{"id":193,"date":"2019-06-04T13:15:04","date_gmt":"2019-06-04T17:15:04","guid":{"rendered":"https:\/\/pressbooks.library.ryerson.ca\/iwacc\/?post_type=chapter&p=193"},"modified":"2019-07-30T11:09:08","modified_gmt":"2019-07-30T15:09:08","slug":"2-3-seizures-and-physical-reactions-level-a","status":"publish","type":"chapter","link":"https:\/\/pressbooks.library.torontomu.ca\/iwacc\/chapter\/2-3-seizures-and-physical-reactions-level-a\/","title":{"raw":"2.3 Seizures and Physical Reactions (Level A)","rendered":"2.3 Seizures and Physical Reactions (Level A)"},"content":{"raw":"
\r\n

Guideline 2.3<\/strong> Seizures and Physical Reactions<\/h2>\r\nDo not design content in a way that is known to cause seizures or physical reactions.\r\n\r\n<\/div>\r\nWhat do I need to know so that content does not cause seizures?<\/strong>\r\n\r\nPeople who have epilepsy can have seizures when exposed to flashing or flickering lights. There are three causes of flickering lights on computer screens:\r\n
    \r\n \t
  1. Flashes can be caused by the display.<\/li>\r\n \t
  2. Flashes can be caused by the computer and how it renders images and other content.<\/li>\r\n \t
  3. Flashes can be caused by the content itself.<\/li>\r\n<\/ol>\r\nAlthough web authors have no control over the first two, they can ensure that flicker is not caused by the content, such as a movie of strobe flashes or an animation of rapid-fire explosions.\r\n\r\nTo conform at Level AA, WCAG 2.1 describes how to determine safe values for flashing content. To conform at Level AAA, a page must avoid all flashing content.\r\n
    \r\n

    Success Criterion 2.3.1<\/strong> Three Flashes or Below Threshold<\/h3>\r\nLevel A\r\n\r\nWeb pages<\/a> do not contain anything that flashes more than three times in any one second period, or the flash<\/a> is below the general flash and red flash thresholds<\/a>.\r\n\r\n<\/div>\r\n

    Three Flashes or Below Threshold Explained<\/h4>\r\nPeople with a form of epilepsy called photosensitive seizure disorder have seizures when exposed to flashing or flickering lights. Many colours can cause these seizures, but flashing red lights are known to trigger seizures more readily than other colours, and the larger the area flashing, the greater the possibility of inducing a seizure.\r\n\r\nTo meet this requirement, web authors must ensure that flickering is not caused by the content itself, such as a movie of strobe flashes or an animation with rapidly switching colours.\r\n\r\nSC 2.3.1<\/strong> allows content to flash if it is dim enough and confined to a small area. W3C publishes formulas for determining safe values for \u201cgeneral flash\u201d and \u201cred flash\u201d thresholds. But even these so-called safe values can trigger seizures. Research suggests the greatest sensitivity to flashing is between 10 hz and 25 hz. The next section, SC<\/strong> 2.3.2<\/strong>, is more stringent.\r\n
    \r\n

    Try This: Flashing Screens<\/strong><\/h4>\r\nHumans can only detect flashing (i.e., the flicker rate) up to about 50 hz (or 50 flashes per second), after which a screen or content appears continuous. You may recall seeing computer or television screens flickering or the rolling bar that cycles across the screen in cases where a monitor is being viewed in a video or TV program. The cause is the difference in the flash rate of the monitor and the flash rate of the camera recording the monitor.\r\n\r\nEven though you cannot see it due to the flash rate being faster than the eye can detect, all screens do flicker.\r\n\r\nSee the flash of a video recorded monitor:\r\n\r\n\r\n\r\n[embed]https:\/\/youtu.be\/Fp9u6D5mvfw[\/embed]\r\n\r\nVideo: <\/strong>How to make screens look good on camera<\/a> by Kyle Lawrence\r\n\r\nMovie and television producers are now quite aware of flashing screens. Technology is used to prevent it from occurring, which is why you may need to go back to fairly old movies or television programs to see it happening.\r\n\r\n<\/div>\r\n
    \r\n\r\nSuggested Reading:<\/strong>\r\n