OneTaskBy Tim King · Updated
Flash and Flicker Checker
Count General and Red Flashes Per Second Against the WCAG Three-Flash Limit
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How the Flash and Flicker Checker works
This flash and flicker checker scans a video for the rapid flashing that can trigger seizures in people with photosensitive epilepsy. It counts general and red flashes in every second of the clip, compares them with the WCAG three flashes threshold and shows exactly where the video goes over it.
How the flash and flicker checker measures flashing
The video is sampled up to 30 times a second and each frame is split into a grid. For every cell the tool works out relative luminance, the same measure used in WCAG, and watches for swings of at least 10% where the darker state is below 80%. A swing one way and back again is one flash. When enough of the frame flashes together, the flash is counted.
The three flashes rule
WCAG 2.3.1 and broadcast guidance such as Ofcom's say content should not flash more than three times in any one second unless the flashing area is small. The tool slides a one-second window across the clip and reports the worst second. It also checks saturated red flashing separately, because transitions to and from deep red are considered especially risky.
Reading the chart
Each bar is one second of video. Blue bars are within the limit, red bars go over it, and the dashed line marks three flashes. Download the CSV to get a row for every second with general and red flash counts and a pass or fail.
Area
The default counts a flash only when at least a quarter of the frame changes, a common screening approximation of the WCAG area rule. Lower it to be stricter for footage watched close up or full screen.
FAQ
Does a pass mean my video is safe for broadcast?
No. This is a free screening check that catches most problem flashing, but broadcasters require a certified analyser such as Harding FPA. Use this tool to find problems early and fix them before formal testing.
What should I do if part of the video fails?
Find the failing seconds in the chart, then slow the flashing down, reduce the contrast between the light and dark frames or shrink the flashing area. Trimming the section out entirely is the quickest fix, then run the check again.
Is my video uploaded?
No. Every frame is analysed by your browser on your own device and nothing is sent to a server, so unreleased footage stays private while you check it.
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