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3.10.3 Thermopile Detector Temperature Effects
Thermopile (Newport High-Power) detectors are significantly affected by
temperature fluctuations arising from air flow disturbances. As the detector is
a temperature measuring device, air flow disturbances set a practical lower
limit on the power that a detector can measure. In order to get the most out
of any thermopile detector, be careful to shield the detector from air flow
disturbances. Common sources of disturbance are: air conditioners and
people walking past.
3.10.4 Energy Detector Temperature Effects
Pyroelectric (Newport Energy) detectors are AC coupled devices and thus are
not susceptible to temperature induced DC signal offsets or noise floor changes.
One generally does not need to take much precaution with pyroelectric detec-
tors except to make sure that their damage threshold is not exceeded.
3.10.5 Ambient and Stray Light
Ambient and stray light striking the detector should be considered when
making a measurement. Ambient light can be distinguished from dark current
(or the detector/meter noise floor) by either turning off or blocking the source
and covering the detector face with opaque material such as a piece of black
rubber.
Using the human hand to cover the detector is not advised because it emits a
significant amount of infrared radiation and radiates a temperature signifi-
cantly different from ambient. With the detector covered, a reading of the
dark current may be made. Next, remove the material which is covering the
detector and take another reading. The difference is the ambient light level.
NOTE
Changes in ambient light levels can occur from such factors as turning room
lights on or off, or by moving people or equipment. Remember, if you can see
your detector element, then your detector can see the light bouncing off you.
The effects of ambient light are greatly reduced when using a fiber-connectorized
signal input to the detector. If free-space beam measurements are desired,
using an attenuator will often improve the signal to ambient signal noise level.
Wavelength-specific filters, such as optical cutoff, bandpass, or spike filters
can be used if the signal wavelength spectrum permits. Other techniques to
reduce stray light include using apertures, placing the detector in a box or
other housing to shield the surface from light (or air currents) and turning off
room and other polluting light sources.
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