Simple projection systems require light to pass through an image stored on a transparent film, then through a magnifying lens system, and finally projected onto a screen for viewing.
As systems become more complex, the light from the lamp may pass through a series of lenses before reaching the film. These lenses – or condensers – collect and direct the light so that as much as possible passes through the whole area of the film, then onto the magnifying lens. Reflectors behind the lamp also ensures that all light from the lamp is directed toward the film.
How it works
The overhead projector is very simple to operate and maintain. A light source is directed through a piece of transparent acetate (the transparency or film) or a liquid crystal display (LCD) panel, and the image is then projected onto a screen. An LCD panel can be used for projecting anything on a computer screen – e.g. text, data, graphics and animation.
If the light source is a quartz halogen lamp (see diagram below), it may either have a separate concave reflector or the reflector may be part of the lamp. The light is directed to a Fresnel lens which condenses the light and directs it through a glass plate or stage on which rests the film or LCD panel. The light shines through the image and is collected by lenses at the head assembly. Some overhead projectors have the mirror needed to project the image as a separate part of the head assembly. Others have the mirror as an integral component of the projection lenses in the head assembly. Several methods are used for focusing. Whatever method is used, there will be a focus knob. Some overhead projectors are focused by moving the head assembly up and down the post, others by moving the post up and down, and still others by rotating the projection lens in its carrier.
Because of the heat output of the lamp an overhead projector usually has a fan assembly. Some fans turn on and off with the lamp switch, others are turned off by a thermostatically controlled switch. To prolong lamp life, allow the fan to cool the lamp.
Many overhead projectors have a safety device to stop the power to the unit when the lid is open. When the lid is open (or even ajar) the lid switch is disengaged. The lamp does not work and the fan does not run. When the lid is properly shut, the lamp and fan both operate.
Examine an overhead projector, and identify the following parts. Your overhead may not have all these parts or controls.
- lamp on/off switch
- fan on/off switch
- Fresnel lens
- mirror (may be part of the head assembly)
- projection lens(may be part of the head assembly)
- head assembly
- focus knob
- lid safety shut-off switch
Regardless of the model, here are some principles to follow when setting up an overhead projector (OHP).
- Place the OHP on a table, stand or trolley where it will not be bumped or knocked.
- Place the OHP so that it faces the screen. The presenter will face the audience. Make sure that the OHP does not block the presenter’s view of the audience.
- Plug the unit into the power point, using an extension cord if you need to.
- Make sure that the power cord and extension cord are out of the way of people stepping over them. If you cannot run the power cord around the edge of the room, either tape it to the floor or cover it with a mat.
- Turn the unit on at the controls. Some OHPs have separate controls for the fan and the lamp.
- Before the audience arrives, test the OHP to ensure that the lamp is working.
- Check the projected image in all areas where the audience will sit. If the projected image is wider at the top than at the bottom (vertical keystoning), either raise the OHP to reduce the angle of the head assembly to the screen, or tilt the screen forward. If the projected image is wider at one side than the other (horizontal keystoning), move the OHP so that it squarely in front of the screen.
- Adjust the focus so that all areas of the projected image are focused.
- Turn off the lamp.
- After use loosely coil the power cord. Many OHPs have hooks for you to wind the power cord around.
- Keep the OHP covered when it is not in use to keep it free from dust.
Care and maintenance
- Do not place the projector in a place where sunlight may shine on it directly, as the heat may warp the Fresnel lens.
- Never carry the projector by the projector arm or post.
- Keep all components in the light path clean and free from dust. Clean using a lint-free cloth and gentle window cleaner.
- Ensure that the Fresnel lens is installed the right way up. There will be an indication on the lens as to which way to install it. If it is not installed correctly the light will be concentrated through the centre of the image but not around the edges. You will see projected a bright circle on a dark background.
- All parts in the light path must be inspected and replaced where necessary – e.g. a scratched lens must be replaced to obtain the best projected image.
- Ensure that the lamp works. Replace the main lamp and spare according to the manufacturer’s guidelines and your organization’s policy. Take care not to touch lamps with bare fingers. Many organizations require that a spare lamp is always available with equipment. Others have a policy of not keeping spares with equipments to reduce pilfering or incorrect replacement if the equipment is available to the public.
- Inspect the power cord and plug for electrical safety.
- Keep the fan assembly free from dust by dislodging dust with a small brush and vacuuming inside the cabinet.
- Do not move the projector while the lamp is still hot. Allow the fan to cool the unit first.
TroubleshootingComplete the following table, suggesting possible solutions to operating problems. You may find it useful to refer to a user manual.
|No light after turning power on|
Plug projector in
|Dark edge with light in the centre of the projected image||Place Fresnel lens correctly|
|Dark spot on screen||Clean dirty compartments|
Check for warped Fresnel lens
|Inability to focus entire image||Check for warped Fresnel lens|
Ensure focus knob is operational
Check for broken projector arm
|Not enough light on screen||Clean Fresnel lens/lamp glass|
Replace old/broken lamp