Today, more and more interiors are illuminated by fluorescent light sources. And that’s why this issue of fluorescent light flickering is becoming more and more important to photographers. You may or may not have noticed that your videos can contain wavy dark bands, falsified colors, or frame-to-frame blackouts when you shoot them under fluorescent lighting. This is known as flickering of fluorescent light, commonly referred to as flicker. Many people often confuse this flickering with the flickering 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 does the flicker occur:-
To understand flicker in video frames, we’ll first need to understand luminance flicker in fluorescent lights. The frequency of the commercial electrical network at which fluorescent lights operate is standardized at 50 Hz or 60 Hz (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 flows first 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, since the commercial grid frequency in Europe is 50 Hz, fluorescent lights in Europe flicker at 100 times per second, and since the grid frequency in the US is 60 Hz, in the US . USA blink at 120 times per second. However, with tungsten filament lights, the flickering is negligible because the tungsten remains hot and therefore gives off light even when the current is momentarily zero.
Now, this variation in luminance from fluorescent lights is not apparent to human eyes when looking directly at a fluorescent-lit scene due to the natural “persistence of the 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 thus 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 frequency of the AC power supply. For example, in the case where we shoot a video with a frame rate of 60 FPS in a scene illuminated by fluorescent lighting operating at a commercial frequency of 50 Hz, the brightness fluctuation of the individual frames occurs as a result of the luminance fluctuation of fluorescent light.
Solution:-
This flickering problem is solved by adjusting the camera’s frame rate so that it is a divisor of the fluctuation rate of the fluorescent light. For example, for a network frequency equal to 60 Hz, the fluctuation rate of the fluorescent light will be 120, and therefore we should set the frame rate to 15, 30 or 60. This would mean limiting the exposure times 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 blink periods in an image, and since frame rate is in sync with 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 main electrical system. For example, if a camera is configured to operate in the US, which has a network frequency of 60 Hz, but the user of the camera travels to India, which has a network frequency of 50 Hz, then the user of the camera must 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 be only a partial AC cycle, and there is no guarantee that successive frames will be 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 put up with overexposure to remain flicker-free.