From: SFPhysics@aol.com
Date: Fri Oct 12 2001 - 10:12:21 PDT
From: SFPhysics@aol.com Message-ID: <b2.7f1de.28f87e75@aol.com> Date: Fri, 12 Oct 2001 13:12:21 EDT Subject: LED Oddities/CCD strobed sampling
Hi Eric:
What you and the class were witnessing is the phase difference between the
60Hz AC of the bulb and the sampling rate of the CCD (charge coupled device)
that is the photo sensor taking the picture. Television video has a scan
rate of 30Hz but it is not synchronized to the 60Hz line voltage so the bulb
will glow continuously in your eyes due to persistence of vision while the
camera samples at 30Hz and comes up with no light, some light, full light,
some light, no light as the phases between the two change in their time
domain alignment. While our eyes look continuously at something, the CCD of
the video camera samples only for a few thousandths of a second. If that
sample is taken while the LED is dark then that is what the camera "sees."
With a period of 10 seconds for the whole on/off repetition your frequency
difference between the line current and the camera sampling works out to
0.1Hz which is about right. As the power grid is stressed near maximum use
you will see this change when the 60Hz current drops to say 59.9Hz. Later in
the evening the power grid will go up to 60.1Hz to bring up the total cycles
delivered for the day to average out at a perfect 60Hz and this is to keep
electronic and electric clocks on time. The camera sample rate is determined
by an internal quartz crystal oscillator that varies very little.
Ever heard of a stroboscopic effect? The LED/CCD camera was a good example.
The old tuning fork stroboscope that has a shutter mounted on the prongs of a
tuning fork will do the same thing with the LED on AC.
Have your students wave their hands across a large screen television and
count their fingers. A computer monitor will work also. Do this with the
hand about an inch in front of the monitor and wave vigorously from side to
side. Most of our modern video addicted children have already discovered
this so they will be aware of the effect. Ask them how this might explain
the action of the LED?
Best wishes to everyone on the list,
Al Sefl
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