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Name: Yen Vi T.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 2001-2002


Question:
I have two nagging questions that have puzzled me forever! When I watch a TV show that has a shot of a computer monitor, the monitor appears to flicker.

When I was in a convenience store, I noticed that the surveillance video had a shot of the store front which captured the store's neon sign. When viewed from the surveillance video, the colour of the neon sign changed periodically from dark to light.

Can you please explain these two phenomena? Are they related?


Replies:
Yen,

Actually they are related through how a television screen works. If you could operate a television screen or computer monitor in VERY slow motion, you would notice it to be a little light flying across the screen, left-to-right, one thin line at a time top-to-bottom. The pattern would be similar to how your eyes move when you read one page of a book. As the little light scans across, it changes color. We don't see this because it happens so fast.

If you take a powerful magnifying glass to a TV screen, you discover one more thing. That little light isn't steady as it moves. It is little dots flickering on and off in succession. A narrow beam of electrons shoots from the back of the TV to the screen, scanning across the screen. Electric magnets in the screen control the pattern. When the beam hits red dots on the screen, they glow red. When it hits blue dots, they glow blue. When it hits green dots, they glow green. When between dots, nothing glows.

When watching a television on a television, the two screens do not flicker at exactly the same times. Your screen may be between dots when the screen on TV flashes red. Your screen may be ready to flash blue when the screen on TV is between dots. As a result, you do not see a steady picture on a screen on TV.

As for a neon sign, it is powered by an alternating current. This means that the voltage and current of the sign reverse repeatedly (actually 60 times per second in USA). The sign flashes brightly when the current is strong forward and then again when it is strong backward (120 times per second). Our eyes cannot see such quick flickers. If the light flashes brightly while the TV screen is the right dots, the light looks bright. If the light flashes while the screen is between dots, the light looks dim.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


Dear Yen Vi,

The answer to both questions is that the light source goes on and off at a frequency slightly different from the frequency between frames recorded by the TV camera.

In the case of the monitor, it is refreshed at a rate rapid enough, typically about 30 times per second, so it does not appear to flicker to the human eye. This feature of the human eye is, of course, also used to make moving pictures appear smooth. Similarly, a neon light goes on and off 120 times per second since it is driven by 60 cycle alternating current. The TV camera records a picture about 30 times per second and so may sometimes record the neon light when it is bright and sometimes when it is off. If the TV camera is very close to a submultiple of the AC frequency, the picture of the neon light may move slowly from bright to dark and back again.

This is closely related to the very common phenomena of wheels appearing to turn backwards in moving pictures. If a spoke moves almost all the way, but not quite, to the position of the spoke ahead of it between two adjacent frames of the movie, the eye is fooled into believing the leading spoke has moved backward slightly. If this happens 30 times per second, the eye (and brain) will be convinced that the wheel is rotating backward. Watching a movie of, say, a wagon slowing down, you can often see the wheel appear to change direction several times as the time between frames of the movie changes from, say, slightly more than three spokes to less than three spokes to more than two spokes, etc.

I hope this is helpful.

Best, Dick...



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