The Over-Head Projector is used for:

Concept — Projected teaching aids are grouped by the way the image reaches the screen. A machine that projects by transmitted light needs the image carried on a clear base, so the beam can travel through the material itself; a machine that projects by reflected light needs an opaque surface for the beam to bounce off; and an electronically generated display needs a signal source instead of any physical image placed on the machine.

Application — Applying that to the overhead projector, whose material rests on a horizontal glass stage:

1. A lamp and a Fresnel lens sit directly below the glass stage, so the light travels upward through whatever is laid on the stage.

2. A head assembly above the stage — an objective lens with a mirror set at 45° — turns that beam through a right angle and throws the enlarged image onto a screen behind the presenter.

3. Because the beam must pass through the material resting on the stage, that material has to transmit light rather than block it, which means a clear written or printed acetate sheet.

4. A marker-written or copier-printed acetate sheet of exactly this kind is what is called a transparency, so that is the medium the machine is built to display.

Cross-check — Cross-check against the other media by their own nature:

• Photographs and printed charts carry their image on opaque paper, so they are enlarged by reflected light in an epidiascope or opaque projector, not by a beam sent through them.

• A film reel needs an intermittent frame-advance mechanism and a shutter, while electronic video needs a decoder and a timed display refresh, to run frames in sequence; the overhead projector stage by contrast is static and holds one sheet at a time.

• A presentation file is stored data rather than a physical image, so it needs a computer driving an LCD or DLP digital projector.

Result — the Over-Head Projector is used for transparencies.