How to: Flash lights and accommodate people with photosensitive epilepsy

I read about Liana Ruppert having a seizure, triggered while playing Cyberpunk 2077 for review. CD Projekt Red was quick to add a warning and a fix, but there are good practices to minimize risk in the design phase. There wasn’t a clear guide when I needed one for the SPRK project, so I’m making this: For anyone flashing lights for effects, grabbing attention, or any other reason—in person or on screen—here is what the Epilepsy Foundation and IEEE lay out:

For screens, aim to meet the British Independent Television Commission (ITC) guidelines¹ at a minimum:

1. Use fewer than 3 pulses per second (Hz)

2. Flashes should affect less than 25% of the screen

Assuming the initial version Liana played is similar to this IGN preview⁴, the flashing effect can cover most of the screen, and I counted ~11 Hz.

Cyberpunk 2077 flashing lights

Multiple factors contribute to and compound the risk^2,3. Though flash rate (#1) and visual field (#2) are the papers’ most mentioned considerations, if meeting one isn’t possible—say for a stage’s front-row audience—further risk reductions include:

3. Add background lighting to reduce light contrast to under 10%²

4. Avoid light-dark stripes^1,2

5. If #4 isn’t possible, limit to five moving stripes or eight stationary stripes¹

6. Avoid deep or saturated red^1,2

7. Move light away from central vision to the periphery²

8. Stay under the brightness threshold of 20 nits¹, or for further risk reduction, 1 nit²

I don’t like “The best way to win is not to play”-type guidance, because it implies that no one should ever try. In the case of the SPRK project, the flashing pattern I devised is what makes it immediately recognizable as a flare (important when someone is driving 70+ MPH). However, putting the work into an intricate chase or gradient pattern can achieve a better effect than simple flashing, so if that’s achievable, seriously consider this mitigation:

9. Don’t flash lights

All the best to CD Projekt Red and their first week with Cyberpunk 2077’s release. As I was looking for articles and screenshots, I found that this goes far beyond them and this release.

Be good to your users.