Ask A Scientist General Science Archive

name         Yen Vi T.
status       educator
age          20s

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?
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The above answers are correct.  Sometimes this effect is referred to as
"strobing."  It has to do with the fact that the camera is taking a series
of still pictures, and the object being photographed is pulsing at nearly
(but not exactly) the same frequency.

Normal TV in the United States is 29.97 frames per second.  This is  the way
it comes from the TV broadcaster and this is the speed of camcorders.
Normal theatrical movies are 24 frames per second.  Monitors connected to
computers can be almost any frequency, depending on the speed of the
computer.   (LCD monitors don't blink at all so they show no strobing. )

Incandescent light bulbs don't flicker or blink.  Fluorescent bulbs brighten
and dim 120 times per second (Hz) but never go completely dark in between
the bright periods.  In contrast, neon lamps tend to go on and off at 120
Hz.  If the TV camera has a 29.97 Hz shutter and the neon sign pulses at 120
Hz, it will appear as if the neon sign slowly gets bright and dim. That is
because the TV camera will see every 4th "blink" of the neon sign.  When the
camera is looking at the "bright" parts of the blink, the sign will look
bright.  When the TV camera is looking at the "dark" parts, the sign will
look dark.

You will not see flickering of lights and TV screens on major TV shows and
movies.  In big productions, the TV monitors and lamps are synchronized to
the speed of the movie camera.

Bob Erck
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