| Today more and more of the indoors are lit by | | | | video frames are captured at a frequency which is |
| fluorescent light sources. And that is why this topic of | | | | significantly different from the AC power supply |
| fluorescent light flicker is becoming more and more | | | | frequency. For example, in case we shoot a video |
| important to photographers. You might/might not have | | | | with a frame rate of 60 FPS in a scene lit with |
| noticed that your videos may contain rolling dark bands, | | | | fluorescent light operating at 50 Hz commercial power |
| falsified colors or blackouts from frame to frame | | | | frequency, the brightness fluctuation of individual |
| when you shoot them in fluorescent light. This is known | | | | frames occurs resulting from the luminance fluctuation |
| as fluorescent light flicker normally termed as flickering. | | | | of the fluorescent light. |
| Many people normally confuse this flickering with the | | | | Solution:- |
| flicker or black rolling bar that occurs in frames when | | | | This flickering problem is solved by setting the frame |
| you shoot a video of your TV set or your computer | | | | rate of camera such that it should be a divisor of |
| monitor. But actually flickering has to do with | | | | fluorescent light fluctuation rate. For example, for mains |
| fluorescent light sources. | | | | frequency equal to 60Hz, the fluorescent light |
| Why flickering occurs:- | | | | fluctuation rate will be 120 and thus we should set the |
| To understand flickering in video frames, we will first | | | | frame rate to 15, 30 or 60. This would mean limiting the |
| have to understand the luminance flicker in fluorescent | | | | exposure times of the sensor to multiples of the period |
| lights. The mains frequency of commercial electrical | | | | of the AC power cycle, which lets the camera gather |
| power at which fluorescent lights operate, is | | | | light over the duration of integer number of (n) flicker |
| standardized at either 50 Hz or 60 Hz (frequency at | | | | periods. This in effect averages the varying light level |
| which alternating current is transmitted from power | | | | over the n complete flicker periods into one image, and |
| plant to end user) depending on geographical region. | | | | since the frame rate is synchronized with the flicker |
| 50Hz AC (alternating current) changes direction 100 | | | | rate, each frame has the same apparent light level. To |
| times per second as during every cycle out of 50, first | | | | facilitate this, cameras today come with anti-flicker |
| current flows in one direction, then in the other direction. | | | | control. You will need to remember to make the light |
| Similarly, 60Hz does so at 120 times per second. The | | | | frequency setting equal to the electrical frequency of |
| luminance of the fluorescent lights fluctuates according | | | | the local main electrical system. For example, if a |
| to direction change of AC, instead of being relatively | | | | camera is set for operation in USA, which has mains |
| constant. So for example, as the commercial mains | | | | frequency of 60 Hz, but the camera user travels to |
| frequency in Europe is 50Hz, so fluorescent lights in | | | | India, which has mains frequency of 50 Hz, then the |
| Europe flicker at 100 times per second and as the | | | | camera user must change the light frequency setting |
| mains frequency in US is 60Hz, so in the USA they | | | | of the camera to compensate for the different |
| flicker at 120 times per second. However, with | | | | fluorescent lighting condition, or else suffer from |
| tungsten filament lights the flickering is negligible | | | | flickering problem. |
| because the tungsten stays glowing hot and thus | | | | Where anti-flicker control is not preferred? |
| giving light even when the current is momentarily zero. | | | | There could be cases however, where light levels may |
| Now, this luminance variation of fluorescent lights is not | | | | be very much brighter in which the required exposure |
| apparent to human eyes when one looks directly at a | | | | time is lesser than one AC cycle. But such exposure |
| scene lit by fluorescent light because of the natural | | | | times would not be effective in reducing flicker, |
| effect "eye persistence". Our eyes perceive such | | | | because each exposure time would be only a partial |
| lighting as constant. But a video camera is not able to | | | | AC cycle, and there is no guarantee that successive |
| perceive such persistence. Hence, if successive video | | | | frames would be in the same part of the AC cycle. |
| frames are exposed during different periods of the | | | | This is why anti-flicker modality in cameras locks the |
| cycle of AC, they will have considerably different | | | | exposure time to a minimum of 100/120, and thus in |
| luminance, and the resulting picture will therefore | | | | very bright scenes, the user has to bear |
| appear to flicker when projected. This happens if | | | | over-exposure to remain flicker-free. |