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The ergonomics of lecturing with 35mm slides

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Academics and professionals often share important information and concepts through conferences and lectures and make use of 35 mm slides or other visual material. If these are poorly designed or presented, there is a serious waste of costs in time, effort, and travel. This paper examines briefly some interactions between speaker and audience and some basic requirements for the design and presentation of 35 mm slides.


This paper is in two parts. The first is concerned with the triad of relations between speaker, audience, and slides, and the second with some elementary errors in making and projecting slides. With their knowledge of task analysis, ergonomists should understand much better than others the needs of an audience. It is regrettable that they are as clumsy as many other professionals in the technique of speaking with slides when they should provide a lead in this area.

There are elementary rules for the design of text on 35 mm slides, such as no more than 7 lines of text, a legible typeface, and a contrasting lighter background. More simply, an audience in suitable surroundings is likely to be able to read a slide projected onto a screen if its text can be read with the naked eye when it is held in the hand.

Several important factors are not discussed here because of limited space, especially human perception, information processing, and typography. These are dealt with in standard texts on Human-Computer Interaction. Screen Format Design also shares several principles of good text design.

Who is the audience?

There is a huge difference between speaking to colleagues about matters of common interest, and speaking to an audience with a different native language using concepts and terms new to them, possibly through an interpreter. They may also be tired and distracted by new surroundings and competing activities.

The expectations audiences have of a speaker influences their listening. A speaker known to be informative and entertaining or introduced strongly is given a halo. Experienced speakers know that if they grip the attention of a strange audience with a vivid introduction in the first half minute, the arousal level of their audience, and the interaction with it, will mean a more successful occasion. However what is badly missing is some method of assessment of this success or failure. Material should suit the audience, where necessary with graphics translated accurately, and the lecture rehearsed carefully with the interpreter. Where difficult words are to be used which have different possible translations, such as "model", two or three different versions of the slide can be prepared and the best one chosen with the help of the interpreter. The triadic relation of speaker, audience, and content is a complex one, the subject of Semiotics (Sless 1986).

Who is the speaker?

Speakers vary enormously in many respects. If their voice is too soft, then everybody's time may be wasted. The head of a department should rehearse members of staff who are to give papers elsewhere through local presentations which are criticized ruthlessly. To a large extent the speaker is an actor. This concept is underlined by the growing metaphor of HCI as theatre ( Laurel 1991).

Effective speakers are perennial students of techniques of audio-visual presentation and of their own performance, and grateful for destructive criticism, however depressing this may be at times. In an old proverb, your enemy is your best friend, making you try harder. Good speakers are rewarded by personal satisfaction, professional promotion, and better acceptance of their ideas.

Some details of information design

Throughout history, there must be few outstanding orations whose success required visual aids, and many lectures today do not need them. The roles for such devices are:

1. To focus attention of speaker and audience, especially where Short Term Memory is overloaded with new or

detailed information. Too much detail means it should be presented in written rather than oral form.

2. To present pictures or diagrams, said to be worth a thousand words, especially for the new or unfamiliar.

3. To assist metaphors and humour. These are powerful tools but dangerous if they are not culturally appropriate and sensitive.

Rules for designing good slides are given in many standard texts (Bligh 1972, Calnan and Barabas 1981, Kemp 1980), Kodak 1987). Also, slides should be over-exposed by half to one f-stop). This is for two reasons. First, reflected light from the screen (especially a light background to text slides) makes a dark room less dark, and the audience less likely to sleep (Harvey et al 1983). Second, some lecture rooms have stray daylight which makes darker slides harder to see.

The content of a slide should be clear and concentrate on its subject. Any irrelevant content is distracting and should be excluded during the original photography or by masking the slide. Too much blurring or contrast makes a slide hard to understand and irritating and it is best to omit it.

Making good text slides requires a single-lens reflex camera, a macro lens which keeps the corners in sharp focus, a blue filter, and a copy-stand. The text should be double-strike on a laser printer, and over-exposed by 2 or 3 stops to ensure a crisp white background.

The venue

The distance from the back of the audience to the projection screen helps determine the size of screen, assuming other factors are satisfactory. Normal visual acuity, so called 6/6 or 20/20 vision, is the ability to identify alphabetical characters occupying 5 minutes of arc, equivalent to 8.7 mm at 6 metres, or characters slightly under 1 unit high at 600 units distance. Using a common engineering safety factor of 3, a ratio of character height to distance of 1:200 is therefore a good rule for ensuring that text is legible.

At 10 metres distance to a screen with seven double-spaced lines of text, letters should be at least 5 cm. high, giving a screen height of 1.5 m. Some guidelines suggest a ratio of screen diagonal to room depth of 1:8, which on further calculation gives a ratio of a similar order of magnitude.

The height of the screen, slope of the floor, and arrangement of the seats should allow a clear view of the screen, with the further assumption that the line of sight has an angle of greater than 45 degrees to the screen. The front of the lecture room should be dim enough not to obscure the screen image, but the back of the room with more lighting. Common problems include an entrance for the audience near the front of the room, outside noise, and poor design and labelling of controls used by the speaker.

Other aspects

Many other practical factors influence the success or failure of a presentation. These include clear guidelines for conference assistants and speakers, and supervising these. Kodak-type carousels should be standard. Some speakers and assistants do not know the simple rules for unloading them from the projector. There may not be spare projector bulbs handy, or the know-how to change them. Instructions to speakers should include a diagram of a good text slide (and overhead transparency). Common problems today will be replaced by worse ones, if new technology using projection of computer images is managed as badly as slides are now.

In conclusion, the International Ergonomics Association has a special responsibility to develop and disseminate effective guidelines in this area.


Bligh, D.A., 1972, What's the Use of Lectures? Penguin
Calnan, J., and Barabas, A., 1981, Speaking at Medical Meetings, 2nd ed: Heinemann, London
Harvey, R.F. et al, 1983, Dreaming During Scientific Papers, Br.Med.J., 287, 1916-1919.
Kemp, J. E., 1980, Planning & Producing Audiovisual Materials, 4th ed: Harper & Row.
Kodak, 1987, Making Effective Lecture Slides: Publication No. M3-106, Eastman Kodak Company, Rochester, New York.
Laurel, B., 1991 [in press], The Art of Interface Design: Addison Wesley.
Sless, D., 1986, In Search of Semiotics: Croom Helm, London.


Who is the audience?
Who is the speaker?
Some details of information design
The venue
Other aspects







The ergonomics of lecturing with 35mm slides

Michael Patkin and David Sless
The Whyalla Hospital, South Australia, and the Communication Research Institute of Australia