Epileptic Safe Flash Hz

Epileptic-safe flash frequency (Hz)

Flashing or strobing content can set off a photosensitive seizure. WCAG 2.1 success criterion 2.3.1 (Level A) — Three Flashes or Below Threshold says a web page must not contain anything that flashes more than three times in any one-second window, unless the flash stays below the general flash and red flash thresholds. The rule itself is short: flashes_per_second ≤ 3 Hz. Anything from 3 Hz up to 30 Hz lands in the danger zone, and that means checking it against luminance and area thresholds too.

The best-known case is the Pokemon "Dennou Senshi Porygon" episode that aired in Japan on December 16, 1997. A run of red/blue flashes at around 12 Hz put 685 children in the hospital with photosensitive seizures, vomiting and convulsions. The W3C later released the PEAT (Photosensitive Epilepsy Analysis Tool), a free desktop app that goes through video files frame by frame using the Harding flash and pattern algorithm that broadcasters rely on around the world. Brazil's ABNT NBR 17060 standard carries the same thresholds over into digital accessibility.

Applications

Run animated banners, video intros, CSS keyframe animations, game cutscenes, high-contrast loading strobes and any auto-playing media through this calculator. When a flash sits above 3 Hz, you have a few options: drop the frequency, soften the luminance delta, shrink the flashing area to under 25% of a 10° visual field, or swap the strobe for a fade. And always give people a pause control they manage themselves, plus support for the prefers-reduced-motion media query.

FAQ

Is 3 Hz a hard ceiling? It is the general threshold. Flashes at or below 3 Hz are taken as safe. Go above it and you also need to check luminance and the red-flash thresholds with PEAT.

Why is the 3–30 Hz range called the danger zone? Most photosensitive seizures fire between 15 and 25 Hz. The protected range still stretches from 3 to 30 Hz to stay on the safe side, since sensitivity differs a lot from person to person.

Does this apply to CSS transitions? It does. Any quick back-and-forth of colors, opacity or transforms counts as a flash once the luminance change gets big enough. Lean on easing curves rather than discrete keyframes.