[Delphinus]

Fantastic info, DJ88.

Just to add a few points. PAR is "photosynthetically active radiation." These are the wavelengths of light, that provoke the most photosynthesis activity in chlorophyll. The more PAR you have, the more photosynthesis is possible.

We measure PAR in units of PPFD (photo-something flux density). Sorry, don't really remember what both P's are for offhand.

So, more PAR == more growth of corals.

But PAR isn't the whole story. You don't really get the spectacular colouration with PAR. (Unless you consider "brown" spectacular :D ). To get really cool colours, you need to have so much intensity of light, that the coral grows pigments to reflect away unnecessary light. What people are now finding that the more blue light there is, the better colour they have on their corals.

So, what to make of this? If your goal is "growth but don't care about colour" then go for a high PAR bulb. If your goals is "colour but not necessarily growth" then go for the blue bulbs. If your goal is "both growth AND colour" then you need a mixture of both bulbs.

We are now finding out that if run on certain ballasts, the Radium 20000K's bulbs in the 400W size give both a decent PAR (or PPFD) output, and a nice blue light for colouration.

What a lot of people have misinterpreted in the MFA2002 article by Sanjay is that they now think a 150W DE bulb gives better PPFD than a 400W bulb. The only point Sanjay was trying to make with this finding is that the efficiency of a bulb can be greatly improved with a proper reflector. You cannot compare a 150W bulb to a 400W bulb. The intensity and spread will be completely different.