Mr. Wilson said:
The basic rule of thumb is to have at least 200 PAR on the substrate. SPS will do well in the top 2/3 of the tank and LPS & soft corals will dominate the lower 1/3. Tanks that are deeper than 3' (1 metre) will have very high PAR for the first 12" but by the time the light reaches the reef structure it will be reduced to a safe intensity.
LED fixtures with focused optics (lower than 90˚) may cause corals to burn. Some LED manufacturers use optics as narrow as 40˚ in order to score high in PAR tests. The trade off of narrow optics is spotlighting, poor coverage, and burnt corals. Raising the light will spread the light better, but at the cost of PAR.
The LED fixtures over Peter's tank have 90˚ optics. They also offer 120˚ lenses for tanks that are 24" or shorter. You really need to look at spectral graphs, CRI and colour temperature in order to establish the PUR (photosynthetically usable radiation) value. PAR is less important as it is only a measurement of photosynthetically available radiation.
Mr. Wilson, I moved over here so as not to get off topic on Chinchai's thread. You answered my question on best to try for a PAR that you Want at mid tank, and let the top and bottom adjust. My question is my tank is 42"d. top of sand to top of water + 9" more to top of hood. I would think I would need optics to hit say 300/400 PAR mid tank ? What optics would you suggest with my tank? Also I would assume I would need to put the leds closer together when using optics? What should the spacing be? One more, my tank is 96x35x42d. How many leds do you suggest I use? I'm thinking Cree XP-G's CW. and XR-E's RB. with ELN-60-48P power. Any input will surly be appreciated! Bond