Normally one thinks of 'eyestrain' as being caused by either the
accommodation sytem or the oculomotor system.  The symptoms of presbyopia,
for example, are brought about by the increase in the proportion of ones
amplitude of accommodation needed to focus a near object.  This increase
occurs when the total amount of accommodation available declines (with age).

Our bodies are often pretty lazy, and the eye uses just enough
accommodation to bring an object into 'good enough' focus, rather than
precise focus.  When focus is not precise, a point source will be imaged on
the retina as a 'blur circle' .  The circle has the shape of the eye's
pupil, and its dimensions are determined by 1) the amount of defocus and 2)
the pupil size.

In a bright environment the pupil size decreases - which is why uncorrected
presbyopes can perform near tasks so much better under higher light levels.
 In a dark environment the pupil will enlarge, and so for a given amount of
blur the amount of accommodation needed will be greater - with the
potential for more discomfort.  The closer the visual tasks, the greater
will be the problem.

If the workers prefer to work under near-dark conditions, then there is
something wrong with the lamps.

Peter Howarth



At 10:51 05/07/02 +1000, McFarlane, David wrote:
>Dear all,
>
>I recently had an inquiry from a computer firm where some of the staff
>apparently prefer to work in rooms that are so dark they can only just see
>the keys on the keyboard. I was asked if this could cause eyestrain. I did a
>quick search and found no evidence one way or the other. I suspect that
>slowness of dark accomodation coupled with the need to see other things as
>well as the screen might be the reason for any eyestrain. I remember half a
>century ago  (in the fifties) when television was still a novelty in England
>(and I was still a child) I was told not to watch the TV in a dark room as
>it "draw my eyes", that is to say, tire them needlessly. Is there a simple
>answer to this question?
>
>Regards, David McFarlane.
>
>




Peter A. Howarth
Visual Ergonomics Research Group (VISERG)
Department of Human Sciences
Loughborough University
Leicestershire LE11 3TU
England

'phone:  (U.K.) (0)1509-223040
http://www.lboro.ac.uk/departments/hu/groups/viserg/viserg1.htm