Web pages do not contain anything that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds.
On this page:
Rationale
The intent of this success criterion is to allow users to access the full content of a site without inducing seizures due to photosensitivity.
Individuals who have photosensitive seizure disorders can have a seizure triggered by content that flashes at certain frequencies for more than a few flashes. People are even more sensitive to red flashing than to other colors so a special test is provided for saturated red flashing. These guidelines are based on guidelines for the broadcasting industry as adapted for computer screens, where content is viewed from a closer distance (using a larger angle of vision).
Required development techniques
Compliance with this checkpoint requires all of the following techniques to be met.
These techniques are defined in the WCAG 2.0 Level A Success Criterion for Guideline 2.3.1 (link resides outside of ibm.com).
1. No flashes between 2 and 55 times per second: Ensure that no component of the content blinks or flashes at a frequency between 2 times per second and 55 times per second.
2. Small flash area: Keep the flashing area small enough.
Examples for General developers
1. No flashes between 2 and 55 times per second: Ensure that no component of the content blinks or flashes at a frequency between 2 times per second and 55 times per second.
To comply with this technique, all of the following examples must be implemented.
Example 1
Ensure that content does not violate the general flash threshold or red flash. Displays which flicker or flash can cause photosensitive epileptic seizures in susceptible individuals, particularly if the flash has a high intensity and is within the frequency range between 22 times per second and 55 times per second. This includes flashing text, turning graphics on and off or repeatedly changing between different images on the screen.
An animation of a thunderstorm shows six flashes of lightning. The flashes are so fast and large that the general flash threshold is violated when tested with a flash analysis tool. The animation is modified to create a short pause after each pair of lightning flashes. After the changes are made, the animation does not violate the general flash threshold.
2. Small flash area: Keep the flashing area small enough.
To comply with this technique, all of the following examples must be implemented.
Example 2
Keep the flashing area small enough.
1. To convert viewing distance to rectangle size, multiply the viewing distance by 0.1745 (10 * Pi / 180) to get the width of the rectangle, and multiply the viewing distance by 0.1309 (7.5 * Pi / 180) to get the height of the rectangle. (This calculation can be done in inches, or millimeters, or any other unit of length.)
2. Determine the size of a 10 degree angle of view in pixels.
To do this, multiply the width and height of the rectangle from step 1 by the resolution of the screen, in pixels per unit length, to get the horizontal and vertical size of the rectangle in pixels.
- For a 1080p widescreen display (which is 1920 by 1080 pixels), the resolution of the screen in pixels per inch is 2203 divided by the diagonal screen size, in inches.
- For a 720p widescreen display (which is usually 1365 by 768 pixels), the resolution of the screen in pixels per inch is 1566 divided by the diagonal screen size, in inches.
- For an LCD computer monitor which specifies the pixel pitch in millimeters / pixel, the resolution of the screen in pixels per inch is 25.4 divided by the pixel pitch in millimeters.
For any display, if you know the actual diagonal screen size in inches, and the horizontal and vertical resolution of the display in pixels, then the resolution of the screen in pixels per inch is the square-root of ( (horizontal resolution in pixels) * (horizontal resolution in pixels) + (vertical resolution in pixels) * (vertical resolution in pixels) ).
3. Multiply the width of the rectangle by the height and divide by 4.
An author creates an animation that will be displayed on a screen in the entrance lounge at a company. Using the size and resolution of the display and the closest distance that a person can stand when viewing the display, they calculate the size of 25% of the 10 degree of central vision in pixels (using the formula above). This would be the small safe area. They then are careful to never flash any area larger than the small safe area.
Recommended development techniques
Implementation of recommended techniques is not required to comply with this checkpoint, but these techniques should be reviewed since they can improve the accessibility and usability of the application. See WCAG 2.0 Additional Techniques (Advisory) for 2.3.1 (link resides outside of ibm.com) for a list of techniques and examples.
Required test techniques
The following manual test techniques are required to test this checkpoint
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Use the following accessibility verification test (AVT) to validate the Web content. It is recommended that these tests be performed in order. |
| Action | Result | |
|---|---|---|
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Verify the compliance criteria for this technique as follows.
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Pass:
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Additional test hint for flashing content When testing this requirement:
For more details, including information on general flash and red flash thresholds and how to calculate small safe area see the required development techniques for this checkpoint. |
®2009 IBM Corporation
Last updated August 01, 2009.
W3C Recommendation 11 December 2008: http://www.w3.org/TR/WCAG20/ (link resides outside of ibm.com)
Copyright 1994-2009 W3C (Massachusetts Institute of Technology, European Research Consortium for Informatics and Mathematics, Keio University), All Rights Reserved.