I have a question about where should the measurement be made with respect to the lamp arc position. It's lumenlab lamp, so it's relatively big. The question is when I measure the distance of this lamp arc and the reflector, is it right in the middle of the arc, or should be on the edge. I would assume it's right at the center. The same question for the distance between this and the condenser lens. Is it from the center of the arc, and the center of the condenser lens? The condenser lens is thick, so if measure to the center of the flat surface is not the same as half way between the flat and the curve surface.

Most references will (should) specify measurements to the "plano" side of the condenser (the flat side). As for the arc - use the geometric centerpoint of the 3 dimensional arc tube as your reference point. Note that a lamp used in a horizontal burn position may have the arc slightly displaced from the geometric centerpoint of the arc chamber but I'd guess that amount of slop is tolerable. In the end you'll find the real sweet spot empirically - it doesn't normally wind up where theory dictates.

Thanks for the reply. One of my concern was that I use the LL reflector and it touches the LL lamp. Someone said it almost touch the lamp. I wonder if adding a gap to this would resulted in sharper images. The question is how much gap. One way is to experiment. I would assume that I would get better image by adjusting this, without adjusting the triplet for example, so the adjustment is relatively easy. If moving this, and the triplet has to move, it would be much harder to adjust.

I think the theoretical perfect gap is like 1 millimeter between outer shell of lamp and rim of reflector which I don't believe will make a very significant difference in image quality. There are too many other deviations from theoretical perfection to make such a small "error" in lamp arc to reflector alignment super-critical. Just my opinion. I won't sweat over 1 millimeter "error" in my projector's lamp/reflector alignment. Others may argue.