Using the OHP and Data Projector in class

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Hardware: Notes on dealing with problems with the projector and its setup Software: Notes on how to design OHTs

The Overhead Projector, or “OHP” is one of the most frequently used (and misused) tools in education and training today. It may be facing a challenge from the data projector, but it is still more flexible and useful for all except uninterrupted, rehearsed presentations.   

Notes on use

The use of the OHP or data projector sends a message: “This has been prepared.” This is in contrast to using the whiteboard or flip-chart, which sends “This is spontaneous,” or, "I am reacting to you". There is a case for both kinds of message, of course, under different circumstances.

OHP vs. Data Projector

Data projectors are becoming increasingly popular and more frequently built in to classrooms. If you have to choose, consider these factors:

OHP Data Projector
Not much to go wrong: and if it does, you can often fix it yourself Requires a computer as well, and a link: there's a lot of temperamental hardware.
Restricted in the range of media it can use: OHTs and a few transparent or silhouetted objects Can use static or animated graphics, sound and even movie clips, with a sufficiently powerful machine
Can easily take OHTs out of sequence, to re-cap or jump ahead to respond to a question. Needs a dual-monitor set up, if students are not to see the nuts and bolts of the set-up when you want to select a slide out of order (learn to blank the screen while you do it)
OHTs are bulky and rather fragile You can put an entire lecture sequence's material on one floppy—your entire teaching career's output on CD or DVD.
Temptation to use badly-designed OHTs: pages of books copied onto transparencies, for example. Your presentation package makes it harder to produce a bad show than a good one.
You can write on and otherwise amend OHTs on the fly. On the whole you are stuck with what you have prepared.
Students can use write-on OHTs for reporting back from groups, etc. Not really practicable, except with a smart-board

The essence of the OHP and the Data Projector

In order to put the following tips into context, consider a few points about the OHP in the classroom:

  1. The OHP is usually under the control of the teacher: this means that it is by default an instrument of teacher-centred instruction. This is not necessarily a bad thing, and teachers can develop its use as a tool of session management. Switching it on, for example, is likely to send the message, "Now get ready for some input from me", and can be used to curtail discussions which are veering off the point. Similarly, switching it off when a discussion starts is a way of acknowledging that the class group is making the running.
  2. One teacher I observed recently worked mainly on a flip-chart: she just used the data projector to project single key words about the topic the class was working on. It was very effective in helping the students to keep track.

  1. It presents a series of images in sequence: the timing and ordering is again controlled by the teacher. Indeed, having a series of OHTs can readily substitute for the teacher having to have her own notes, and free her up to pay more attention to the class, and to expound the material more spontaneously.
  2. But the fact that each image is shown for a limited period of time means that its content needs to be digestible within that period. So apart from sheer legibility issues, there should not be too much on any one OHT.
  3. It is, of course, a visual medium: this requires attention to the needs of students who may not find it accessible. These include not only students with visual impairments, but also dyslexic students. There is some evidence to suggest that the high contrast of the screen, which is a help to many students, can hinder the reading of those who have some forms of dyslexia. Students with hearing impairment may find it difficult to follow the lips of a lecturer and look at the screen at the same time.
  4. Conventional text is linear: we all know it takes time to read. Graphics, including some non-conventional text such as mind-maps, and labelled diagrams, are perceived at once as a whole, and then "read" in unpredictable orders by different students. This comes more naturally to some students than others. You have two channels of communication going at the same time, assuming that you are talking at the same time. You need to consider how they interact.
        • Are they parallel? Do the two channels carry the same information, with the one reinforcing the other? That may be useful on occasion, but if you are reduced to reading out verbatim the contents of the OHT (except when it is for the benefit of students with difficulties), the redundancy level may be too high.  That is the root of "death by PowerPoint".
        • Are they complementary? Do the two channels carry different information? This is often the case when the OHT is used for graphics. More visually-oriented students may find it difficult to pay attention to what is being said when concentrating on the screen: more aurally-oriented students may find their natural tendency to listen to the voice hinders their ability to make sense of the image. Give the students a few moments to digest what they are seeing before commenting on it.
  1. You are requiring the students to look: when they are looking at the screen, most of them will not be able to make notes, so a lecture or class which uses a lot of OHTs may also require handouts, if it is the kind of session which calls for notes. Remember, too, that copying diagrams or charts from a screen takes much longer than making text notes. Keep graphics simple. Be explicit about what is worth noting and what not. However, if leaving time for copying breaks up the session too much, think again about providing the information in other ways, too. If your material was prepared in one of the standard presentation packages, it could be exported directly to an intranet or Virtual Learning Environment (VLE), for example, for consultation after the session. (Presentation packages may provide an automatic "Export as Web Page" facility).

Tips for use

  1. It's sucking eggs time again! Get the OHP set up for optimum image size, focused, etc. before the class starts. Make sure everyone can see.

  2. Once you are sure of the above, you have no need to look at the screen. You can concentrate on the class. Needless (?) to say, don't get in their way.
  3. So do not point to material on the screen itself. Use a pen or other pointer on the stage of the OHP. If your hand is at all shaky, lay the pointer down on the OHP stage. Standing in front of the screen to point out salient topics:
      • means that you are tempted to talk to the screen and not the class  
      • means that some of the image is projected onto you, and there is a large obscuring shadow on the screen
  1. The only exception is when you are projecting onto a whiteboard, and writing/drawing on the board to elaborate on the image. (This is also the only occasion when you should project onto a whiteboard, which will also reflect back the light source, and may well form a "hot-spot" obscuring the image for some students.) Even this can usually be dealt with more effectively by writing on an overlaid transparency on the OHP stage.
  2. Do not leave an old OHT showing when you have moved on to talk about something else. Switch off between OHTs, or at least between sequences.

      When I started using the OHP years ago, this was not advocated, because it shortened the life of the bulbs, and a blank screen was a small price to pay to avoid the risk of a bulb blowing and the disruption that caused. Modern halogen lamps are less sensitive (although it is still not advisable to move an OHP until it has thoroughly cooled down).

OHP Hardware OHP Software


To reference this page copy and paste the text below:

Atherton J S (2013) Learning and Teaching; [On-line: UK] retrieved from

Original material by James Atherton: last up-dated overall 10 February 2013

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