- From: Lee Roberts <leeroberts@roserockdesign.com>
- Date: Fri, 20 Dec 2002 23:47:32 -0800
- To: "'Web Content Guidelines'" <w3c-wai-gl@w3.org>
It is true that non-US electrical frequencies are 50Hz. However, this does not indicate that the flickering or flashing of a screen from a web site is based upon this frequency. Flickering can be caused by poor electrical supplies, problems with the computer or monitor, or more often found the poor judgment of designers creating flashing banners. The US National Fire Protection Agency, in order to keep with the Americans with Disabilities Act and from causing photosensitive epileptics seizures their standards are established at 2Hz maximum. This standard has been accepted in the majority of countries. "the flash rate be kept to under 2 hertz with breaks every so often between flashes"[5] This more strictly applies to our concerns for screen flicker. "Video games may induce paroxysmal discharges in subjects who are photosensitive. 30% of reported cases are not photosensitive (40). The increased sensitivity of video game patients to IPS at 50 Hz indicates that display flicker may underly video-game seizures (41) 100 Hz TV screens should be recommended in patients with TV-induced attacks (42)."[4] "The most effective frequency of stimulation for evoking paroxysmal discharges is usually about 15-20 flashes /sec, but the range may be from 5-80 flashes /second..."[4] "In general, flash rates faster than 5 per second and screen changes faster than 3 per second may pose a risk to people with photosensitive epilepsy. This effect seems to diminish when flash rates exceed 70-100 flashes per second." [6] "The horizontal scan lines on a high frequency (100 hertz) TV are harder to see on small screens and therefore less likely to trigger a seizure."[5] "It is important to note that about 48% are sensitive at 50 flashes/s and that about 15% are sensitive at 60 flashes/s, which are also the frequencies of AC current in Europe and North America respectively."[7] The generalized experience is that 5Hz to 20Hz causes the most problems. However, it is also noted that 48 - 50 percent of photosensitive epileptics still seizures at 50Hz. It is also noted that 15 percent of photosensitive epileptics are found to be sensitive to 60Hz. Another interesting note would be the fact that a small percentage of photosensitive epileptics encountered seizures at frequencies of 100Hz. My recommendation would be 2Hz to 60Hz for the following reasons: [A] The laws governing disability protection rate the 2Hz as the low margin. [b} Photosensitive epileptic seizures still rate 15 percent of the population at 60Hz. Therefore, a rate of 2Hz to 63Hz is more acceptable than the 50Hz or 59Hz margins applied by Section 508. Please note that the Indegene Link [4] was a study performed using US Air Force personnel. As noted above, the Indegene Link [4] reported that they found personnel affected at 100Hz. I was not able to find the link again that recommended 63Hz, but with this information we should be able to put the majority of persons at less risk if we exceed the 60Hz margin. Thanks, Lee UK Resources (added for value) [1] http://www.epilepsynse.org.uk/pages/info/leaflets/photo.cfm#lights [2] http://www.epilepsynse.org.uk/pages/info/leaflets/photo.cfm#computers US Resources [3] http://www.inter-dimensions.com/en-us/dept_36.html [4] http://www.indegene.com/Opt/FeatArt/indOptFeatArt6.html [5] http://www.epilepsyfoundation.org/answerplace/factsheets/photo.html Canadian Resources [6] http://epilepsyontario.org/faqs/work/photo.html France Resources [7] http://www.john-libbey-eurotext.fr/articles/epd/2/3/129-36/ -----Original Message----- From: w3c-wai-gl-request@w3.org [mailto:w3c-wai-gl-request@w3.org] On Behalf Of Jason White Sent: Friday, December 20, 2002 3:39 PM To: Web Content Guidelines Subject: RE: Checkpoint 2.3 Avoid causing the screen to flicker. Lee Roberts writes: > > Photosensitivity actually has a range of 2Hz to 63Hz. However, the > majority of standards states the recommendation is 2Hz to 60Hz. Are these North American or international standards? According to Gregg (when this topic was last discussed), 50 hz is the frequency of AC power supplies in Europe, and it would be pointless and impractical to specify an upper threshold in excess of this value. At least, that was the argument.
Received on Saturday, 21 December 2002 00:48:11 UTC