So, you've got a 2000 lumen OHP and you put your LCD on it, and are getting 150 lumens. Why?

Well, LCDs simply cannopt pass a lot of light.

First off, LCDs need polarised light. This is how they work, and you can't do it without. So you put a polarising filter on the light. Light that is in the correct orientation passes, light that isn't in the correct orientation doesn't. Given a completely random input, this means that about 50% of your lamp's light stops right there.

Next, unless you're completely colourblind, you probably want a colour image. That means that the light has to be filtered into it's component red, green and blue parts. With an ideal RGB light source, each of these filters would stop 67% of the remaining light, but since we don't have an ideal RGB light source, it's going to be closer to 75%

So we lose half at the polariser, and three quarters of what's left at the colour filter.
0.5 * 0.25 = 0.125.

This leaves us with a maximum of about 12.5% of the light, with absolutely ideal conditions.

We'll lose another 10% or so of what's left (Call it 1.5% just 'cause) becasue we'll lose some to the glass and liquid crystal. So that's a maximum of 11% left.

Now we have to realise that the colour filter and LCD cells don't cover 100% of the LCD, I'd say that we lose at least another 15% to these black bits in between the sub-pixels.

What's left is about 9.5% of the light that hits the LCD leaving the other side of it (Assuming that the liquid crystal is de-energised to make the light leaving it as white as possible.) This is assuming htat you have an "ideal" LCD, with absolutely nothing blocking more light than it should.

The best LCD that I know of was measured with about 9% of the light that hit it getting passed through to the screen, which is in line with this analysis.