn them on and adjust them for the desired brightness/color you want for just your viewing pleasure. No they don't do anything for the coral, but I think this is one of the next steps for LED fixtures - we can provide plenty of PUR now with LEDs, lets focus on getting complete coverage and then being able to dial the brightness and color to what looks pleasing to our eyes.
