OHP GUIDELINES
Audio Visual Services and Development
Information Systems and Services
AVSD HELPLINE - 2627
OVERVIEW
An Overhead projector (OHP) is one of the most common and simplest forms of audio-visual equipment. Basically, it’s a big box
with a light inside, a couple of lenses and a mirror, and is used to project ‘transparencies’ onto a screen. All common-user
teaching rooms are equipped with an OHP; these often vary in model, shape and size, but they all do the same job and have
primarily the same functions.
OHP FUNCTIONS
On the whole, the OHP is straightforward to use. Just point it toward the screen, switch it on and lay the transparency on the
glass. There are however a few functions which will make sure you get the best possible quality performance and image.
Lamp change lever: The majority of OHPs we have are equipped with two
lamps. The reason for this is that if one lamp fails at the start of a lecture, the
lecturer can quickly bring the spare lamp into operation usually by simply
moving a lever, causing minimal disruption for the class. The ‘blown’ lamp can
then be replaced by a technician at a more convenient time.
Focus control: All OHPs have a focus control which will give the sharpest
possible image when adjusted properly. The focus knob is usually located on
the ‘head’ of the OHP and will appear to move the head up and down the track
on the vertical stem when turned. You can usually be sure that you will get the
sharpest image when you can see the concentric circles of the fresnel lens
clearly on the screen.
Fringe Control: If your OHP image has either a blue or orange tinge to the
edges, this can usually be rectified by adjusting the fringe control on the OHP.
Adjusting it until there are no visible orange or blue edges will ensure the best
projection possible.
Mirror Control: The height of the image on the screen is usually determined by the angle at which the mirror lid is lifted.
Basically, the straighter the image is projected, the better it will look. Unfortunately, in a lot of teaching rooms this is not possible
and the image height has to be raised by adjusting the mirror which causes a ‘keystone’ effect making the image have a ’sloping’
appearance. It is also very important to make sure the mirror lid is kept open at all times, because operating the OHP with the
mirror lid closed will cause it to overheat and can cause the lens holder to melt!
CHANGING A LAMP
When a lamp has blown and needs replacing the first thing needed to do is find out what type of lamp is needed to fit. Our OHPs
use either 250w or 400w lamps and in time the technician will be able to recognise which is which at a glance. Until then, the
lamp required can usually be found out by either checking the OHP itself, where there is often a label which indicates what type
of lamp is needed, or by checking the lamp that is removed. Once the required lamp has been identified:•
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Disconnect the OHP from the mains.
Open the OHP lid.
Remove the blown lamp taking great care that the lamp is not hot and removing it slowly and gently to ensure it doesn’t
break (leaving the pins stuck in the lamp holder).
Carefully fit the new lamp making sure that you do not touch the bulb with your fingers; skin contact will cause the
lamp to bubble and discolour when the OHP is used.
Close the lid, plug the OHP into the mains, switch on and test.
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