David,

I'm enclosing a short comment about Owens' response, because I sometimes
come across confusion about the terminology of 'contrast sensitivity'.

From the name, one would imagine that in measuring contrast sensitivity
(i.e. measuring the "Contrast Sensitivity Function") one was evaluating a
person's sensitivity to contrast.  However, the term is conventionally
applied to the use of sine-wave gratings to assess sensitivity to spatial
frequency (or temporal frequency in the case of the Temporal Contrast
Sensitivity Function) and NOT sensitivity to contrast.

In plotting the CSF, contrast is the *dependant* variable, plotted on the
y-axis, and is *not* the independant variable.  The x-axis is the sine-wave
spatial frequency, and the measurement recorded to produce the CSF is the
reciprocal threshold contrast (modulation) at each spatial frequency.

The name CSF is unfortunate, as it does not describe what is being
assessed, and one should not confuse Contrast Sensitivity with ones
sensitivity to contrast!

cheers

Peter






 At 12:30 31/07/02 +1000, McFarlane, David wrote:
>Dear Colleagues,
>
>Some of you have been wondering what was the upshot of my recent query about
>looking at computer screens in near darkness. I have taken a little while
to obtain permission to quote you but most people were happy to permit
this.By way of thanking you all publicly here is the question and a summary
of the best replies;
>
>The Question:
>
>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 accommodation 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.
>
>The Answers:
>
>The most helpful answer I received was from Owen Evans. He pointed me to
the reference in Grandjean. Figure 161 on page 272 shows that a background
5 times darker than the centre of the visual field reduces contrast
sensitivity to 77% of the value found in a no-contrast situation. This is
based on research by S. Guth,  (1958),  "Light and comfort", Industrial
Medicine and Surgery, 27, pages 570 -574.
>
>Owen commented that contrast sensitivity is a measure of our ability to
distinguish small objects with varying contrast between the object and the
background is a topic that is well-covered by Salvendy "Handbook of Human
Factors and Ergonomics".
>He also provided a "Rule of thumb": if it is screen intensive work, buy
the biggest and sharpest screen that you can afford, and update them
frequently.
>
>        *       Thanks, Owen! I had always thought - incorrectly - that
this contrast problem only arises with glare problems where the brightest
light was peripheral to main visual task.
>
>Jenny Long said;
>
>The first question you should be asking is "what is the polarity of the
screen?". If the workers are using a light coloured screen (e.g. black
characters on a light background) and they are working in the dark, then
there is a large amount of contrast between what they are viewing and their
surrounds. This can cause a problem with dark adaptation (especially seeing
reference material or their keyboard after viewing the screen) which could
result in eyestrain symptoms. It also means more work for the muscles in
the iris controlling the pupil size, as the pupil would need to change size
frequently to cope with the bright screen and the dark surrounds. (By the
way, this is the basis for the instruction you received as a child, David,
about not watching the TV in the dark.)
>If, however, these workers are using a dark screen with light coloured
characters, then this might explain why they prefer to work in the dark. As
a rule of thumb, it is best to keep the ambient illumination levels similar
to the brightness of the screen.
>
>
>
> One person commented that;
>
>If the screens are a bit old, or the users are trying to use them at
maximum resolution, there may well be a focus issue - as you turn up the
brightness, the pixels defocus a bit, sometimes a lot. If the screen is
old, the phosphor may not be as efficient as it once was, requiring higher
brightness settings than usual. In order to get the sharpest image that
they can to work with, the users may be turning the intensity down, and
then need lower ambient light levels in order to minimise reflections and
achieve adequate contrast.
>
>Terry Simpson commented;
>
>"I have had some experience with this scenario. Users sometimes say that
it they prefer it darker. Often this is a dislike of the current (poorly
specified and maintained) lighting rather than a preference for low light.
It also sometimes indicates poor display equipment. Low light levels will
open the iris and put a greater demand on visual correction. This might
result in symptoms of eyestrain."
>
>Denny Ankrum commented;
>
>"Theory and the research would suggest that they would have a problem. The
trouble with that is that the research, etc., does not conform to what
happens. Hmmm. I guess reality must be wrong. I've run into situations
similar to what you describe. My advice would be to let them continue.
Suggest that if they do feel discomfort that they might want to raise the
ambient light levels a bit. In any event, they are not going to do
permanent damage to their eyes."
>
>
>Tom Mayfield commented;
>
>"Reading by torch light under the sheets was always my mother's cue for
saying I'd harm my eyesight.  I also remember the old TV's and that
observation.  John is right about reducing contrast generally for ease of
readability and I seem to remember some info in the old VDT Manual.
>
>There is the issue of your peripheral vision being more sensitive in
darkeness, so there could be a constant switching of attention to
peripheral movement (if there is any) and the consequent shutting and
opening of the iris as the eye moves from the lighted screen to the dark
room.  This would also happen if the user is a "hunt and peck" typist.
>
>It is an interesting question, as I have tried working with my lap top in
the dark, and the constant accommodation changes that Peter Howarth
mentioned become unbearable for me after a very short time (I'm a "hunt and
 peck" typist!).  It would be interesting to look at your programmers'
error rates compared to working in lit conditions.
>
>Peter Howarth commented;
>
>"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.
>
>Many thanks for all your contributions!
>Regards, David.
>
>




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