Note: This page is now of course way out of date. It has been retained partly for curiosity value and partly to avoid failed links if it were to be removed from the site. Navigation links may well not work.
The OHP is a simple machine: it has a lamp in the base
(although there are portable versions which project a lamp downwards from the lens housing
onto a reflective stage), usually cooled by a fan. The lamp may have high and low
settings, and a "fringing" control to position it centrally. Some machines have
a slide device to change bulbs semi-automatically if one blows. There will be a cut-out
system which switches off the power automatically if you open the base.
The light is distributed by a fresnel screen to illuminate the stage evenly,
and then focused by a lens and mirror arrangement (often enclosed), onto the screen.
The focus knob moves the lens assembly up and down a column.
The OHP needs to be square to the screen: otherwise the
image will be distorted.
Note, too, that the
reflectivity of screens may fall off rapidly as the viewer moves to the side. Providing a
good view necessitates attention to seating arrangements. (Yes, I know it's obvious and
even patronising, but teachers do not always think about it at the start of the class, and
having people move to be able to see in the middle of the session is unnecessarily
On older machines, the lens can move from being parallel with
the stage, resulting in selective lack of focus which cannot be cured with the focus
control. Loosen the holding screw and adjust it, or use discreet brute force (which is
probably how it got misaligned in the first place — and not many teachers routinely
carry a screwdriver)
If something like the image on the right appears, it is
because the lamp is misaligned under its condenser lens. Some machines have a slide or
knob control on the front to cope with this: if not, call the technician! Re-alignment is
not difficult, but something of a distraction from the business of the class.
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