Frequency at which the eye perceives pulsing light as continuous — roughly 50–60 Hz depending on brightness. Below that, you see flicker from HMI or fluorescents.
Your eye fuses light pulses into a uniform brightness above a certain frequency—below this threshold, you perceive the flicker distinctly. This critical frequency is around 50–60 Hz, but it varies significantly depending on the brightness of the light source, your individual vision, and the angle at which you perceive the light. In film practice, this means: If your HMI lights or fluorescent tubes are not running in sync with the camera frame rate, a disturbing flicker pattern will be visible in the image—especially noticeable during fast pans or in post-production on the monitor.
On set, this is a real problem. You typically work with 50 Hz power grids in Europe or 60 Hz in North America. Your camera runs at 24p, 25p, or 50i. If these frequencies don't harmonize, visual noise is created—the flicker becomes visible because the camera's exposure cycles "look into" the pulsation of the power frequency. This is particularly problematic with LED panels, older fluorescent tubes, or inexpensive dimmers. High-speed cameras make it even worse: at 120 fps or higher, the gap between the pulses becomes more pronounced, and the eye could theoretically perceive flicker again, even if the power frequency is actually high enough.
The practical solution on set: Invest in flicker-free LED systems (at least 3200 Hz pulse frequency) or secure your HMI lights with a frequency converter that adapts the mains frequency to your camera frame rate. Many modern cameras also have a menu option for "Anti-Flicker"—this allows you to set the frame rate to match the local power frequency. When shooting 24p footage in 50 Hz countries, you often have to work with 25p or accept that certain old light sources will become problematic. In editing, flicker is difficult to repair—you would have to resort to conversions or frame interpolation, which compromises image quality. It's better to avoid it from the start.
An additional tip: Smartphone videos also suffer from the flicker fusion threshold when shot under artificial light. Your eye often doesn't perceive it because the phone's refresh rate is high enough—but on a large monitor or projector, it becomes obvious. Therefore, in modern productions, it's standard practice to consider the local power frequency and the characteristics of your light sources during the planning phase.