Have you ever noticed why headlights and taillights of cars and other vehicles seem to flicker On and Off constantly while on a video clip or on camera? If you would watch the lights with your naked eye, they would seem normal but under the camera, the spooky effect would resume. This is all the case with the Boundary Screens in cricket fields, even the screen of your laptop or PC and any other such LED screen.
LED Flickering of car headlamps;picture:krmlight.com
The explanation for this weird process is quite technical. Brief answer, LED lights are not "always on". Instead, they pulse on and off, faster than the human eye can recognize. However, camera refresh rates catch this flicker. As a result, the lights appear to switch between On and Off.
LED flickering in video occurs when the camera's frequency refresh rate is greater than that of the lighting that the camera is trying to capture. This isn't really an issue when the LEDs are fully on, but becomes an issue when LEDs are dimmed down, and the frequency slows down.
DImmed LEDs cause flickering;picture:ee.co.za
Car headlights, taillights and DRLs often use an LED in two modes. For example, Stop (bright) and Tail (dim), DRL (bright) and Front Position (dim).
For the dim function, the LED could be biased to operate at a lower DC current.
But when the LED is made to operate at one high DC current, operating at a lower DC current again injects uncertainty of light output.
Since most LEDs are only operated for brightness at one relatively high current, these LEDs which also perform dim functions are operated in PWM (Pulse Width Modulation) mode to achieve the dim mode.
The human eye integrates the amount of light over time, and even though the LED is ON only at a high intensity and OFF the rest of the time, it appears to our eyes as being dim.
Flickering captured by camera shutter speed;picture:bhphotovideo.com
To eliminate flickering from a camera shoot, decrease the rate of capture (if you can) to ~100Hz and you *should* avoid any flicker.
Watch the following video to better experience the flickering of LED lights.
Different types of flickering of LED lamps;picture: Youtube-Flaming Photo
Have you ever observed the wheels of a vehicle while in motion? They appear to rotate forward during the forward motion and appear to rotate backwards whilst in the reverse motion. Right, everybody is aware of such a process and most of the time it is deliberately shown in movies and advertisements.
As the speed goes up, the rotation seems to become faster and so far, so good. But suddenly everything goes screwy and the motion of the wheels appear to slow down. Slowly and slowly it comes to a halt and when it resumes the wheels appear to rotate in the reverse direction, even though the car is still moving forward.
How is this possible? You may think that it must be in your head and the rotation of the wheels is in the right direction, but it isn’t. You observed it correctly. The wheels were turning in the opposite direction, or at least they appeared to do so.
Actual Motion vs Perceived motion in the visual effect;picture:maycontainmaths.wordpress.com
This phenomenon is called the 'wagon wheel effect' or the 'stroboscopic effect'.
It occurs when the view of a moving object is represented by a series of short samples, such as a set of pictures, different from a continuous view and the moving object is in rotational or other cyclic motion at a rate close to the sampling rate.
It also accounts for the "wagon-wheel effect", so-called because on video, spoked wheels on horse-drawn wagons sometimes appeared to be turning backwards.
Stroboscopic Effect explained;picture:henryaudio.com
Cameras record footage not continuously, but by capturing a series of images in quick succession, at a specified "frame rate."
When the frequency of a wheel's spin matches the frame rate of the camera recording it (say, 30 revolutions per second), each of the wheel's spokes completes a full revolution every 1/30 seconds, such that it ends up in the same position every time the camera captures a frame.
Wagon wheel effect explained with angles;picture:quora.com
The result is a footage in which the wheel in question appears motionless.
So when a wheel seems to spin in a direction opposite to its actual rotation, it is because each spoke has come up a few degrees shy of the position it occupied when it was last imaged by the camera.
Watch the following video to better experience the wagon-wheel effect.
Another explanation for this phenomenon, when experienced in real life, is that when we see something that is moving, we do not actually see every point in the motion. We capture an image 10-12 times every second and we use the data that we capture to fill in the blanks. Since the rotation is faster than what the eye can capture, it may appear that the rotation is actually in the opposite direction. As the speed of rotation changes, the apparent direction of rotation will also change. Other examples of the stroboscopic effect include a strobe fountain and the fan of a helicopter.
I hope that now when you encounter such phenomena, you won’t feel like pulling your hair out of confusion and will be able to boast about it in front of your friends!
Stroboscopic effect captured on video;picture:Youtube-Default Name
Helicopter Blades also display wagon-wheel effect;picture:en.wikipedia.org