Buying An Overhead Projector--Heads?

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 <(Amazon.com product link shortened)>, and <(Amazon.com product link shortened)01673401/sr=2-3/ref=pd_ka_b_2_3/102-1905724-8880112>.
Also see the 3M 1711, 1720 and 1730 on <http://mpsuperstore.com/computer/group173.htm . Doublets? Open/closed heads?
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Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
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They do pretty much the same thing. The mirror is lighter, cheaper to build, and probably still under patent.
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