From: SFPhysics@aol.com
Date: Thu Oct 25 2001 - 03:16:21 PDT
From: SFPhysics@aol.com Message-ID: <fe.e16cd88.29094075@aol.com> Date: Thu, 25 Oct 2001 06:16:21 EDT Subject: LED vs.CCD part 2
Eric:
The CCD (Charge Coupled Device) is the chip that takes the picture and is a
sampling device as I said before; but, I didn't state that the sample time is
varied. Modern cameras have no iris to control the light coming through the
lens so they vary the time the chip is enabled to do the sample. The sample
time is long when light is low and short when light is bright. In the dark
room the sample time can become the whole time the frame of the picture is
being assembled so that the LED is going to be on during some portion of the
sample period. When the flashlight was used, the CCD chip sample time
dropped to just a portion of the frame so that it began blinking again on the
monitor at a rate that was the sync difference.
The LED will indeed be off when you are either reversed biased or when the
forward bias falls below the 1.6V threshold voltage. It will still be
pulsing at 60Hz. The actual time it is on is called a duty cycle. If DC
were being applied you would have a 100% duty cycle. Subtract 50% for the
reverse bias time of the AC and then subtract the time it takes for the
forward biased wave to reach the 1.6V and the time after it falls back below
the 1.6V. Most likely you have something like a 38% duty cycle depending on
the transformer output. In the dark your CCD of the camera may be enabled
near 100% of the 60Hz frame rate so that the 38% duty cycle of the LED shows
up solidly. In the bright room or with the flashlight the camera may drop
back to 30%, or even down to 1% of the frame time and the blinking comes back
as the two event times "beat."
Clear as mud, right?
Al Sefl
Factoid: Modern optical astronomy is extremely dependent on CCDs. The
Hubble and even Mt. Palomar has been equiped with CCDs for a number of
reasons. The old photographic plates required many photons to be gathered to
change just one silver halide molecule. The CCD is so sensitive that a
single photon will register a charge. This lets the modern optical
astronomer sit in a nice warm room while a computer assembles an image. The
CCD also lets optics become active so that the shape of the lens is altered
in real time to cancel out star twinkle caused by atmospherics. The CCD has
truly revolutionized optical astronomy.
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