whitescreen.tv
← Blog

Flashing prank screens and photosensitive epilepsy: how to prank safely

· 9 min read · The WhiteScreen.tv team

A red warning triangle with an exclamation mark on a dark striped background
The Virus Screen prank. Its alert glow is slow and soft; the table below shows what we found in every prank.

This article is general safety information, not medical advice.

A good screen prank makes someone jump, laugh, and then forgive you. It should never put anyone in hospital. For a small number of people, rapidly flashing images can trigger a seizure, so before you run a flashing effect on a colleague's monitor or a party projector, it is worth knowing which effects flash and how to use them safely.

We went through every prank on WhiteScreen.tv to find out. Most do not flash at all. One effect, the Glitch Screen, could flash faster than the standard guideline, so we changed it. It is still a fast, high-contrast effect, so the advice below still applies.

Photosensitive epilepsy in brief

Photosensitive epilepsy is a form of epilepsy in which seizures are triggered by flashing or flickering light, or by some high-contrast patterns.

  • It is uncommon. The UK's Epilepsy Society says "around 1 in 100 people has epilepsy and, of these people, around 3% have photosensitive epilepsy" (Epilepsy Society). A 2005 review for the Epilepsy Foundation of America put the prevalence of seizures from light stimuli at about 1 in 10,000 people, or 1 in 4,000 among those aged 5 to 24 (Fisher et al., 2005).
  • It mostly affects young people. "Most people who develop photosensitive epilepsy are between the age of 7 and 19 years old", and it is more common in women than men (Epilepsy Society).
  • Some flash rates are riskier than others. "Frequencies of 15-25 Hz are most provocative, but the range is 1-65 Hz", and red color is also a factor (Fisher et al., 2005). The Epilepsy Society gives 3 to 30 flashes per second as the common range.
  • People often do not know they are at risk. In the best-known case, an episode of the Pokémon cartoon broadcast in Japan in 1997 sent 685 children to hospital after a sequence of red and blue flashes (Fisher et al., 2005); most seizures happened at a scene where red and blue frames alternated 12 times a second (Takada et al., 1999). Only 24% of the children who had a seizure during the cartoon had ever had one before.

That last point is why "nobody here has epilepsy" is not a complete safety check.

The three-flashes rule

Broadcasters and web designers work to the same basic limit: no more than three flashes in any one second.

The web standard, WCAG 2.2 success criterion 2.3.1, says: "Web pages do not contain anything that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds" (W3C). The detail matters:

  • A flash is a pair of opposite changes in brightness, bright then dark or dark then bright, big enough to matter: a change of at least 10% of the maximum relative luminance, where the darker state is below 0.80 (W3C Understanding 2.3.1).
  • Small flashes are allowed. Flashing is acceptable if it covers less than about a quarter of any 10-degree area of the visual field. On a 1024 x 768 screen, W3C uses a 341 x 256 pixel rectangle as the test area.
  • Red is treated separately. Any pair of opposing transitions involving a saturated red counts as a red flash, whatever the brightness change.

UK television follows the same idea. Ofcom's guidance says flashing is not permitted when the flashes cover "more than one quarter" of the screen and there are "more than three flashes within any one-second period" (Ofcom). It also limits regular patterns of "more than five light-dark pairs of stripes in any orientation", and warns that flashing lasting more than five seconds "might constitute a risk even when it complies with the guidelines". The international broadcasting recommendation, ITU-R BT.1702, sets out the same limits (ITU). Ofcom is candid about what the rules can achieve: "it is therefore impossible to eliminate the risk".

Which of our pranks flash

We read the code of every prank and timed every repeating animation. For the one effect that changes quickly and unpredictably, the Glitch Screen, we also recorded it frame by frame in a 1024 x 768 browser window and counted general and red flashes in every 341 x 256 area, following the WCAG definitions, at 60, 120 and 144 frames per second. That is our own approximation of the test, not a certified analysis with a tool such as the Trace Center's PEAT.

PrankWhat moves, and what we found
Glitch Screen, Chaos styleRandom color bursts, some across the whole screen and some red, plus tearing and static. Before our fix, it went over the limit from intensity 2: on a 60 Hz display we counted about 3 to 4 flashes per second at intensity 2 and about 7 at intensity 5, and more on faster displays. Now the bright bursts share one timer and are at least half a second apart, so we counted at most 2 flashes per second at every intensity, at 60, 120 and 144 Hz.
Glitch Screen, Digital styleColored blocks and torn lines. Before the fix it reached up to 3 flashes per second at maximum intensity on a 120 Hz display. Now we counted at most 1.
Glitch Screen, Analog VHS styleScanlines and faint static. No flashes found, before or after.
Virus ScreenA soft colored glow over the dashboard fades in and out over a fraction of a second, automatically about once every 30 to 55 seconds, and when you press some keys. Do not hammer the arrow keys.
FBI Lock ScreenThe red warning banner blinks once per second. It is a banner, not the whole screen, and it stops blinking if the device asks for reduced motion.
Hacker TyperThe cursor blinks and the ACCESS GRANTED banner glows, each once per second.
White NoiseAnimated TV static. Every pixel changes every frame, but the overall brightness stays constant. WCAG does not count very fine noise as flashing, though some people find static uncomfortable to watch.
DVD ScreensaverThe logo changes color when it bounces. No flashing.
Broken Screen, Fake Blue Screen, Fake Windows Update, Fake macOS Update, Matrix, Fake Terminal HackStill images, progress bars, scrolling text or falling characters. No flashing.

Why did the old Glitch Screen flash more on fast displays? It decided on every animation frame whether to fire a burst, so a 120 Hz screen got twice as many chances per second as a 60 Hz one. The new version is timed by the clock instead of by frames, so every display behaves the same. It also calms down, with fewer bursts and no full-screen color, when your device asks for reduced motion.

A cap is not a guarantee. The guidelines themselves say that no limit removes all risk, and the Glitch Screen is still fast, high-contrast and full of movement. Treat it with care around anyone who is photosensitive.

How to prank safely

  1. Choose a non-flashing prank when in doubt. The Broken Screen, the fake updates and the Fake Blue Screen are just as convincing and do not flash. If you know or suspect that someone in the room has epilepsy or is sensitive to flicker, use one of those.
  2. Keep fast effects off big screens. Larger images fill more of the visual field, so the same effect matters more on a TV or projector than on a laptop. If you use the Glitch Screen on a big screen, pick Analog VHS at a low intensity, or choose a still prank instead.
  3. Keep the room lit and keep your distance. The ITU recommendation advises watching "in a well-lit room from a distance of at least two metres" (ITU). Glitch effects in a dark room at close range are the riskiest combination.
  4. Keep it short. A few seconds is enough to get a reaction, and long flashing sequences add risk.
  5. Watch the person, not the screen. If anyone looks unwell, confused or dazed, end it immediately with Escape. The Epilepsy Society notes that "covering one eye completely with your hand can reduce the photosensitive effect", and advises not to close the eyes, as that can cause a flicker effect (Epilepsy Society).

If someone has a seizure

The NHS advice is to:

  • only move the person if they are in danger;
  • cushion their head if they are on the ground, and loosen any tight clothing around their neck;
  • note the time the seizure starts and finishes;
  • turn them on to their side once the convulsions stop, and stay with them, talking calmly, until they recover.

"Do not put anything in their mouth, including your fingers" (NHS). And do not hold them down; the Epilepsy Society's advice is to "allow the seizure to happen" (Epilepsy Society).

Call an ambulance (999 in the UK, 911 in the US) if it is their first seizure, if it lasts more than five minutes when you do not know how long their seizures usually last, if they do not fully wake up or have another seizure, if they are seriously injured, or if they have trouble breathing afterwards (NHS, CDC).

In short

Most of our prank screens are still images or slow animations. The Glitch Screen was the exception; it is now capped below the three-flashes limit, but it is still the fastest effect we have. Pick a calm prank when you are not sure about your audience, keep the room bright and the prank short, and know the first-aid basics. A prank is only funny if everyone is fine at the end of it.

Sources

More from WhiteScreen.tv: the screensaver collection and the changelog. How we source and check articles: editorial standards.