Why does a CRT monitor flicker when shot with a digital camera

"As the electron beam scans the screen from top to bottom, the pixels at the top of the screen lose their light emission, before the beam jumps back to the top of the screen and scans the pixels thus lighting up the pixels once again. This  process happens so fast that the naked eye is not able to detect the pixels going dark, however a digital camera picks up this subtle difference in their pictures, which appear as flickering"

 

To understand the flickering of monitors when shot with a digital camera, it is necessary to understand the working of a CRT monitor. A CRT monitor makes use of a cathode ray tube which has an internal structure as shown in the figure below. The cathode at the base of the funnel shaped tube is capable of shooting rays of electrons which fall on the fluorescent screen in the front of the tube. When the electron beam falls on the fluorescent screen(phosphor), due to phosphor's property of luminescence, the spot where the electron beam falls on the screen lights up thereby illuminating the monitor. In a CRT monitor each pixel is composed of a group of three phosphor dots, each dot representing either of the colors – red, blue or green, together called a triad and depending on the intensity of the electron beam on each phosphor dot, different colors can be produced. 

crt WORKING

The CRT monitor receives data for the picture to be displayed through an analog cable, which is decoded by a display controller. Depending on the decoded information the cathode gun fires electrons, which is directed to the screen with the help of deflecting coils, which makes use of magnetic field to direct the electron beam onto the desired spot on the screen.

Raster scan

Now inorder to display a complete image on the screen, the entire fluorescent screen is scanned repeatedly line by line. The electron beam starts from the top left corner and sweeps to the right end of the screen, before jumping to the next line and so forth till it reaches the bottom right corner of the screen. Once it reaches the bottom right corner, the electron beam once again starts scanning from the top left corner and the whole process is repeated.

By the time the electron beam scans the last line of the screen, the first couple of lines would have lost its light emission. But before the naked human eye can detect this loss of light on the pixels of the first line, the electron beam finishes scanning the last line and jumps back to the top left corner and starts scanning the first line. However when a camera is used to take a picture, there may be some parts of the screen which do not emit light at the instance when the camera captures the picture. And this loss of light on some parts of the screen appear as flicker.

References


[1] http://www.bit-tech.net/hardware/2006/03/20/how_crt_and_lcd_monitors_work/

[2] https://en.wikipedia.org/wiki/Cathode_ray_tube

[3] http://www.thehindu.com/todays-paper/tp-features/tp-sci-tech-and-agri/monitor-flicker/article2146214.ece