Today more and more of the indoors are lit by fluorescent light sources. And that is why this topic of fluorescent light flicker is becoming more and more important to photographers. You might/might not have noticed that your videos may contain rolling dark bands, falsified colors or blackouts from frame to frame when you shoot them in fluorescent light. This is known as fluorescent light flicker normally termed as flickering. Many people normally confuse this flickering with the flicker or black rolling bar that occurs in frames when you shoot a video of your TV set or your computer monitor. But actually flickering has to do with fluorescent light sources.
Why flickering occurs:-
To understand flickering in video frames, we will first have to understand the luminance flicker in fluorescent lights. The mains frequency of commercial electrical power at which fluorescent lights operate, is standardized at either 50 Hz or 60 Hz (frequency at which alternating current is transmitted from power plant to end user) depending on geographical region. 50Hz AC (alternating current) changes direction 100 times per second as during every cycle out of 50, first current flows in one direction, then in the other direction. Similarly, 60Hz does so at 120 times per second. The luminance of the fluorescent lights fluctuates according to direction change of AC, instead of being relatively constant. So for example, as the commercial mains frequency in Europe is 50Hz, so fluorescent lights in Europe flicker at 100 times per second and as the mains frequency in US is 60Hz, so in the USA they flicker at 120 times per second. However, with tungsten filament lights the flickering is negligible because the tungsten stays glowing hot and thus giving light even when the current is momentarily zero.
Now, this luminance variation of fluorescent lights is not apparent to human eyes when one looks directly at a scene lit by fluorescent light because of the natural effect “eye persistence”. Our eyes perceive such lighting as constant. But a video camera is not able to perceive such persistence. Hence, if successive video frames are exposed during different periods of the cycle of AC, they will have considerably different luminance, and the resulting picture will therefore appear to flicker when projected. This happens if video frames are captured at a frequency which 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 scene lit with fluorescent light operating at 50 Hz commercial power frequency, the brightness fluctuation of individual frames occurs resulting from the luminance fluctuation of the fluorescent light.
This flickering problem is solved by setting the frame rate of camera such that it should be a divisor of fluorescent light fluctuation rate. For example, for mains frequency equal to 60Hz, the fluorescent light fluctuation rate will be 120 and thus we should set the frame rate to 15, 30 or 60. This would mean limiting the exposure times of the sensor to multiples of the period of the AC power cycle, which lets the camera gather light over the duration of integer number of (n) flicker periods. This in effect averages the varying light level over the n complete flicker periods into one image, and since the frame rate is synchronized with the flicker rate, each frame has the same apparent light level. To facilitate this, cameras today come with anti-flicker control. You will need to remember to make the light frequency setting equal to the electrical frequency of the local main electrical system. For example, if a camera is set for operation in USA, which has mains frequency of 60 Hz, but the camera user travels to India, which has mains frequency of 50 Hz, then the camera user must change the light frequency setting of the camera to compensate for the different fluorescent lighting condition, or else suffer from flickering problem.
Where anti-flicker control is not preferred?
There could be cases however, where light levels may be very much brighter in which the required exposure time is lesser 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 would be in the same part of the AC cycle. This is why anti-flicker modality in cameras locks the exposure time to a minimum of 100/120, and thus in very bright scenes, the user has to bear over-exposure to remain flicker-free.