Physical Layout
Writing on the Board
OHP use





OverHead Projector “Software” — Transparencies

Note: This page is now of course way out of date.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.
Although prepared transparencies (OHTs) are the most common medium on the OHP, you can get silhouettes of solid objects, and interesting effects with shallow transparent dishes of liquid (with due regard for electrical safety)

Kinds of OHT

There are three kinds of OHT:

  • Write-on film: the old-fashioned "acetate". You can also write on the other kinds, but you cannot do anything but write or draw on this kind. In particular, never try to put it through a photocopier or a laser printer: it will melt and gum up the works very expensively. Nowadays, acetates are chiefly used when students need to prepare OHTs in class (such as writing up the results of syndicate discussions: it has advantages over flip-charts in that it is easier to take away and type up, but because only one report is shown at once it cannot readily be used for comparisons between reports), or for overlays on printed OHTs. A few years ago, an acetate roll fitment was a common and very useful feature of OHPs, but it has largely disappeared.

  • Pens for write-on film come in two kinds: permanent and water-based. Now that most OHTs are prepared on computer, you only really need the latter. Their marks can be washed off laser transparencies for re-use, but not of course off ink-jet transparencies, but see below for a note on this.
  • Laser transparencies: also used for photocopying. These are thicker and heat-resistant, and often come with a backing sheet. Printing on them generates large amounts of static. Most prints (unless you are well-heeled) are black only. Make sure you tune your printer-driver to give the best rendition it can of half-tones. When highly magnified, grey-scales can appear like old "dotty" newspaper photos.    
  • Ink-jet transparencies: still the most expensive (50p per sheet retail, about half that through institutions) but the best for colour. One side of this transparency has a slightly "gritty" covering: that is the only side which will take the ink, so make sure it faces the print-head. They can take a long time (15 mins +) to dry, and a fast printer may deposit a new sheet on top of one which is still tacky, often with unfortunate results. So do not leave your printer unattended, and remove each sheet as it emerges. Do not try to put this kind of transparency through a photocopier or laser printer. Note that using a template from a computer package which gives you a tinted background will greatly increase the printing time, and usage of ink. Again, explore the printer-driver options for best quality: most printers allow you to specify that you are using transparencies.   
    • Most ink-jet printers allow you to print the content reversed left to right. If you use that option, the image will be on the back of the transparency when projected properly. That means you can annotate it with water-based pens on the front as you are using it, and then wipe the notes off and re-use it.

Designing OHTs (and PowerPoint™) presentations

1 Conceive of your OHTs as a sequence, rather than as "one-offs".  Do not try to cram too much on each one. See notes on using the OHP, and on the reveal technique, below.
2 KISS: Keep It Simple, Stupid.
3 If at all possible, use a presentation package to prepare them. They feature in all the major office suites on all platforms, and they almost impose good practice on you.

If you must use a word-processor:

  • Use landscape rather than portrait format, with wide left and right margins (12 pica, 5cm, 2 inch). This stops the line getting too long, so the eye has difficulty in following it, and also ensures that the border of the OHP stage do not cut off the edges of your OHT. Landscape rather than portrait makes it less likely that you will try to cram too much in.  
  • Use a minimum text size of 18pt: this is recommended for legibility. Dedicated packages will encourage large sizes by default.     
  • Use a sans-serif font such as this one (Verdana) or Arial, for a cleaner look, unless you know what you are doing.
4 Use both UPPER and lower case for your text: this shows the shape of the word more clearly and greatly enhances legibility. PRINTING may be clearer for poor handwriting, but that is not an issue with computer-prepared OHTs


5 Three or four points per OHT is generally enough—certainly no more than six.
6 Use bullet points and paragraph spacing to distinguish the points: all packages encourage you to use a hierarchy of bullet points to show the relative importance of the topics
7 You can also use bold face and larger sizes or both (and colour) to emphasise key words or phrases. Do not use underlining: it interferes with the shape of the word.
8 On the other hand, be selective about what you emphasise: too many competing emphases interfere with comprehension. Putting everything in bold does not necessarily help legibility.
9 Better still, use graphics. I don't mean the clip-art which is so readily and seductively available in most packages—it rarely adds anything to the sense. I mean diagrams, charts, and the occasional scanned picture. All packages have simple but adequate drawing tools, and an hour's playing with them will pay great dividends.
10 If you use pictures, use vector graphics if possible (broadly speaking, the products of "drawing" rather than "painting" applications). They can be re-sized without loss of definition, so that you can fill your OHT with them, and they look better when projected.

(Photographs and other figurative pictures are "bitmaps", made up of pixel dots, which get chunkier if you enlarge them, or run into each other if you reduce them: unfortunately, all but the most sophisticated Web graphics at the time of writing are bitmaps—although all my diagrams started life as vector graphics—so don't expect too much if you put Web pictures onto your OHTs.)

11 In particular, use mind-mapping to give an overview of your session. You can keep returning to the mind-map OHT to help students keep track of what has been covered. You can also give it to them as a handout to help them to structure their own notes, or as an aide-memoire.

There are free copies of mind-mapping software available from here and here

12 A popular technique for using OHTs is that of the "reveal". You start with the OHT concealed by a piece of paper, which you withdraw paragraph or point by point, as you talk about them.
  • If you are still using the reveal technique, you probably have too much on each OHT   
  • It does have the advantage of keeping previous points on screen as you proceed...   
  • but it is usually clumsily done, and the paper falls off as you reach the bottom of the OHT, revealing the final points—ready or not!    
  • Instead, why not use the facility to copy content in your presentation package, to build up a sequence of slides, in which the earlier points have been de-emphasised by making them smaller, or greying them out, so that they do not compete with the current point?
  • If you must use it, put the cover paper under the transparency: it stops it falling off until you reach the very bottom.


In particular, there is no excuse for photocopying blocks of text from a book onto an OHT. If you need direct quotations, scan them and use an Optical Character Recognition (OCR) program to convert them to text so you can format them properly.

OHTs are also known as "transparencies" (risking confusion with 5x5cm slides) or "viewfoils" (which I have never understood because there's no foil involved)





Can't afford MS Office™? Download OpenOffice for free: it has a very good Presentation package which produces compatible (.ppt) files

This is 18pt (although browsers render it inconsistently)









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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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