Buying An Overhead Projector--Heads?

[W. Watson]

I'm thinking about buying a projector. I see the head suspended above the glass plate can be at least of two varieties. One seems like a simple mirror and the other seems like a headlight enclosure (called a closed head). Can anyone give me some idea what that's about? For example, look at ,

and .

Also see the 3M 1711, 1720 and 1730 on . Doublets? Open/closed heads?

[default]

They do pretty much the same thing. The mirror is lighter, cheaper to build, and probably still under patent.