Today, more and more interiors are illuminated by fluorescent light sources. And that’s why this issue of fluorescent light flicker is becoming more and more important to photographers. You may or may not have noticed that your videos may contain wavy dark bands, false colors, or frame-to-frame blackouts when you shoot them under fluorescent lighting. This is known as fluorescent light flickering, commonly referred to as flickering. Many people often confuse this flicker with the flicker or black bar that occurs in frames when recording video from your TV or computer monitor. But actually the flickering has to do with fluorescent light sources.

Why flickering occurs:-

To understand flicker in video frames, we’ll first need to understand luminance flicker in fluorescent lights. The commercial power grid frequency at which fluorescent lights operate is standardized to either 50 Hz or 60 Hz (the frequency at which alternating current is transmitted from the power plant to the end user) depending on the geographic region. 50 Hz AC (alternating current) changes direction 100 times per second, since during every 50 cycle, current first flows in one direction and then in the other direction. Similarly, 60 Hz does it at 120 times per second. The luminance of fluorescent lights fluctuates as the AC changes direction, rather than being relatively constant. So, for example, because the commercial grid frequency in Europe is 50 Hz, fluorescent lights in Europe flicker at 100 times per second, and because the grid frequency in the US is 60 Hz, in the US. The US flashes 120 times per second. However, with tungsten filament lights, flicker is negligible because the tungsten remains hot and therefore gives light even when the current is momentarily zero.

Now, this variation in luminance of fluorescent lights is not apparent to the human eye when looking directly at a scene lit by fluorescent lighting due to the natural “persistence of eye” effect. Our eyes perceive that lighting as constant. But a video camera is not capable of perceiving such persistence. Therefore, if successive video frames are exposed during different periods of the AC cycle, they will have considerably different luminance and therefore the resulting image will appear to flicker when projected. This happens if the video frames are captured at a frequency that is significantly different from the AC power supply frequency. For example, in case we shoot a video with a frame rate of 60 FPS in a fluorescent light-lit scene operating at a commercial frequency of 50 Hz, the brightness fluctuation of individual frames occurs as a result of fluctuation. of fluorescent light luminance.

Solution:-

This flicker issue is resolved by setting the camera’s frame rate to be a divisor of the fluctuation rate of fluorescent lighting. For example, for a mains frequency equal to 60 Hz, the fluorescent lighting fluctuation rate will be 120 and therefore we should set the frame rate to 15, 30 or 60. This would mean limiting the exposure times sensor to multiples of the AC Power Cycle period, which allows the camera to collect light for an integer number of (n) blink periods. In effect, this averages the varying light level over the full n periods of blinking in an image, and since the frame rate is synchronized with the blink rate, each frame has the same apparent light level. To facilitate this, today’s cameras come with anti-flicker control. You will need to remember that the light frequency setting is equal to the electrical frequency of the local mains electrical system. For example, if a camera is set to operate in the US, which has a 60Hz network frequency, but the camera user is traveling to India, which has a 50Hz network frequency, then the camera user the camera needs to change the light frequency setting of the camera. to compensate for different fluorescent lighting conditions, or suffer from a flickering problem.

Where is anti-flicker control not preferred?

However, there could be cases where light levels are much brighter where the required exposure time is less than one AC cycle. But such exposure times would not be effective in reducing flicker, because each exposure time would only be a partial AC cycle, and there is no guarantee that successive frames are in the same part of the AC cycle. This is why the cameras anti-flicker mode locks the exposure time to a minimum of 100/120 and therefore in very bright scenes the user has to endure overexposure to remain flicker-free.