Use of transparencies with the overhead projector
When ready-prepared transparencies were expected
Before computer-aided projection, a collection of prepared transparencies (also known as acetates) for use with the overhead projector was almost a rite of passage for a professional educator. It was usual to arrive for a formal presentation with a stack of ready-prepared transparencies to give an air of professionalism.
Use of transparencies with less formal presentations
Often, particularly in regular teaching, there was more educational value in writing for an audience while working through what one was saying - rather like with the old-style blackboard and chalk system.
There was a similar facility with the OHP. There was always a roll of thinner, cheaper and disposable acetate which could be written on and rolled along to provide more space. This can be seen in the picture on the OHP page.
In theory the roll could even be washed afterwards for later use, provided that non-permanent pens had been used. However, washing a roll of plastic and drying it was not for the faint-hearted, and I doubt if anyone ever did it. In fact, I tried to wash regular transparencies from time to time, but it never worked successfully. Dried drops of water always seemed to show if the transparencies were allowed to dry naturally or fabric fibres seemed to be left if any sort of fabric was used for drying. Also some coloured smears always seemed to be left.
Techniques for using transparencies
How presenters used their transparencies was very much a matter of personal preference, just as how they might choose to use computer-aided projection today. However there were accepted norms and guidelines:
Revealing bullet points one by one
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 as one talked about each point. The paper, which could be of any type, blocked out the light, making anything beneath it invisible to viewers. I always thought it looked messy and preferred to display all the bullet points and point to whichever one I was talking about.
Any type of pointer could be used, but it always cast a shadow, so was best laid flat.
Today, with computer-aided projection, the bullet points can simply be made to appear as needed.
Numbers of transparencies per presentation
It is well known that, within reason, the more visual aids in a presentation the better. However, a stack of transparencies was bulky and rather heavy. I used to give numerous presentations overseas where it was necessary to pack to a limit of weight and size limit. Yet prepared transparencies were essential. How much better today, just to pack a memory stick!
There was also regarded as an ideal number of transparencies to show every one or two minutes. I can't remember the number now as I always used common sense.
Resource implications for schools, colleges, etc
For the institutions that had to provide the overhead projectors and their associated equipment, the resource implications were considerable.
The OHP pens were an expense because new ones had to be bought quite frequently as they were used up or dried out. Replacement packs of blank transparencies or rolls had to be bought on a regular basis. This in addition to the cost of the OHPs themselves which were too bulky to be moved from room to room on a regular basis.
Irritations for presenters and their audiences
In passing, I have mentioned some of presenters' annoyances with transparencies, but there was another major one: the near impossibility of making corrections or adding further thoughts. Either replacement transparencies had to be made or the old transparency had to be lived with.
There were also annoyances for audiences. Seeing text or drawing being whisked away, out of focus and being replaced out of focus or seeing the roll being wound on out of focus was probably the main one - not that audiences knew any difference at the time.
In the area of visual aids, there can be little doubt that what we have today is a considerable improvement on the overhead projector and transparencies.