Do not use flashing or blinking text, objects, or other elements having a flash or blink frequency greater than 2 Hz and lower than 55 Hz.
On this page:
The purpose of this checkpoint is to ensure the software is developed so that it does not induce seizures due to the use of blinking or flashing.
Text and objects that blink or flash can cause photosensitive epileptic seizures in susceptible individuals, particularly if the flash has a high intensity and is in the frequency range between 2 Hz and 55 Hz. This includes flashing text, turning graphics on and off or repeatedly changing between different images on the screen.
Required development techniques
Use one or more of the following techniques to meet Checkpoint 5.2 from the IBM Software Accessibility Checklist:
- If the application must use blinking elements, use frequencies that are less than 2Hz or greater than 55Hz.
Examples of required development techniques
Developers must retrieve the blink rate from the system setting. Windows allows users to customize the cursor blink rates through the Control Panel Accessibility Options. Developers can access these settings through the GetCaretBlinkTime function to emulate the system's cursor blink rate for custom cursors or for customized user interface elements. For more information on this technique, see the article "Flashing User Interface and the GetCaretBlinkTime Function" (link resides outside of ibm.com) from Microsoft.
Recommended development techniques
The techniques above are required; the following techniques are recommended to enhance accessibility:
- Minimize the area of the screen which is flashing. Smaller areas are less likely to cause seizures.
- Avoid flashing that has a high level of contrast between the states. Some individuals are more susceptible to high intensity flashing.
Required test techniques
Test the software to ensure that it complies with accessibility requirements.
Required test software
No additional software is required for this test. A program that displays the time interval between blinking or flashing text may be used if you are unable to estimate the blinking rate of flashing elements.
The following techniques are required to verify this checkpoint:
|1||Manually verify that the software does not use blinking or flashing elements.||
The software does not implement blinking text or objects.
The software displays blinking text or objects. If the software has blinking elements, perform the next test to see if the blinking is between the range of 2 Hz to 55 Hz.
|2||Display the blinking elements in the software. Run a program that displays the time interval between blinking text.||
The time interval of the blinking text is less than 2 Hz or greater than 55 Hz.
The time interval of the blinking text is between 2 Hz and 55 Hz. This interval can cause photosensitive epileptic seizures in susceptible individuals.
It is somewhat difficult to test the exact rate that something is blinking or flashing. These guidelines provide a practical method of deciding if flashing content is within the 2 to 55 Hz range
For slow blinking items: Watch the blinking item, and count the number of blinks that occur in one second. (Or count the number of blinks in 10 seconds, and divide by 10). Verify that no more that two blinks per second occur.
For fast blinking items: Most people cannot see a blink rate of more than about 60 Hz. (This is why most computer screens have a refresh rate of 60Hz or faster.) For anything that flashes at a rate of about 55 Hz or more, you will probably not be able to tell that it is flashing at all. Therefore, verify that your eyes cannot perceive any blinking or flickering for the element, unless it is blinking at a rate of 2 times per second or slower.
Any noticeable blinking or flashing elements are blinking at a rate of 2 times per second or slower.
Blinking elements can be visually detected and they are blinking faster than 2 times per second.
©2009 IBM Corporation
Last updated October 07, 2009.