RE: Checkpoint 2.3 Avoid causing the screen to flicker.

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

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

"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

"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
[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

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.


UK Resources (added for value)

US Resources

Canadian Resources

France Resources

-----Original Message-----
From: [] 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

Received on Saturday, 21 December 2002 00:48:11 UTC