Skills for Access

Why this is important

While any online content viewed on a monitor where the display is generated by a cathode ray tube is, by its nature, flashing, refresh rates of the monitor itself should be imperceptible to the end user. However, for people who have photosensitive epilepsy, media content that flickers or flashes at specific frequencies, or rapidly moves between light and dark shades (strobe-style flashing) may trigger seizures, and steps must be taken to ensure that these people are not unexpectedly exposed to flashing content at these danger frequencies.

General Principles

There are two accessibility solutions for this barrier. The first option is to avoid providing media content that flashes or flickers within the danger rate of 2 and 59 Hertz (flashes per second), and to avoid using rapidly flashing alternate light and dark shades. The figures of 2 to 59 Hertz are based on the figures of 4 to 59 Hertz specified by the W3C Web Content Accessibility Guidelines), and 2 to 55 Hertz specified in the Section 508 Standard referred to in the US Rehabilitation Act.

The second option is to provide any content that does flash within this range on a page linked from another page warning that flickering content is present. This gives people who may be at risk of experiencing seizures the option of avoiding the flickering content.

Before you continue

The advice on this page helps you avoid introducing a specific accessibility barrier, but it's not a magic formula. To avoid attempting to follow a technical solution that is not appropriate to the resource and its intended purpose, you need to know the context in which the multimedia resource is being used:

1. The purpose or aim of the multimedia resource in question, and whether it is being used to supplement another resource in the learning environment, or whether its use is required by students.
2. The target audience, their knowledge and expectations, and the type of browsing and assistive technology that they may be using.
3. Whether the information and experiences provided by the multimedia technology are already available in an equivalent, alternative form.

For more background on this approach, see our Guide to the use of multimedia in accessible e-learning.

Technique Details

If flashing media must be used, for example in a video clip or animation, then do not automatically launch the media without providing a warning (text and/or audio) that flashing content is provided. For a web site, this warning could be provided in text on the page from which the media is accessed; for a kiosk based application, a warning may be more challenging, particularly if the media is on a loop. The warning may need to be provided on the kiosk itself, or be present (in text and audio) on the media presentation itself in advance of the appearance of the flashing content.

Testing

Tools do not yet exist to automatically test the flicker rate of media content, although in drafts of version 2.0 of the Web Content Accessibility Guidelines, the W3C suggests that such a tool is in development at the University of Wisconsin. Without an automated tool for diagnosis, human judgement is required to ensure this issue has been addressed.