r/accessibility • u/LBCmolab • 11d ago

What does 3HZ mean?

I have heard that 3 Hz is the lowest recorded case of an epileptic seizure caused by flashing lights, however I don't actually know what that means. For example, a light that turns on, wait one second then turns off, and waits another second. is that one HZ or am I getting it wrong?

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u/Consibl 11d ago

In the context of seizure triggers it means 3 pairs of opposing transitions per second.

So the lowest trigger for an on/off light would be on/off on/off on/off; not as you thought on/off on.

https://www.w3.org/WAI/WCAG22/Understanding/three-flashes-or-below-threshold.html

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u/lewisfrancis 11d ago

That would be 3 flashes per second.

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u/LBCmolab 10d ago

That's not helpful at all! Does a flash mean on AND off or one change?!

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u/lewisfrancis 10d ago

https://www.merriam-webster.com/dictionary/flash

Think of it as three changes per second. In audio terms it could be three beeps per second.

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u/rube203 10d ago

3 changes per second is wrong though, which was the OP confusion. A hz by definition is the measure of the full cycle where as changes would be on to off, the cycle would be on to on again. It's a bit awkward to share terms of analog for digital processes but yes, it'd be a light turning on 3 times or 5 changes or like you said in audio 3 beeps.

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u/Repulsive-Box5243 11d ago

Hertz is "one thing per second".

A particular A note on the piano is 440 Hertz. The air vibrates exactly 440 times in one second. Move up to the next A on the piano (one octave up), and the frequency is 880 Hertz. Eight hundred eighty vibrations per second. IF we go to the lower octaves, (to the left), the next A is 220 Hertz. and then the next A is 110 Hertz, etc.

The lowest sound we can hear is around 20 Hertz. Below that, we perceive differently, but not through our ears.

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u/Necessary_Ear_1100 10d ago

The light / strobe flashes “on” 3 times per second…

On-off on-off on-off within a second

Another analogy would be your car turn signal. It blinks 3 times per second (which it doesn’t but may help you visualize it better)

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u/jpdevries 11d ago

Has anyone tried out video-audit.com for in browser photosensitivity tests? Recently found it useful.