Universal Design for Web Applications

Flicker and Patterns

In 1997, an episode of Pokemon airing in Japan made the world take note of the phenomenon of photoepilepsy. Some 685 Japanese children and adults were taken by ambulance to hospitals, suffering from seizures, convulsions, blurred vision, headaches, dizziness, and nausea, after watching a scene of the cartoon in which much of the screen flashed blue and red at a rate of 12 hertz (12 times per second).

The same potential to cause seizures exists in web content as well, and WCAG 1.0 restricts flashing content, instructing authors to avoid anything that flashes between 4 and 59 Hz. Section 508 also bans flashing content in the range of 2–55 Hz.

In the aftermath of the Pokemon incident, however, much research has been done to determine what colors, at what saturation levels, at what flashing frequencies, and in what percentage of the viewable field can trigger these seizures. With this information in hand, WCAG 2.0 now specifies more granular advice. Here’s what it says:

Web pages do not contain anything that flashes more than three times in any one-second period...

OK, that’s the rule of thumb. If you need to dig deeper than that, prepare for some hard science. Here are the qualifiers if you want content that flashes faster than 3 Hz:

• A flash or rapidly changing image sequence is below the threshold (i.e., content passes) if any of the following are true:

  • There are no more than three General Flashes and/or no more than three Red Flashes within any one-second period; or

  • The combined area of flashes occurring concurrently occupies no more than a total of .006 steradians within any 10-degree visual field on the screen (25% of any 10-degree visual field on the screen) at typical viewing distance

where:

• A General Flash is defined as a pair of opposing changes in relative luminance of 10% or more of the maximum relative luminance where the relative luminance of the darker image is below 0.80; and where “a pair of opposing changes” is an increase followed by a decrease, or a decrease followed by an increase; and

• A Red Flash is defined as any pair of opposing transitions involving a saturated red.

  Exception: flashing that is a fine, balanced pattern such as white noise or an alternating checkerboard pattern with “squares” smaller than 0.1 degree (of visual field at typical viewing distance) on a side does not violate the thresholds.

There. Got that?

That “10-degree visual field” is the important part. As the WCAG 2.0 Guidelines explain, that amounts to a 341x256 rectangle on a 15” to 17” 1024x768 display, viewed from 22”–26” away. If it’s smaller than that, you’re good to go. For anything larger, you may need to limit the flash rate. Tools to analyze your content’s conformance to this requirement are available and will likely become easier to find as support for WCAG 2.0 increases.