Hi, thanks for the reply. I know it's not of vital importance because we have a vast array of lighting to choose from in our hobby, and we can get perfectly good results with them.
My thinking is that chlorophyll a has a peak response at something like 430nm, but we are providing our tanks with light spectrum peaking in the blue portion of visible light - by the time it hits the corals in the water, some of this spectrum will have shifted into the ultra violet portion of light that is not strongly absorbed by water. There has been a lot of research into the affects of ultra violet on xoozanthellae, and it may be beneficial at low intensity.....but I was considering a trial at home making my own led set up which shifts the peak output to somewhere around 580-600nm. Bearing in mind the refractive index of our tank water could be roughly taken to be 1.33, our corals would actually be receiving a spectrum of light peaking somewhere around 450nm.
The reason I asked the question is that I have probably missed something really obvious about why our lights are designed as they are, but it seems like they are optimised for plants grown out of water. Is it just that Cree leds with a certain output are commonly used, and this is what's available to manufacturers, and this is good enough?
If I ever get round to contacting any light manufacturers I will post any responses if anyone is interested.