Low-priced halogen lamps are normally sufficient for lectures in rooms with up to 30 people. Metal halide lamps are recommended for a larger audience, or if the reproduction of true colours is of special importance. Ohps with metal halide lamps bring more contrast to the image and, due to the spot-like light source, an even better picture sharpness.
OHPs are daylight projectors which can be used without darkening the lecture room. In order to reach a good projection quality, a minimum light output on the screen is needed.
The recommended light output of the projector, measured in lumen for a small room (1m x 1m picture) is 1000 lumen, for a classroom (2m x 2m picture) would be 3500 lumen.
If the lamp dies during a presentation or teaching session this would be inconvenient and disruptive. Automatic lamp changer will switch to the second lamp without any interruption of your lecture.