Eugene is correct. I showed those charts to show how the bulbs we choose for reef applications spike in the same wavelengths as the most efficient ones for coral photosynthesis.
PAR meters measure all light between 400nm and 700nm. However, 500 to 600nm light does not produce as much photosynthesis as light around 450nm or 650nm to 700 nm. So while overall PAR may seem to stay the same or drop somewhat overall PUR (Photosynthetically Usable Radiation as shown in some of the articles you linked to) may drop more than PAR since the decrease in blue is probably more than the decrease in the green to red part of the spectrum. So we should probably look more at PUR than PAR but few people have spectrophotometers that can do the more complex analysis. What is intersting is that if you measure PAR of a cool white LED and compare to a blue LED, they put out very similar PAR numbers. I would be really interested to see what a PAR meter measures for a green LED of similar wattage. I suspect that PAR would be very close to the others but that's not PUR and corals would not do well under pure green LEDs.
MH and florescent bulbs degrade relatively quickly (within 1 to possibly 2 years with the better bulbs) such that PUR decreases as the overall spectral output changes (or shifts depending on your frame of reference) to be more warm. It is fairy well accepted that nuisance algae grow better at warmer colour temperatures and old bulbs can promote their growth, as I have seen first hand with my old T5 bulbs. If things did not change in that way why change bulbs so often? So to summarize, blue decreases a fair bit but green to red increases somewhat so the loss of blue in the PAR is offset somewhat by the increase in green and red. But that does not mean PUR stays the same, it will decrease and the corals will not be photosynthesizing as efficiently.
I still contend that LEDs have a great advantage. A 30% decrease in 50,000 hours means that with a 10 hour a day lighting schedule you could theoretically get 13.5 years of use and PUR will drop 30% in that time. But the CCT will stay the same and you should not have problems with nuisance algae as the LEDs degrade. Realistically, how often do you change your MH bulbs and what do they cost each time? My T5s would realistically last no more than a year and replacement costs were $250 to $300. Let's say the LEDs last 10 years then I am saving $2500 to $3000 in bulb costs. Then add the electricity savings as well as the reduced likelihood of needing a chiller (more an issue with MH than T5) along with the stability of the CCT and LEDs look pretty good.
This whole debate reminds me of the T5s vs. Metal Halide debate. There was great resistance to T5s with people strongly believing that you could not have a thriving SPS tank under T5s and that T5s were inferior to MH (heck, that debate is probably still ongoing). But we now know that is not the case. The same accusations are being leveled at LEDs but I do believe that time will show that LEDs are a very good option for lighting reef tanks.
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