Flicker Fusion Test

What it measures

This test estimates your critical flicker-fusion (CFF) threshold — the fastest rate of flicker at which a blinking light stops looking like it is flickering and appears to merge into a single, steady glow. It is a rough indicator of how quickly your visual system can process rapid changes in brightness over time.

How it works

A small square on the screen switches between bright and dark many times a second. You slowly raise a slider to speed up the flicker until the square stops looking like it is blinking and instead looks like a solid, steady patch — your fusion point. The catch is hardware: a web page can only redraw at your monitor's refresh rate, so the fastest flicker it can physically show is about half that rate. The engine measures your display's refresh rate, caps the slider at half of it, and reports your result as a relative flicker-fusion band rather than a true clinical CFF figure.

Tips for an accurate result

• 1Sweep slowly around your fusion point — Move the slider gently back and forth near the rate where flicker just disappears. Pinning the exact point this way is more reliable than a single fast pass.
• 2Test when rested for a fairer baseline — Fatigue, alcohol, and low blood sugar all lower flicker fusion. Testing when alert gives a result that better reflects your typical visual processing.
• 3Compare yourself to yourself, not to others — Because the result is capped by your display, the most useful comparison is your own score over time on the same screen — for example rested versus tired.