As kids, most of us probably played with flipbook animations—a notepad corner filled with slightly different drawings, flicked with the thumb to bring a character to life. The faster you flipped, the smoother the motion; too slow, and it became a series of disjointed stills.
Monitors work essentially the same way: how many times it "flips" the image per second defines its refresh rate, measured in Hertz (Hz). 60Hz means 60 flips per second.
It sounds simple, but this "flips per second" concept hides interesting physiological and physics questions. A widespread myth claims the human eye can only perceive 24 frames per second, making anything higher wasteful. Is that true?
Busting a Myth: Your Eyes Aren’t a Camera
"The human eye sees only 24 fps" is a misconception repeated endlessly. Its origin is actually practical—in the 1920s, 24 fps for talkies was a compromise between film cost and sound sync quality, not an "eye limit."
Your eyes aren’t a camera that snaps discrete photos; they’re more like a continuous light-processing pipeline. Light hits the retina, photoreceptors react, signals reach the brain, taking tens of milliseconds. The process doesn’t "reset"—the previous frame lingers as the new one overlays it, a phenomenon called persistence of vision, lasting about 1/16 of a second.
This persistence makes discrete images in flipbooks, films, and monitors appear as continuous motion. As long as flipping is fast enough (roughly above 15 fps), the brain skips recognizing "two separate images" and just fills in the motion.
But "seeing continuous motion" and "perceiving smoother motion" are different. Studies show trained individuals can detect differences well above 60Hz, and fighter pilots can even identify aircraft in 1/200-second flashes. So high refresh rates aren’t mystic—your eyes truly feel them, just not as "frame-by-frame," but more through subtle sensations like "smoothness," "responsiveness," and "less ghosting."
Untangling Another Knot: Why 24 fps in Movies Feels Smooth but Choppy in Games?
You might wonder: if eyes sense high refreshes, why does 24 fps look fine in movies but feels like a slideshow in games?
The answer lies in "blur." Movie cameras keep shutters open during each frame’s exposure, creating natural motion blur that fills gaps between frames, making it smooth. Game frames are rendered sharply per frame, with motion "empty" between them; at 24 fps, this results in jarring jumps, hence choppiness. So at 24 fps, movies trick eyes with "real blur," while games expose discontinuities. This explains why games demand higher refresh and frame rates than movies.
Then Why Does 60Hz Still Look Okay?
If higher is smoother, why have we used 60Hz screens for decades without feeling laggy? It ties back to the other half of refresh rates—motion blur.
In reality, object movement is continuous, with light signals constantly hitting your eyes. Screens are discrete: between frames, there’s a tiny gap where objects "jump." This jump leaves a brief blur trail on the retina.
Higher frame rates mean smaller jumps, shorter blur, and sharper images. That’s why switching from 60Hz to 120Hz feels "not much different, but just comfortable"—that comfort comes from reduced blur and less mental effort to fill missing frames.
A more intuitive example: a white dot moving steadily across a black screen. At 60Hz, it jumps 60 times per second, each jump leaving a noticeable tail; at 240Hz, it jumps 240 times, barely moving each time, the tail vanishes, and the dot looks "solid." This is what UFO tests illustrate.
Fun fact: the term "refresh" has history. Early CRT monitors used an electron gun to scan lines across the screen, phosphors lit briefly then dimmed, requiring constant "re-refreshing" to maintain the image—hence "refresh." Today’s LCDs and OLEDs no longer use electron guns, but the term persists, with "how many times per second" still applied—now to the entire panel’s pixel state.
"Production" vs. "Display": Two Stations on an Assembly Line
Now, let’s clarify the classic distinction between refresh rate and frame rate. Many think they’re the same, but they’re two stations on a production line.
Frame rate (FPS) is how many images the GPU "production station" creates per second; refresh rate (Hz) is how many the monitor "display station" shows per second. One produces, the other displays.
Ideally, they match perfectly. In reality, they run independently, leading to two issues: if the GPU produces too fast and the monitor can’t keep up, extra images are discarded, or each shows half, causing screen tearing—where the image splits. If the GPU produces too slow, the monitor has nothing new, so it reuses the last frame, causing stutter—where motion freezes.
Vertical Sync (VSync) forces the GPU to "wait" for the monitor, eliminating tearing but possibly adding latency. Later, G-Sync and FreeSync are smarter: instead of the monitor waiting for a fixed rhythm, it dynamically adjusts refresh to match GPU output—you produce 80 frames, I refresh 80 times. This is Variable Refresh Rate (VRR), now a key selling point for high-end screens.
How Browsers "Count" Your Refresh Rate
After the theory, back to the practical: how can a web-based test know your screen’s Hz?
It uses a clever browser API called requestAnimationFrame. Designed to sync animations with screen refresh, it precisely calls your "draw next frame" instruction each time the screen is about to refresh. In short, this API is "welded" to the monitor’s sync signal.
The test logic is simple: start a stopwatch and count how many times this API is called per second. 60 calls means 60Hz; 144 calls means 144Hz. To avoid skewing from occasional hiccups, it usually averages over several hundred frames, then rounds to familiar tiers like 60, 120, 144, 240.
Interestingly, this counting can reveal off-screen issues. If your count drops to 30, it’s likely not the screen changing, but the browser throttling refresh to save power when you switch tabs. In multi-monitor setups, browsers sometimes conform to the lowest refresh screen, causing high-refresh screens to read lower too. These are byproducts of counting, but they help diagnose the entire display chain’s health.
The Invisible "Flicker" Behind Refresh Rates
Finally, let’s talk about something often overlooked behind refresh rate numbers: backlight flicker.
You might think screens emit steady light, but many backlight by rapidly alternating on and off—lower brightness means longer off periods. This is PWM dimming, common in OLED screens. Because it flickers extremely fast (hundreds to thousands of Hz), your consciousness doesn’t notice, but your optic nerves and ciliary muscles continuously react subtly.
This explains why some people feel eye dryness or strain after prolonged low-brightness OLED viewing without knowing why. Flicker tests with alternating black-and-white screens exaggerate this effect, letting you "see" otherwise imperceptible flicker. It doesn’t test refresh rate itself, but whether the dimming mechanism behind it is eye-friendly.
So next time you see "144Hz" or "240Hz," don’t dismiss it as marketing talk. Behind these numbers lie clever designs involving persistence of vision, motion blur, and production-display rhythm coordination—and you can verify with a web test whether your screen executes them well.
That’s the beauty of science: once you understand the hidden machinery behind everyday smoothness, the world looks a little different.