Four issues have not been addressed yet:
1) Relationship of light out-put and distance of fixture to tank. If you want homogenous light from the top to the bottom of the tank, then the light source has to be further away from the tank. Remember inverse square law of light attenuation with distance. But, with greater distance you also lower overall intensity. This is a typical trade-off situation.
2) Color rendition index, i.e., how well does the artificial light match the natural sun-daylight. CRI of 100 is perfect match, regular cool white T12 is around CRI of 60, full spectrum HO fluorescent can be up to about CRI of 90, some MH are up to CRI94-96.
3) How much light do you need? Usually light requirements on culture sheets are for noon-midday, so peak intensity. But morning and evening light is lower. Artificial light is fully on or entirely off. Converting the natural light fluctuation to a square intensity function (i.e., squaring the "circle", though a parabola is the better approximation), roughly half of the culture-sheet requirement is about right for artificial light intensity.
4) What about LED's? they are up and coming in the coral reef community, and corals need A LOT of light.
I use on a 5'L x 2'H x 1'D 4x 80W 60" HO T5s. Seems to be enough, even with having all four on only for 3h a day. I do not grow very high light intensity plants (vandas etc.). If I would be in the market for a new light, I would very much look at LEDs. Don't know about their CRI.
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