Tutorials Accessibility

Flashing Content and Photosensitivity: What to Avoid

By Infinite Creation · Intermediate · ~15 min

A 1960s mid-century comic book style editorial illustration showing an accessibility specialist and a colleague testing with a screen reader, with a colour grading panel with trackballs, a camera on a tripod visible in the frame.

Overview

Flashing imagery can trigger seizures in people with photosensitive epilepsy, and unlike most accessibility issues the consequence is medical rather than inconvenient. It is also one of the easiest problems to avoid once you know what causes it. This guide covers what actually creates risk (rate, area, contrast, and colour) the thresholds used in broadcast and web guidance, and a practical check to run before publishing anything with strobes, transitions, or rapid cutting.

What You Need

Any video containing flashing, strobing, or rapid cutting
A way to review it at full size rather than in a small preview
Awareness of where it will play, autoplay contexts raise the stakes
Willingness to change an effect you like if it fails the check
A warning card if flashing is essential to the piece
No specialist equipment. The thresholds are about content, not tools

Steps

Identify anything that flashes more than three times a second

The widely used threshold across web and broadcast guidance is three flashes in any one second period. Below that, risk is substantially lower. Above it, risk rises sharply. Scan your edit for strobe effects, rapid cuts between very different brightness levels, flickering graphics, and camera flashes at press events.

Consider how much of the screen is affected

A small flashing element in a corner is far less risky than a full-frame strobe, because risk scales with how much of the visual field is involved. Guidance defines a small central area of the screen as the region that matters most. A full-screen flash is the worst case and should simply be avoided.

Watch the contrast of the transition, not just the rate

What matters is the size of the luminance change. Alternating between two similar mid-tones many times per second is much less hazardous than alternating between black and white. If you need a rapid effect, reducing its contrast is often enough to bring it into safe territory while preserving the intent.

Treat saturated red as a special case

Transitions to and from saturated red carry elevated risk relative to other colours at the same brightness, and guidance singles them out. If an effect flashes red, it deserves more caution than the same effect in another hue. This is the detail most often missed.

Reduce rather than remove where you can

Most flashing effects survive being slowed below three per second, reduced in contrast, or confined to a smaller area. The creative intent (energy, impact, a transition) is usually preserved. Outright removal is rarely necessary once you know which variable to adjust.

Warn clearly if flashing is genuinely essential

If the content cannot work without it (documenting a real event, or a piece where the effect is the point) put a warning at the very start, before the flashing occurs, and in the description. A warning that appears after the fact serves nobody, and one buried in the description alone will not be seen in an autoplay context.

Pro Tips

Autoplay raises the stakes considerably, because the viewer had no chance to decide before it started.
Camera flashes at press events and concerts are a common accidental source: check event footage specifically.
Lowering contrast is usually a better fix than lowering rate, because it preserves the feel of the effect.
Small preview windows hide the problem. Review at full size before deciding an effect is safe.
When in doubt, slow it down. Almost no effect is improved by being above the threshold.

Tested: the three flash rule, contrast and red, and the site's own checker

We tested the page's claims on 2026-09-29 against the WCAG 2.2 definition of a flash, which we read on the W3C's own page, and against the site's Flash & Photosensitivity Checker. The clips are computer-generated frame sequences at 60 frames per second, four seconds each, alternating between two states. No real video, no video codec and no browser was involved. A flash is two opposing transitions, so one full cycle is one flash. The definition passes content with no more than three general flashes and no more than three red flashes in any one second.

Clip	By the WCAG definition, busiest second	Site checker (fixed 6 October 2026), busiest second
Black to white, 1 per second	1 general (passes)	1 general (passes)
Black to white, 2 per second	2 general (passes)	2 general (passes)
Black to white, 3 per second	3 general (passes)	3 general (passes)
Black to white, 4 per second	4 general (over)	4 general (over)
Black to white, 5 per second	5 general (over)	5 general (over)
Black to white, 6 per second	6 general (over)	6 general (over)
Grey step of 0.05 in luminance, 4 per second	0 general (passes)	0 general (passes)
Grey step of 0.08 in luminance, 4 per second	0 general (passes)	0 general (passes)
Grey step of 0.12 in luminance, 4 per second	4 general (over)	4 general (over)
Grey step of 0.30 in luminance, 4 per second	4 general (over)	4 general (over)
Light grey to white, both above 0.80, 4 per second	0 general (passes)	0 general (passes)
Saturated red to grey of equal luminance, 4 per second	0 general, 4 red (over)	0 general, 4 red (over)
Saturated red to dark grey, 4 per second	4 general, 4 red (over)	4 general, 4 red (over)
A white patch on black, 8 percent of the frame, 6 per second	6 general (over)	6 general (over)

The three per second rule holds as the page states it. Black to white at 1, 2 and 3 flashes per second passed and at 4, 5 and 6 failed. Three per second passes with no margin, since a fourth flash in the same second fails, which is why the page calls it a ceiling.

Contrast matters as step 3 says, and the line falls where the definition puts it, at a change of 10 percent of full luminance. A step of 0.05 and one of 0.08 at 4 per second passed. Steps of 0.12 and 0.30 failed. The definition adds a second condition the page does not mention: if both states are lighter than 0.80 in relative luminance the change is not a flash. Light grey to white, 0.85 to 1.0, at 4 per second passed for that reason.

Saturated red is a separate test, and it can fail a clip that has no luminance flashing at all. A saturated red alternating with a grey of the same luminance, 4 per second, had no general flashes and 4 red flashes, so it fails on colour alone. This is step 4's point that red is a special case. The red test used is Note 3 of the definition: one state with R/(R+G+B) of 0.8 or more and a chromaticity difference above 0.2.

The site's checker, linked from step 1, did not work when we started. Its two main functions had been written onto the end of a comment line, which made the whole script a syntax error, so the page loaded and the tool did nothing. That had been the case since the file was first committed. It was fixed on 6 October 2026 and then rewritten, and the right hand column is from the rewritten code.

Before the rewrite the checker was stricter than the definition and missed red. It counted every change of direction, so a full cycle counted as two and black to white at 2 per second read as 4. It missed the red to grey clip completely, because it decided on luminance alone, and it judged the whole frame, so a white patch covering 8 percent of the frame at 6 per second counted nothing, though Note 1 of the definition puts the area allowance at 25 percent of a 10 degree field, about 2.8 percent of a 1024 by 768 screen. The rewritten checker follows each 4 by 4 pixel block of a small copy of the frame on its own, counts pairs of opposing transitions, counts a flash only where at least about 2.8 percent of the frame shares it, and tests saturated red by the ratio and chromaticity rule above. On these same clips it now matches the definition in every row, including the red to grey clip (0 general, 4 red) and the 8 percent patch (6 general). A patch of 1.7 percent at 6 per second passes, as the allowance says it should.

Validated. Run 2026-09-29. The definition is read from W3C's WCAG 2.2 glossary entry for the general flash and red flash thresholds. The general flash and red flash counts are our own implementation of it on generated frames, not a certified tool, and the area allowance was applied by arithmetic for one patch only. The rewritten checker's detection code was run in Node on generated frames, so the tool in a browser, with real video, was not run. This is a screening test of a screening tool. Photosensitive epilepsy is a medical matter, and nothing here replaces a broadcast analysis tool or a clinician's advice. We did not test step 1's claim that risk is lower below three per second, or how any clip looks.

Knowledge Base

What You'll Learn

This is the one accessibility topic where getting it wrong can physically harm someone, and it is also among the easiest to get right. Below: what creates risk, and what the thresholds actually say.

What Actually Creates Risk

Photosensitive seizures are triggered by specific visual patterns rather than by "bright content" in general, and four variables interact.

Rate is the most cited. Risk rises in a band of flash frequencies, with common guidance drawing the line at three flashes per second. Faster is not automatically safer.

Area matters because risk scales with how much of the visual field is involved. A full-screen flash affects far more of the retina than a small element.

Contrast, the magnitude of the luminance change, is what makes a flash a flash. Low-contrast alternation at the same rate is substantially less hazardous.

Colour, specifically transitions involving saturated red, carries elevated risk independent of the other three, which is why guidance calls it out separately.

Regular high-contrast striped patterns can also trigger symptoms, which is worth knowing if you work with graphic patterns or moiré-prone fabrics on camera.

Where the Thresholds Come From

The three-flashes-per-second threshold appears in both web accessibility guidance and broadcast regulation, and its prominence has a specific history: a 1997 television broadcast in Japan triggered seizures in a large number of viewers simultaneously, which prompted broadcasters internationally to formalise limits that had previously been informal.

Broadcast guidance is generally the more detailed, specifying limits on flash rate, the proportion of screen area, luminance change, and saturated red transitions, along with rules about regular patterns. Web guidance is simpler and expressed as a pass or fail condition.

For anyone publishing online rather than broadcasting, the practical position is that the thresholds are a floor rather than a target. Staying comfortably below them costs almost nothing creatively, and the downside of being marginally over is not a compliance finding. It is someone having a seizure.

Where This Fits

A flash check belongs in the same pre-release pass as captions and contrast, and Publishing Accessible Video lays that pass out in order.

FAQ

Q: What is the flashing threshold I should stay under?
A: Three flashes in any one-second period is the widely used line in both web and broadcast guidance. Treat it as a ceiling to stay well below rather than a target to approach, and remember that area, contrast, and saturated red all affect risk independently of rate.

Q: Do rapid cuts count as flashing?
A: They can, if consecutive shots differ substantially in brightness and the cutting is fast enough. A rapid montage alternating between very dark and very bright shots produces the same luminance pattern as a strobe. Check fast-cut sequences specifically, since they are rarely thought of as flashing effects.

Q: Is a warning enough if my content has to flash?
A: A warning is necessary but not sufficient on its own. It must appear before the flashing, be visible rather than only in the description, and account for autoplay contexts where the viewer had no opportunity to read anything. Reducing the effect is almost always preferable to warning about it.

Q: Why is red singled out?
A: Transitions to and from saturated red carry elevated risk compared with other colours at similar brightness, which is why both broadcast and web guidance treat them as a separate condition. If an effect flashes red, apply more caution than you would to the same effect in another hue.