Photosensitivity affects less than 5% of epileptics according to the
British Ep. Society. It is a frequency induced phenomenon in which
the person exhibits some or all of the following: sweating,
headaches, anxiety, feeling high, sleeplessness. The most likely
range is between about 30 and 80 Hz so the mains is slap in the
middle. Thus fluorescent lights and TVs are problems. Tungsten lights
are preferred as they do not flicker.
A recent client claimed he had no problem with TV but could not use
computer monitors even up to 80 Hz refresh rate. The BES say that
viewing TVs at distance is usually ok but close viewing causes the worst
effects. His experience was of watching TV at a distance under tungsten
light at home, while looking at PC screens at less than two feet
under fluorescent light in college. The BES also say that small
monitors are better than large for this group.
He believes that his sensitive range is as high as 100 HZ and that
the solution for him is a high frequency refresh rate screen. He
referred us to several claiming rates up to 120 or 150 Hz. With
multi-scan monitors the actual scan rate depends partly upon the resolution
you use. High resolution low/frequency, low resolution/high
frequency. But when Paul Dilley checked them out, the manufacturer's
own manuals conflicted with the published upper limits quite
considerably, or he was told that to achieve frequencies over 100 Hz
would need resolutions like bricks and well below anything offered by
Windows.
This suggests that our client has not experienced what he thought he
had- a high frequency monitor. (Incidentally, we know about LCD
displays, he doesn't like them either.) Whatever the paper spec for
the monitor, it would not have been showing what he thought.
Given his reaction to TV at home, which is counter to expectation,
and his bad reaction to all screens in college viewed under flourescent
lighting, is it possible that his sensitive range is much lower than he
thinks and that he is experiencing a beat effect between the lighting and
the screen? A screen operating at 85 Hz under a fluorescent light at 50 Hz
will create a beat effect of 85-50 or 35 Hz. A TV under tungsten will
exhibit 50 Hz at a distance or 25 close up.
I offer this partly for information and partly to ask if anyone else
has had a similar case and what did they do about it. For those who
want more details about the screens you should contact Paul Dilley.
Dave Laycock
Head of CCPD, Chair of NFAC
Computer Centre for People with Disabilities
University of Westminster
72 Great Portland Street
London W1N 5AL
tel. 0171-911-5161
fax. 0171-911-5162
WWW home page: http://www.wmin.ac.uk/ccpd/
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