A basic portable overhead projector comprises a large box with a cooling fan and a bright light. Attached to it is an extended arm that has a mirror at the end which catches the light and reflects it onto a projector screen.
The primary purpose of this unit is to enable an audience to view images that have been enlarged and then projected onto a chosen surface. This is accomplished by placing a transparency onto the top of the unit so that both the audience and the speaker can see the image that is needed to be displayed. These kinds of overhead projectors were at one time extremely popular and could be seen in many different venues ranging from school classrooms to high end business meetings. In the last few years the more modern computer based models seem to have gained popularity which has led to a decline in favour towards the overhead versions.
The portable overhead projector dates back to World War II where one of the first uses was seen as a great training tool for groups of servicemen. It made its way into the school classroom during the 1950s and 60s as it was found to be a perfect educational tool and very soon crossed over into the business world to help as a training aid. During the 1990s virtually every classroom or business in America was utilizing the overhead projector and it was at this time that it reached its sales peak.
The get the optimum viewing of a an image to be displayed you need your unit to be approximately 2 to 4 metres away from the wall or projector screen you want to display it on. There are some custom built models available that will enable you to increase the distance between the unit and the display surface but these will come at a much higher price. The projector can also be sat on an overhead projector cart to make moving it around much easier.
There are many makes and models of overhead projector that are available but the main difference can be seen in the different types of optics they use.
Folded Optics or Chamber Optics
These types of optics use a mirror between the lamp and the lens. The main benefits of using a folded optic as opposed to the more basic or common direct optic projector is that the resolution is far clearer and when the desired image is projected there is much more light used. There is approximately 50% more light available when using a folded optic projector in comparison to the direct optic model.
The Reflected Light Optic
The specially designed lens of the reflected light optic is backed by a mirror. The light source directs light to the lens and then this light is reflected by the mirror. This reflected light is then directed back through the lens and onto the main projective lens. The light is now focused onto a second mirror and finally onto the wall or projector screen.
It's most likely that a portable overhead projector will use a reflected light optic, with the cooling system, the light source and the main projection optics fitting quite neatly into a single unit.
Although not a necessity, in order to utilize the full potential of your projection unit it would be advisable to keep these 2 things in mind.
1 - For ease of moving around have the unit sat on an overhead projector cart and
2 - To get the most out of viewing your images it's advisable that you use a dedicated projector screen.
This kind of image display unit may not be quite as popular as it was at its peak but it can still be found in many classrooms and business meetings around the world. Having said that, many users still prefer to stick with the more established design of the portable overhead projector as opposed to the more modern computer based models because they feel they remain far more reliable and are easier to use.
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To read more about which type of Portable Overhead Projector would be best suited to your needs, you can find more information here:
http://www.portableoverheadprojector.net
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