As someone who has worked with optics for projectors, I can tell you
that it won't work. You need an immense amount of light intensity and it
has to be focused just so (if you've ever fired up the projection lamp
with the housing open, you will know just how bright it is - you can't
see anything else for a few minutes). Many man hours of research go into
producing efficient optics for the projection beam. The lamps are
discharge (arc) types and have control gear in the projector - you can't
just replace them with any old lamp.

What you can sometimes do is buy the lamp itself separately, as the
manufacturer replacement often includes the reflector and housing. This
can cut down the cost considerably. Find the manufacturer code from the
lamp itself and google for it.

Hmmmm, that bodes badly then.

The bulb and reflector come as a single unit (see links below) so I
imagine the majority of the cost is in the bulb. I can't help getting
the feeling though that I'm being massively ripped off and its the
housing not the bulb that is expensive. I wouldn't be at all surprised
to find that practically every Philips projector uses this exact same bulb.

As for the light being focused. Yep I agree that it is focused but, and
here's the interesting bit, there is obviously a reflector in the bulb
but there is also a lense in front of the bulb that diffuses the light.
I am wondering just how accurately I would have to mimic the real bulb
to get a good solution.

I think there might have been a slight misunderstanding as well. I
wasn't intending to just stick a low voltage halogen into the hole
occupied by the PJ bulb as I realise that wouldn't be even close to good
enough and wouldn't work anyway. I was thinking more like a dozen in an
internally mirrored and forced air cooled box that sits under the
projector with a mirror array to take the light into the PJ. Surely that
would be bright enough!

http://www.crazysquirrel.com/images/bulb/complete.jpg
http://www.crazysquirrel.com/images/bulb/label.jpg

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