Skip to Content
NASA - Goddard Space Flight Center    + NASA HomePage
Goddard Section 508 Web Best Practices
NASA HomePage Goddard Space Flight Center HomePage
 
Learning About Section 508 Developing for Section 508 Testing for Section 508
+ Home
 
Developing for Section 508
Section 508 Living Examples
Section 508 Checklist
 
Living Examples - Flicker
(j) Pages shall be designed to avoid causing the screen to flicker with a frequency greater than 2Hz and lower than 55 Hz.

Animated .gif files and videos that flash the same image, or a rapid succession of similar images, can be considered repeated flashing images. Users with photosensitive epilepsy are subject to seizures triggered by such flickering or flashing effects, and even the average user can suffer headaches or dizziness from rapid flashing that can come from animation or video.

Simply put, developers shall not use repeatedly flashing images where the rate is 2Hz (Hertz) to 55Hz (Hertz) flashes per second.

Strobe effects can be described as abrupt changes from light to dark. The more common definition of a strobe effect is when those abrupt changes are repetitive. Users with photosensitive epilepsy are subject to seizures triggered by strobe effects, and even the average user can suffer headaches or dizziness from abrupt color changes.

NOTE: The naturally occurring 'flicker' caused when moving from one page to another is not considered a strobe effect, and therefore does not have to be addressed.

Developers shall not use strobe light effects - abrupt changes from light to dark - or use flashes in any color range at a rate of 2Hz (Hertz) to 55Hz (Hertz) flashes per second.

Users with photosensitive epilepsy are most vulnerable to flicker rates between 2Hz (Hertz) to 55Hz (Hertz) flashes per second range.

NOTE: The naturally occurring 'flicker' caused by the browser when moving from one page to another does not cause a flicker rate between 2Hz (Hertz) and 55Hz (Hertz), and therefore does not have to be addressed.

Guidelines When in doubt about the flicker rate, developers should avoid incorporating any type of flickering mechanism.

Keep in mind that flicker is not the rolling line or similar effect of the 60Hz (Hertz) transmission frequency sometimes seen on older, weaker monitors.
+ Privacy Policy and Important Notices NASA logo

Curator: Courtney Ritz
Responsible NASA Official: Joe Stevens
Last Updated: March 6, 2009

NASA HomePage Goddard Space Flight Center HomePage