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When I was a young adult in the 1970s and 1980s, the overhead projector (OHP) was very much the 'in-thing' to use for anyone delivering information to a group of people. Quite apart from the questionable possibility of it improving learning, it showed that the user was up-to-date with technology.
An overhead projector was normally referred to simply as an OHP.
There were certain advantages of the OHP over what it replaced.
Before the overhead projector, the most common way of delivering a presentation was known as 'chalk and talk'. The person giving the information simply spoke and used chalk to write or draw on a blackboard. This was how children were taught in Victorian times, right through to my own schooling in the 1950s. (Slide projectors with talk and even films were also used occasionally but rather as a special event or treat.)
The overhead projector enabled:
Personally I am not sure that it necessarily helped learning to show pre-prepared material on an OHP, as the delivery was often too fast in consequence. Writing on a blackboard took time which gave an audience time to think and take notes. I must not of course generalise. Certainly pre-prepared material gave presenters a certain sense of security.
I can also remember numerous occasions when the OHP refused to function properly, so time was wasted with the audience sitting waiting while a technician was sought. Often the presenter had to resort to chalk and talk anyway.
Essentially an overhead projector consisted of a boxed lamp which shone onto a transparent plate, so projecting upwards whatever was placed on the plate. This upwards projection was focussed by a lens and reflected by a mirror so that the image could be projected onto a screen of some sort. The focus could be adjusted for a range of distances to the screen and the mirror could be tilted to suit the best orientation of the screen. The tilting of the mirror often meant that the image was not focussed squarely on the screen, but the better screens had an arrangement whereby their tops could be tilted forwards by variable amounts to compensate.
OHPs normally needed some time to warm up once switched on in order to provide a bright enough light.
The slides for projection were normally called either transparencies or acetates. As their name implies, they were transparent sheets, although as can be seen in the photo, there was usually a roll of poorer quality transparent material which was designed for single use at the time of projection.
The sheets were coated with a type of acetate which held writing without it separating out into globules. This coating came off slightly when handled and it affected the skin. So there was a strip at the side for handling. This strip seemed to be missing in later transparencies, so presumably the coating had been improved. Anyway, it was difficult if not impossible to prepare the transparency without handling it outside the special strip and I for one used used to feel my fingers slightly numbed and uncomfortable afterwards. After a good wash they felt back to normal.
Later, transparencies were developed without the strip, so were presumably deemed to be safe for the skin. Transparencies were also developed which took photocopying.
Special pens were needed to write on the transparencies. They were available in several colours and thicknesses and came in two types: permanent and non-permanent. The permanent ones always seemed to come with black outsides, so as to tell the two types apart at a glance.
There were advantages and disadvantages to each type. With non-permanent pens, errors could easily and quickly be erased with a damp cloth, so allowing rethinks, but care had to be taken to keep them away from anything wet. Stories got around of teachers carrying them between buildings in the rain and finding to their obvious consternation that they were blank on arrival.
Permanent pens, while not having this disadvantage did not allow for errors or rethinks.
Of course it was perfectly possible to make a transparency very quickly indeed by simply writing on it with an appropriate pen. However, for special presentations, particularly company meetings, something more professional was expected. (Remember this was at a time when word processors were in their infancy and printers were unsophisticated with only one font size and type at a time.)
One way to get changes in font types and sizes was to use stencils to guide the writing. This was very labour intensive because of having to line up each stencil character with the other characters.
Another way was to use rub-on letters/dry transfers which went by the trade name of Letraset. This was equally labour intensive because of lining up the lettering.
A range of stencils and dry lettering sheets were the tools of the trade for whoever had the task of making professional-looking transparencies. So were special drawing boards with a moveable ruler and clips to hold sheets of paper or transparencies.
How teachers used their transparencies was very much a matter of personal preference, just as how they might choose to use computer-aided projection today.
One point worth mentioning though is that it was common to have all the bullet points in a list on one transparency and to make them visible one at a time by sliding back a sheet of paper. The paper, which could be of any type, blocked out the light, making anything beneath it invisible to viewers.
For presenters, the advantages of modern computer-aided presentation such as PowerPoint are very considerable compared with transparencies on an OHP. In particular, a stack of transparencies is bulky and somewhat heavy. I used to give numerous presentations overseas when packing to a weight and size limit was important. Yet the transparencies were essential. How much better just to have to pack a memory stick.
For the institutions that had to provide the overhead projectors and associated equipment, the resource implications were considerable. OHPs were too bulky to be moved from room to room on a regular basis, so quite a number needed to be bought. The pens and the transparencies were also an expense, compared with computer-aided projection which needs neither.