Principles of Operation 125 125
meter’s amplifier and is typically larger than the theoretical dark current
limit. Silicon detectors are inherently quieter than germanium detectors due
to their higher internal resistance and lower capacitance. The noise or drift in
the dark current sets a lower bound on the measurement resolution that can be
achieved with any given detector. Cooling a detector significantly lowers its
dark current and dark current noise.
The observed dark currents can also be zeroed at any moment via the ZERO
function. Since dark currents drift with temperature, the ZERO should be
adjusted just prior to taking any measurements. If the detector temperature is
constant, sensitivity changes and dark current drifts are significantly reduced.
The 1936/2936 Series of optical power meters can measure the detector
temperature and correct the power readings accordingly. The power
correction happens automatically only for those detectors which are equipped
with a thermistor (Newport offers detectors with thermistors for temperature
compensation. See Newport web-site www.newport.com for more details.)
The thermistor signal is read via the detector DB-15 connector and the
displayed power is adjusted continuously depending on the detector
temperature. The analog output signal shows the detector signal and is not
corrected for temperature.
9.10.3 Thermopile Detector Temperature Effects
Thermopile (Newport High-Power) detectors are significantly affected by
temperature fluctuations arising from airflow disturbances. As the detector is
a temperature-measuring device, airflow 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 airflow
disturbances. Common sources of disturbance are: air conditioners and
people walking past.
NOTE
“Use Prediction to deactivate the power meter acceleration software that provides
the “prediction” response. By using advanced algorithms and known properties of
the detector, this software allows the 1936-R to provide a very accurate power
measurement a few seconds faster than the natural response of a thermopile power
detector. It accelerates the natural response by a factor of 5 to 10.
Turning off the prediction will result in a slower response but it can provide a more
stable measured value in a noisy environment. The default is on. Users are advised
to turn the prediction off if they use a high-power thermopile at low power levels.”
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