An overhead projector typically consists of a large box containing a very bright lamp and a fan to cool it on top of which is a large lens that collimates the light. Above the box, typically on a long arm, is a mirror and lens that focuses and redirects the light forward instead of up. Transparencies are placed on top of the lens for display. The light from the lamp travels through the transparency and into the mirror where it is shone forward onto a screen for display. The mirror allows both the presenter and the audience to see the image at the same time, the presenter looking down at the transparency as if writing, the audience looking forward at the screen. The height of the mirror can be adjusted, to both focus the image and to make the image larger or smaller depending on how close the projector is to the screen.
It's interesting to see a post regarding overheads, they have probably been used by everyone at one point but are rarely talked about. It's even more interesting to see the picture that you guys posted with this post. When I think of overheads I instantly think of the archaic boxes that used to exist in every classroom. I remember thinking when I was in grade school that these things have probably been here for 30 years and would last 30 more years. I used to be amazed by overheads and always got confused when I was trying to project something on them. I can always remember turning the transparent piece of paper the wrong way. This technology is getting phased out in college class rooms at least, the next big thing seems to be the smart boards.
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