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3.10
Measurement Considerations
This section describes detector characteristics, optical and electrical consid-
erations, and environmental influences on optical measurements. In general,
measurement accuracy is limited by the accuracy of the detector calibration.
Accurate measurements, however, are also dependent upon proper set-up,
control of temperature and illumination conditions and understanding the
factors that affect optical measurements.
3.10.1 Detector Calibration and Accuracy
Newport Corporation calibrates its detectors using secondary standards
directly traceable to the United States National Institute of Science and
Technology (NIST) or to Great Britain’s National Physical Laboratory (NPL).
The details and accuracy of the calibration procedure vary with each detector
model but a detailed description of the calibration results is supplied with
each individually calibrated detector. In general, detector calibration accu-
racy varies from 2% to 5% in absolute terms and varies with wavelength. Each
detector will also have some variation in response over its surface. Therefore,
for the most reproducible measurements, light should illuminate the detector
as uniformly as possible over as large an area as practical.
CAUTION
Avoid focusing a light source onto the detector surface. Inaccurate
readings and possible detector damage may result. Consult the detector
manual for saturation or damage thresholds.
NIST traceability requires that detectors be recalibrated on one year intervals.
As individual detector responses change with time, especially in the ultravio-
let, recalibration is necessary to assure confidence in the accuracy of the
measurement. For the most reproducible measurements, the same detector
should always be used for measurements which are to be directly compared.
3.10.2 Quantum Detector Temperature Effects
Semiconductor (Newport Low-Power) detectors, are affected by temperature.
At long wavelengths, quantum detectors typically lose sensitivity with increas-
ing temperature. Additionally, detector dark current increases exponentially
with temperature.
Observed dark current is often dominated by the interaction between the
detector and a 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 which 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.
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