Making your presentations readable


As an optometrist, I have been trained to be many things -- healthcare
practitioner, sales assistant, business person, counsellor, and one
role often forgotten, that of the visual scientist. As a visual
scientist, I have been trained in the basics of illumination
engineering and ergonomics, but rarely apply these skills to anything
more practical than telling patients to have good lighting when they
are reading.

However, there is one problem that I think I can solve which will
eventually affect an academic in one way or another. This is the
problem of how big written material needs to be when projected (or
written) at the front of a lecture room. This, of course, depends on
the size of the lecture room, since even here in the maths department
at UWA, we have rooms ranging from 6 metres (like Maths Lecture Room 2
with capacity 35 people) to 20 metres in length (like the Weatherburn
lecture theatre with capacity 200 people). It also depends on factors
like the brightness of the projector, the distance of the projector
from the projection screen, the quality and angle of the the projector
screen, and other similar things.


*** A sample calculation ***

I would like to present some overhead transparencies to some students
in a small lecture room (like Maths Lecture Room 2). These
transparencies will have some text, some formulae and some figures. I
know that keeping transparencies reasonably sparse will result in
improved readability, but would like to know what size font the text
and formulae should be set.

From general principles of ergonomics, we should cater for at least
95% of the population that we expect to be presenting to, and looking
at aetiological studies, get a target visual acuity of 6/12. This
corresponds to a critical detail size of about 2' of arc, or
equivalently about 10' of arc letter size from the back row. Since we
would like to make reading easy, we will make letters 1.5 times this
size. Since the room is about 6 metres long, the letters then need to
be about 2.6 cm high.

Using a projector about 2 metres away from the projection screen, and
assuming a good, correctly angled projection surface with a darkened
room, allowing good contrast and no distortion at the top or the
bottom of the slide, we can empirically determine that a letter of
height 5 mm high, or equivalently font size 18, is necessary to
satisfy the size requirement.

Often, the projector placement will be further than 2 metres away,
increasing word size, but reducing contrast, thus meaning that font
size 18 is still a good benchmark. Similar problems arise if the
projection surface is not angled correctly, giving distorted areas at
the top or bottom of the transparency.

We now need to realise that mathematical equations are often more
intricate than pieces of text, especially when dealing with small
exponents or subscripts, and thus equations may need to be set in a
larger typeface.

***

Consider now the example of the Weatherburn lecture theatre. Using
analogous calculations, the letter size at the front of the lecture
theatre should be about 8 cm high. While this is larger than one would
normally write on a blackboard, it is not too difficult to write
reasonably quickly and naturally at this size.

Now presume that we are using overhead transparency slides, and the
projector is now 4 metres from the projection screen. We should be
aiming for font size 30, and while the contrast will be less than if
the projector was at 2 metres, most of the audience should still be
able to manage reasonably well, since visual acuity does not drop with
contrast loss until it reaches about 10% (this value coming from the
contrast sensitivity function).

***

I hope that this analysis helps academics improve readability during
presentations, especially day-to-day lectures, since students have a
hard enough time learning without also having to worry about visual
stress.


Last edited: 14 April 2000