Hmm, i noticed, with my 400w GE MV lamp, idk why, but mnercury starts building up in the arctube very fast, and also, my MV lamp dosent brighten all the way it seems. looks dimmer than the 400w HPS or MH. hmm. is the arctube leaking? or what? (btw, i think when it reaches full brighness, it looks 175w instead of 400...

Is the arctube clean?
Is it new or used lamp?
MV end-of-life (so the time when they should be relamped) is not, when the lamp fail completely (or does weird things like cycling or explosions), but when it's lumen output fall below specified level (usually 70% of nominal value).

Then not forgot, MV has lower (~30..40 clean, ~50..60 /DX) efficacy then both MH (60..95, depend on technology and wattage) and HPS (~150 for 400W).
Even if in practical use there are debates about visibility per given wattage, when comparing lamp brightness in close proximity to the lamp, lumens do represent the "impression" very well.

Might be that it IS dimmer than 400w HPS or MV. it looks new. and there are some white lines on the arctube, dont think it is bad though. it mught be working perfectly. hmm.

just wondering.