With the advent of Digital LCD/ DLP projectors, OHP – Over Head Projectors might look like a pretty old technology, but they have their own applications and their own market. They are good at what they do – Projecting Transparency slides on to a bigger screen.
The above image shows exactly how an OHP looks. As you can see there is a box on the base module, over which there is a transparent glass. Below the glass, there are lamps, lamp holding assembly and other circuitry necessary to operate the lamp. A small metallic holder extends and reaches to the header assembly with mirrors on the top.
The function of this mirror assembly is to reflect the image / writing contained in the transparent sheet kept on the OHP glass, over to a screen when the OHP is switched on. This reflection forms a larger image and the size of the reflection can be increased/ decreased by moving the OHP away from the screen/ closer to the screen, respectively.
The reflected image can be best viewed on a screen mounted over a tripod stand or attached to the wall. The OHP transparency sheets are available in departmental stores and one can write on them using marker sketch pens. A portable OHP looks like the one shown below.
Portable OHP’s are light weight and they can fold flat, on to their hard shell case. This makes them easier to store (using a small shelf space) and also easier to carry around (due to their lesser weight). They may have internal storage space to store power cord and transparency sheets.
OHP’s are very useful for educational institutions and also for training purposes in offices/ companies. In many cases, the information (hand written text and diagrams) carried on the transparency sheets are more than enough to convey the message in a presentation/ training session. These transparency sheets are used exhaustively in schools/ colleges as they are sufficient to explain certain concepts that are a part of the syllabus.
Digital Projectors are expensive and are very difficult to maintain. The lamps used in digital projectors are extremely expensive. So, instead of using them for every application, it might be better to use OHP’s for certain simple but effective presentations, that need only simple bullet point text and small hand-drawn images.
Depending on the model, various commonly used OHP’s are rated to give brightness levels between 1000 Lumens and 4000 Lumens. Higher the brightness, better the visibility / clarity of the reflected transparency images.
OHP lamps are pretty inexpensive and they may have an average life of 50 hours – 300 hours, depending on the make and model. It is advisable to have some extra OHP lamps in order to change them when their life is exhausted. OHP lamps can be changed by users themselves easily.
Overheating of OHP lamps are a frequent issue with OHP’s. So, some OHP models have internal circuitry that will sense the heat levels and cut off the power supply to the lamp if they happen to get overheated. Some manufacturers provide two lamps that can be used simultaneously at different times by using one lamp for some time and then changing over to the other lamp (when the first one gets overheated), even while a presentation is going on. This makes sure that one lamp does not get over heated due to excessive usage.
The projection head assembly on the top are made using multiple vari-focal lenses so that the contents of the transparency slides can be precisely and clearly projected on to the screen. The projection head assembly can be tilted to adjust the image position on the screen. The projection head assembly may also be closed in order to prevent dust from entering the optical assembly.
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