Read up on PAR vs PUR (available vs usable). PAR is radiation from 400-700. PUR is a subset of PAR. In a LED fixture with too many white LEDs putting off damaging green/yellow spectrum, the PAR might be very high, but the PUR is terrible and your SPS can hate you or die. Each coral will want a different PUR, so this is very hard to measure.
Spectrum is more important that PAR. By reducing the PAR you are likely reducing the good, as well as the bad since the didoes put out both.
Neither PAR, or the subset PUR, track true UV or IR, which MH and flourescent do emit in varying quantities. Little is know if these have anything to do with coral, but it is totally possible that some deeper SPS absorb the UV and emit a less active spectrum which adds to the color - totally conjecture, but why do we suppose that coral stops using light at 400 just because human eyes cannot see it?
Spectrum is more important that PAR. By reducing the PAR you are likely reducing the good, as well as the bad since the didoes put out both.
Neither PAR, or the subset PUR, track true UV or IR, which MH and flourescent do emit in varying quantities. Little is know if these have anything to do with coral, but it is totally possible that some deeper SPS absorb the UV and emit a less active spectrum which adds to the color - totally conjecture, but why do we suppose that coral stops using light at 400 just because human eyes cannot see it?