Name: Yen Vi T.
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?
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
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.
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Update: June 2012