An open duckbill-type projection head is recommended for most applications. Closed head units are worth considering for school or industrial use. An ultra-lightweight, reflective portable is recommended for remote usage if you wish to project only transparencies. In reflective projectors, the light source is positioned in the projector head. LCD panel projections require the more traditional transmissive overhead, with its light source positioned in the base under the projection stage.
Transmissive overheads are also available in portable format.
The number of lens elements will determine picture crispness. Most basic applications require only a single-element lens.