Modern LED technology
Refresh rate and image refreshing
An LED screen rebuilds its image many thousands of times per second, far too fast for your eyes. But a camera can catch the screen right in the middle of such a refresh, resulting in flicker stripes on video. A higher refresh solves that.
An LED screen redraws its image at lightning speed over and over, thousands of times per second. To your eyes that looks perfectly still.
But if you film the screen with a phone, you sometimes see dark stripes running across the image. That is because the camera catches the screen at the moment it is just ‘reloading’. Screens that refresh even faster show a calm image on video too.
The refresh rate is the number of times per second a screen rebuilds its image, in hertz (Hz). A screen of 1920 Hz looks perfectly stable to the naked eye, but can still flicker on camera. At 3840 Hz the image is virtually flicker-free, even on camera; that is therefore common for rental, broadcast and fixed installations where filming takes place.
Why cameras show flicker
The flicker arises from a timing difference between the camera’s shutter and the screen’s refresh cycle. The LEDs are switched on and off very rapidly via pulse-width modulation (PWM); if the camera shutter opens right during an ‘off’ moment, it captures dark rolling bars. A higher refresh greatly reduces that chance.
Refresh versus frame rate
Do not confuse refresh rate (hardware: how often the screen refreshes) with the frame rate of the content (how many images per second the film contains). Play a film of 24 frames/s on a screen of 3840 Hz, and each film frame is refreshed 3840 ÷ 24 = 160 times. Because of that high refresh, the eye sees no flicker.
Refresh rate and grayscale are coupled via PWM: within one image frame, the driver modulates the on/off time of each LED to create grayscale, while the rows are scanned. A higher refresh gives more time budget to render many grayscale levels flicker-free and reduces visual artefacts.
Camera compatibility
Camera flicker (rolling bars, colour cast) arises when the shutter time is shorter than the screen’s PWM/scan cycle, so that the sensor image registers the ‘off’ phases. Raising the refresh (1920 → 3840 → 7680 Hz) makes the cycle shorter than typical shutter times and eliminates the bars. For broadcast and rental screens, a high refresh (≥ 3840 Hz) combined with sufficient colour depth is the standard.