I think this is the first projector lamp I've actually catalogued in my collection - though I know that there are another couple in there.  Though I'm meaning slide projector here, as I know there's one in this section from an overhead transparency projector as well.

This really does show that lamps come in all shapes and sizes, this one appearing more like something The Doctor would pull out from behind a panel in the TARDIS than a light bulb...fact of the matter is, is that it's a very specialised lamp.

Very early film projectors used carbon rods to produce their light, which while very bright required continual attention, made a horrendous amount of noise, used even more horrendous amounts of power, and produced enough UV to cook anyone nearby who wasn't protected against it.  Needless to say, when filament lamps capable of filling the same role, then followed by discharge lamps, these were innovations very quickly picked up.  It's only since 2004 or so that relatively affordable (LCD) projectors have started to appear in the home, using ultra-high-pressure mercury discharge lamps to produce their light.  Before that, the lamp in a home based projector would almost exclusively have been incandescent.

You couldn't just pick a high wattage incandescent up and stuff it in front of a mirror though.  Projectors need a very precisely focused light source to "inject" the light correctly into their optical system, otherwise you'll end up with one part of the image brighter than another, or just no light at all being apparent on the screen.  Higher wattage lamps (generally for movie projectors) tended to use lamps with a carefully designed filament with a very tightly packed zigzag shape to present a square uniform area of light.  Smaller lamps used a bit more varied methods though, such as this lamp which uses a very small filament for the wattage and a specially designed, highly reflective reflector to deliver a precisely focused beam of light into the projectors optical system.  Of course, having the reflector built into the lamp means that a nice new, clean one is installed with each new lamp.  Especially important in an application such as this where one stray fingerprint could be a real problem.  The key in the base on this lamp would align with one in the socket for the lamp, meaning that it will always slot in in the correct position for operation.

This is a lovely made piece of lighting technology.  Everything about it is very precise, and the reflector is very, very highly polished indeed.