Overhead Projector (OHP): Mechanism & Use in Education
A projector having a flat, transparent top on which a sheet like transparency is placed, and an overhead mirror that reflects the image on the transparency to a screen .A projector capable of projecting enlarged images of written or pictorial material onto a screen or wall from a transparency placed horizontally below the projector and lighted from underneath.
An overhead projector typically consists of a large box containing a very bright lamp and a fan to cool it. On top of the box is a large fresnel lens that collimates the light. Above the box, typically on a long arm, is a mirror and lens that focusses 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.
Focal Length Adjustment
Better-quality overhead projectors offer an adjustment wheel or screw on the body of the projector, to move the lamp towards or away from the fresnel lens. When the mirror above the lens is moved too high or too low, it moves out of the best focal distance for an evenly white image, resulting in a projected image with either blue or brown colour fringing around the outside edge of the screen. Turning the adjustment wheel moves the lamp to correct the focal distance and restores the all-white projected image.
The lamp technology of an overhead projector is typically very simple compared to a modern LCD or DLP video projector. Most overheads use an extremely high-power halogen lamp that may consume up to *750 watts yet produces a fairly dim, yellowed image. A high-flow blower is required to keep the bulb from melting itself due to the heat output. Further, the intense heat usually causes the halogen lamp to fail quickly, often lasting less than 100 hours before failing and requiring replacement. A modern LCD or DLP uses an arc lamp which has a higher luminous efficacy and lasts for thousands of hours. A negative to LCD/DLP technology is the warm up time required for arc lamps.
How to Use
Situate the overhead projector on a low table that is 2 to 3 feet off the floor. Make sure the overhead projector is plugged into a reliable power source. Place an overhead transparency on top of the flat glass surface of the projector. Transparencies can be written on directly with a wet erase overhead pen. Alternatively, a transparency can be fed into a laser printer, and information from programs such as spreadsheets and databases can be directly printed on the transparency.
Turn on the overhead projector. Most machines have a large, circular button on the front for this purpose, but on/off switch locations can vary. Angle the mirror at the top of the projection arm until the image is properly aligned onto a blank wall or drop-down projection screen Move the table holding the overhead projector forward or backward (if necessary) until the projection image approximates the size of the projection screen or blank wall space. Turn the circular knob at the top of the projector arm until the projected image comes into focus. Press the on/off button again when finished with your presentation. Allow the overhead projector to completely cool down before using it again.
Use of Overhead projector in education
The overhead projector facilitates an easy low-cost interactive environment for educators. Teaching materials can be pre-printed on plastic sheets, upon which the educator can directly write using a non-permanent, washable colour marking pen. This saves time, since the transparency can be pre-printed and used repetitively, rather than having materials written manually before each class.
The overhead is typically placed at a comfortable writing height for the educator and allows the educator to face the class, facilitating better communication between the students and teacher. The enlarging features of the projector allow the educator to write in a comfortable small script in a natural writing position rather than writing in an overly large script on a blackboard and having to constantly hold their arm out in mid-air to write on the blackboard.
When the transparency sheet is full of written or drawn material, it can simply be replaced with a new, fresh sheet with more pre-printed material, again saving class time vs a blackboard that would need to be erased and teaching materials rewritten by the educator. Following the class period, the transparencies are easily restored to their original unused state by washing off with soap and water.
Critics feel that there are some downsides as these technologies are more prone to failure and have a much steeper learning curve for the user than a standard overhead projector. While a computer projection system eliminates the need to create hard copy transparencies (which can be quite expensive, particularly if made in colour) of the slide show presentation, many presenters make both in case the computer hardware fails. Furthermore, the overhead projector allows a more direct interaction through live writing on the transparency.
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