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Originally Posted by MarkoD
Before any LEDs I had 4x250 mh and 4x 39 t5
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I was also running 250w MH and T5 before switching to the PS LED,...I started the PS fixture @ 50% and slowly worked my way up to the 90%...then I assumed the Radion was less W Than the PS and there should be no shock when switching the two LED fixtures as previously stated above...I was wrong... LED's are really new in this hobby and from what I have read, we are making a mistake by measuring LED fixtures with PAR....PAR undergrades the blue light spectrums and focus's more on the white/yellows...Higher end LED's used for reef lighting have no yellow as fould in traditional lighting (MH, T5, PC etc) thus measuring in PAR is old tech and not the correct tool for the job when measuring LED...this is also why less expensive 1W LED show more PAR, they have more yellow spectrum to measure yet this yellow is not usable light in reef corals...I think we'll see more about PUR (as oposed to PAR) as the LED fixtures become more common place in this hobby...
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PUR (Photosynthetically Usable Radiation) also known as "Useful Light Energy" is what concerns us as aquarium keepers even considerably more than PAR in providing correct lighting (yet provides a lot of confusion, especially when considering LED Lights).
PUR is that fraction of PAR that is absorbed by zooxanthellae photopigments thereby stimulating photosynthesis. As noted above, PUR are those wavelengths falling between 400-550nm and 620-740nm.
Generally a Spectrograph is used to rate PUR (these are much more expensive than a PAR meter, with the result of many falsely considering PAR and ignoring the more important PUR.)
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