Does this mean that the AB bulb will appear bluer than the XM? Or that the true color emitted from this supposed 10K bulb is really closer to 7K on the XM and 8.2K on the AB bulb?
Don't think either bulb will be "blue" at most you might get a white out of it. But the CCT is a mathematical formula that calculates the closest color temp to what spectrum is given, now with spikes in the blue range (mid 400's) that tends to pull that color temp closer to the higher temp range. The fact the XM10k has a 7026 CCT definately shows a more yellow look to it.
The reason the bluer bulbs (14kK+ sometimes even 12kK) show zero is because there is no accurate color temp for it since it's mostly in the blue.
At what point does the usuable light (PPFD/PAR) provided by a bulb exceed the amount a coral can take in? To put it another way, what is the minimum required PPFD score for corals (SPS) to grow and maintain colors?
Well I think its safe to say that you'd be hard pressed to give TOO much light... atleast if the coral is already used to it, there are many deeper water corals that don't like bright lights, not to mention a coral that's been under 20kK bulbs (great for nice coloring and decieving potential buyers, but horrid light energy) can easily be bleached to death if you blast it with too much light.
here's a chart from Sanjay showing sunlight
as you can see even at 10 meters down the PPFD is over 600, now compare that to what those bulbs put out and you'll see that you should be fine as far as not putting too much light.
End result though is that corals can accept a huge range of lighting conditions, some might keep corals under 400w bulbs, some might keep the exact same corals under 175w bulbs with success. The question is do you want to grow them as fast as possible? if so go with a yellower bulbs. Or do you want to enjoy them from an asthetics standpoint primarily (then go with bluer bulbs).
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