Transparencies for the overhead projector as used mid-20th century
When using an overhead projector, it was and still is important to think about effect one wants to have on a audience. There were several aspects to this which this page explains.
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 neat and ready-prepared transparencies to give an air of professionalism. This may have been more important to some presenters than the educational value.
There was another problem. It was the time it took to prepare formal transparencies and that they all too soon became out of date.
Transparencies composed impromptu in front of an audience
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, except that one could face the audience while writing. There was the facility with the OHP using the 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 did try 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:
It was common practice to have all the bullet points in a list on a single transparency and to reveal them 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. Then I used a pointer on whichever bullet point I was discussing. 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 was 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. It was a tricky compromise. How much better today, just to pack a memory stick!
There was said to be an ideal number of transparencies to show every one or two minutes. I can't remember what the number was as I always used common sense.
Irritations for audiences
For audiences, there were irritations with transparencies. Seeing text or drawings being whisked away, necessarily out of focus and being replaced out of focus or seeing the roll being wound on, again out of focus, was probably the main one - not that audiences knew any difference at the time.
Resource implicationsFor 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 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. Again, how much better is computer-aided projection. It is expensive to set up, but after that it is essentially free to use as much as one wants.
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.