Can I use a Halogen lamp? Yes, with caveats. Halogen lamps are less efficient than MH lamps, and the ones with high enough wattage tend to have very long fillaments. A long filament is sub-optimal for use in a projector, since we want the light source to be as small as possible. The smaller, the better. Halogen lamps for overhead projectors should be acceptable. Halogens usually produce an unacceptable level of heat for the required light output.

Also, in the sheer amount of light, most places peg halogen at about 25 lumens/watt at best, more typically about 20. Many Metal Halide lamps are rated near 100 lumens/watt, but some are closer to 75. This means that you need at least 3 times the wattage from a halogen light to equal the light output of a metal halide. A 150W metal halide lamp will produce more light than a 400W halogen, yet produce less than a third of the heat.