Persistence of vision is the phenomenon by which the eye retains an image for a fraction of a second (about 1/25 s) after it disappears. Combined with the brain’s motion processing, it allows a rapid succession of still images to be perceived as continuous motion.
Discover FizziQ
How to measure it in class
With the FizziQ app, it is possible to explore persistence of vision and its effects.
Steps:
- Observe the stroboscopic effect with the smartphone flash (use a stroboscope app)
- Film a rapidly rotating object and observe the “wagon-wheel” effect
- Rapidly shake the smartphone with the screen on and observe the light trail
- Compare videos at different frame rates (24, 30, 60 fps)
- Use FizziQ’s spectrogram to visualize light beats
Scientific activities on this topic
- Stroboscopic effect
- Thaumatrope and zoetrope
- Screen refresh rate
Learn more
Physiological mechanism:
Persistence of vision has two components:
- Positive (< 50 ms): the image remains briefly visible
- Negative (several seconds): complementary image after intense exposure
It is mainly the brain’s processing (image fusion, motion detection) that creates the illusion of continuity.
Critical frequencies:
| Application | Frequency (Hz/fps) |
|---|---|
| Cinema | 24 fps |
| PAL Television | 25/50 Hz |
| NTSC Television | 30/60 Hz |
| Smooth video games | 60+ fps |
| Flicker perception | 50-90 Hz |
Stroboscopic effect:
When the flash frequency approaches the rotation frequency, the object appears to slow down or stop. If f_flash = f_rotation: apparent immobility.
Formula
Minimum frequency to avoid flicker: f_critical approximately 50-60 Hz (varies with brightness)
Stroboscopic effect: f_apparent = |f_real - n x f_flash|
where n is the integer closest to f_real/f_flash.
Application examples
- Cinema projects 24 images/s, but each image is flashed 2-3 times to avoid flicker
- Fluorescent tubes flicker at 100 Hz (invisible to most people)
- LEDs sometimes flicker visibly on video (beat with frame rate)
- Video game players prefer 60 fps or more for smoothness
FAQ
Q: Why are 24 fps sufficient for cinema but not for video games? A: In cinema, motion blur during capture smooths the movement. In animation or video games, each frame is sharp, making judder more visible.
Q: Can you see screen flicker with FizziQ? A: When filming certain screens or lighting, you can see beats between the screen frequency and the camera frequency.
Q: Does persistence of vision really exist? A: The physiological phenomenon exists (photoreceptors have a response time), but it is mainly the brain that “constructs” motion from discrete images.
Q: How do you explain the wagon-wheel effect? A: When the rotation frequency is close to a multiple of the frame rate, the spokes appear to turn slowly or backwards (temporal aliasing).
Related concepts
Vision - Frame rate - Stroboscope - Cinema - Animation - Motion perception