prism docs · Getting started · Flash safety

Flash safety

Rapid flashing light can trigger seizures in people with photosensitive epilepsy, and a projected wall fills far more of someone's view than a screen. So prism holds every projector to at most 3 full-frame flashes a second, unless you switch the limit off.

The limitAt most 3 flashes a second (WCAG 2.3.1)

DefaultOn, for every projector

WhereDesign, nothing selected ▸ Limit flashes to 3 per second

ModeSimple all projectors at once
Advanced each projector (P1 ▸ Safety)

Actual limiter status 1 · Why it matters 2 · What the limit does 3 · Where to switch it 4 · Which looks are capped 5 · What isn't limited Hints and tips When something looks wrong

1Why it matters

For some people, light that flashes several times a second can trigger an epileptic seizure. Many don't know they're sensitive until it happens. The web's accessibility guidelines set a simple rule for this, WCAG 2.3.1 “Three Flashes”: nothing should flash more than three times in any one second.

Projection makes this matter more. A strobe on a phone is a small patch of light; a strobe across a wall, a stage or a room fills the whole view of everyone in it.

2What the limit does

prism watches each projector's picture as it is drawn, frame by frame.
A sudden jump in the overall brightness counts as a flash. Up to 3 may pass in any second, at least 0.3 s apart.
A flash that would come too soon is held back: the picture is dimmed until it may pass. The rhythm of a fast strobe becomes a gentle flicker.
Slow fades aren't flashes, and a picture getting darker is never held back. A steady strobe at 3 a second passes untouched.

Why on the graphics card. The check happens in the same frame it measures, on the graphics card, so it catches a flash before it reaches the wall rather than after.

3Where to switch it

Every projector at once

In Design, click an empty part of the canvas (or press Esc) so nothing is selected.
On the right, under Output, is Limit flashes to 3 per second. It applies to every projector.

The project's settings: Name, then Output with Brightness, Gamma and Limit flashes to 3 per second switched on — Nothing selected: the project's settings. Limit flashes to 3 per second is the last item under Output; its ? explains why.

Advanced One projector

Each projector keeps its own switch. In Advanced, click the projector's button in the canvas toolbar (P1): its settings end with Safety ▸ Limit flashes to 3 per second.

The projector's Safety section with the flash limit and its explanation — P1 ▸ Safety. The explanation says what is held back: faster strobes, beat flashes and triggers.

Switch it off only for an audience that has been warned. If a show really needs fast strobes, tell people before they come in (a sign at the door is common practice), and consider a version of the show without them.

4Which looks are capped

Look or action	With the limit on
Strobe	Flashes 3 times a second by default, and never faster while the limit is on.
Beat Strobe	Keeps to the beat but skips beats when they come too fast: twice a beat becomes once a beat above 90 BPM, and every other beat above 180 BPM.
Glitch, Lightning, looks that flash on drum hits	Held back by the projector's limit when they would flash too often.
Flash triggers (Tap, “Flash white”)	Held back the same way if you hit them faster than 3 a second.
Everything else	Checked all the time: any look, video or mix that would flash too fast is held back.

5What isn't limited

Blackout and the test patterns: you can always cut to black or check a projector.
The laptop's Camera view in Design: it is your control screen, not a projector. The Projector view and every projector window are limited.

Very old graphics. The limit needs graphics hardware that can draw into floating-point images. On the rare machine that can't, it switches itself off (it notes this in the browser's developer console).

Check the running output

The Flash limit switch requests protection. The status beside it and in the top-bar status panel reports what each output window is actually doing.

Status	Meaning
Active	The output's limiter is running during ordinary content playback.
Ready for show playback	The required graphics support is available, but normal content playback is not currently being limited; for example, the window is displaying a calibration pattern.
Switched off	The setting is disabled for this output.
Unavailable	The necessary graphics feature is unavailable, or the graphics context has been lost. The enabled switch does not make this output protected.
Open projector to check / not confirmed	No current output report. Open or reopen the output window, especially after updating an older running window.

Scan and test patterns are not normal limited content. Recheck the status after restoring a window or GPU context. Limiting is a risk-reduction tool, not a guarantee that a show is safe for everyone; avoid aggressive flashing in your content. See Before a show.

Hints and tips

Design with the limit on. Pulses, chases and colour changes on the beat look great without strobing.
Brightness matters too. A full-white flash is the harshest kind; darker flashes and coloured ones are gentler.
Test the whole show at full brightness before an audience sees it, and watch for anything that flickers.
Tell the venue if the show has strong light effects, even with the limit on.

When something looks wrong

You see	Why	Fix
A strobe flickers dimly instead of flashing	It flashes faster than 3 a second, so the limit holds it back.	Slow it down to 3 a second or less; that's working as intended.
Beat Strobe skips beats	At high tempos, flashing on every beat would pass the limit.	Expected; above 180 BPM it flashes every other beat.
The limit is off and you didn't switch it off	A project or projector from someone else had it off.	Switch it on again in the project settings (and, in Advanced, on each projector).
Flashes are still too intense	3 bright flashes a second can still be a lot.	Lower the output brightness (nothing selected ▸ Output ▸ Brightness), or use softer looks.

prism documentation · Getting started · screenshots taken in prism's simulator · 28 Sept 2026