One of the most commonly used projectors is the overhead projector. This system is an ideal tool to display projected images to your audience. Portable overhead projectors were often used as teaching devices in schools and as presentation tools for businesses. Overhead projectors generally consist of a large box which houses a bright lamp with a fan to cool it. On the top it has a large lens. On a long arm above the box, there is a mirror that shines the light forward instead of upwards.
To display your image, you simply place transparencies on top of the lens. The light from the lamp then shines though the transparency and into the mirror. From here it is projected forward onto a display screen. The presenter and the audience can view the image together because of the mirror. The presenter can see it by looking down at the transparency and the audience can see it by looking at the projection screen.
These projectors are still available, but they aren't used too often because of new computerized and digital technology. The projectors can range in price from about $100 to $500. They are available in various shapes, brands, sizes and styles.