CamCyclopedia
Contents
- What is flicker?
- Why does it flicker?
- How to correct flicker?
- Limitations to correcting flicker
What is flicker?
The amount of light received by the camera sensor depends on its shutter speed. When you are shooting in indoor fluorescent lighting or any other bright lighting with a quick shutter speed, you will notice horizontal bands appearing in the image because of short interval variation in brightness.
The nature of the current supplied is responsible for this phenomenon. The common household electricity supply is alternating current (AC). Alternating current switches-off and on at a speed imperceptible to the human eye, but the camera catches the alternating light and dark intervals. The flicker rate is as quick as 60 times a second.
So, if appropriate corrections are not in place or if a shutter rate faster than a particular value is used for filming, these horizontal bands become visible.
The below images show what happens during flicker.
Why does it flicker?
Flicker can be attributed to two things, i.e. the property of fluorescent lighting powered by AC (alternating current) and the property of image sensors.
The phenomenon of flicker in fluorescent lighting powered by AC supply (fluorescent bulbs, AC LED)
Electrical frequency or AC power is different in each country. In case the frequency is 60hz, it means that the repeated transition between light and dark (flicker) is happening at an interval of 1/60th of a second.
There are two main electrical frequencies of AC, i.e. 50 Hz and 60 Hz. For places like the US, South Korea, etc. 60 Hz is the standard and for Europe, China, etc. it is 50 Hz.
Characteristics of the CMOS image sensor
Image sensors function like human eyes. They are the film in film cameras. It transforms the light intercepted by the camera lens into electric signals and these digital electric signals output the final image with the help of ISPs (image signal processors).
The shutter opens and shuts according to the shutter speed and controls the amount of light that reaches the sensor.
There are two kinds of shutters based on their open-close mechanism. They are the global and the rolling shutter.
Global shutters read all lines on the sensor simultaneously and send the processed signal to the ISP. Rolling shutters read the lines on the sensor one by one before sending the processing signal to ISP.
The Galaxy camera uses the CMOS image sensor which in turn uses the rolling shutter.
As the below images suggest, the light is incident on all lines concurrently. However, there is a slight but significant time difference between the light detected by each line.
So, when the shutter closes swiftly, each line receives a different amount of light based on the variation of brightness at each point in time. This results in alternative bright and dark horizontal bands or flickers.
How to correct flicker?
For a 60 Hz frequency light source, one interval of turning on and off takes 1/120 of a second.
If the shutter speed is regulated to an integer multiple of 1/120 second, there is no variation in the brightness intensity as the light covers all the lines uniformly in time.
- 60Hz light source (1/120 s, 2/120 s, 3/120 s… )
- 50Hz light source (1/100 s, 2/100 s, 3/100 s… )
The Galaxy camera automatically adjusts the shutter speed like above to correct any flicker.
Say, the shutter speed operates at 1/120s against a 60 Hz of electrical frequency and each line of the image sensor detects the light at slightly different intervals, the flicker is still corrected because each line gets the same amount of light. This is illustrated in the example below.
Limitations to correcting flicker
There is a certain shutter speed range to correct flicker.
1/120s can be used for 60 Hz and 1/100 s for 50 Hz. The shutter speed cannot be shorter than the electrical frequency if all lines are to receive the same amount of light.
Shutter speed is quickened to adjust to suitable brightness in a bright shooting environment.
Take a look at the below images. The electrical frequency is 60 Hz and the minimum ISO supported by the sensor is 50. The ISO falls to a minimum and the amount of light and applicable brightness is adjusted by the shutter speed. The intensity of light happens to increase at this time causing the shutter speed to surpass 1/120sec. The correction goes haywire at this point and you experience flicker.
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