A few more, taken from ChatGPT:
LED pwm frequency and flickering in movies
To the human eye, this looks continuous (if the frequency is high enough), but cameras can easily detect that flicker because they record in discrete frames.
So flicker shows up as:
LED pwm frequency and flickering in movies
ChatGPT said:
Good question — this is a real issue when filming LED lights! Let’s go over it clearly.Why LEDs Flicker in Movies
LEDs are usually dimmed using PWM (Pulse-Width Modulation) — turning the LED on and off very quickly to control brightness.To the human eye, this looks continuous (if the frequency is high enough), but cameras can easily detect that flicker because they record in discrete frames.
So flicker shows up as:
- Banding or rolling lines across the image
- Brightness fluctuations (pulsing or strobing)
- Color shifts if RGB LEDs are modulated separately
How PWM Frequency Interacts with Cameras
| PWM Frequency | Likely Effect in Video | Notes |
|---|---|---|
| < 500 Hz | Heavy flicker | Visible even to human eyes in slow motion or periphery |
| 500 Hz – 2 kHz | Often visible in video | Flicker may appear depending on shutter speed and frame rate |
| 2 kHz – 10 kHz | Usually OK for human vision, sometimes visible to cameras | Safer for most shooting conditions |
| > 20 kHz | Practically flicker-free for cameras | Ideal for film/video and high-speed shooting |
Why Cameras Are Sensitive
- Cameras record frame-by-frame, typically at 24, 25, 30, 50, or 60 fps.
- Each frame has an exposure time (shutter speed), e.g. 1/50 s or 1/60 s.
- If the PWM on/off cycle interacts with that exposure time, the brightness captured per frame fluctuates — causing visible flicker or banding.
How to Avoid Flicker in Filming
- Use high-frequency PWM drivers (>20 kHz)
- Many modern LED fixtures for film use 25–30 kHz PWM or DC current dimming instead.
- Match lighting frequency to camera frame rate
- For example, in regions using 50 Hz mains power (Europe) → shoot at 25 fps or 50 fps.
- In 60 Hz regions (North America) → shoot at 30 fps or 60 fps.
- This helps if LEDs are synchronized to mains frequency (common with cheap bulbs).
- Use constant-current dimming instead of PWM
- Professional film lights (ARRI, Aputure, etc.) often use analog current control or very high frequency hybrid dimming to eliminate visible flicker.
- Test under slow-motion
- Even lights that look fine in real time can flicker in slow-motion (high frame rate) shooting, so always check.
️ Quick Example
If your LED uses 1 kHz PWM and you shoot at 24 fps (1/48 s shutter):- 1 kHz means 1000 cycles per second → 1000 / 24 ≈ 41.7 PWM cycles per frame.
- Since that’s not an integer multiple, each frame captures a slightly different average brightness → visible flicker.
