What is a light chopper?...
In many optical measurements the intensity of a beam of light needs to
be continuously monitored. The easiest way to achieve this is to allow the
beam to impinge upon some kind of photo-electric detector (such as a
photo-diode or photo-multiplier tube) and monitor the resultant electrical
output. If the light beam is very weak then the electrical output from the
photo-detector will be very small and therefore some sort of amplification
of this signal will be required.
Now, a continuous optical beam will create a DC signal at the output of
the photo-detector so any subsequent amplifier used to increase this
signal level will need to be capable of amplifying DC. Although this is
perfectly feasible, DC amplifiers do suffer from drift due to temperature
fluctuations. This is particularly evident in high gain amplifiers. Also
any other perturbation of the signal due to other stimuli (stray light for
example) will also be amplified and appear as genuine output.
If the signal of interest (that is the original light beam) could be
made to act as an AC signal then the detector output would be AC and any
further amplification could be carried out with an AC (only) amplifier. AC
amplifiers do not suffer from temperature drift and will not respond to DC
signals. So the only signal that would be amplified is that due to the
(AC) light beam. To make a light beam act in an AC manner it needs to be
turned on and off regularly and accurately. This can be achieved by
chopping.
The most common technique is to pass the beam through a rotating disk
that has holes or slots cut into it at regular intervals. As the disk
rotates it "chops" the beam producing an on/off signal which
when detected by a photo-detector will appear as an AC signal.
The mechanical chopping of the light beam is very precisely controlled
by the chopper and therefore the resultant AC signal due to the chopped
light is at a known and stable frequency which can be monitored and
amplified easily
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