11
CAUTION
Avoid focusing the light source onto detector surface.
Inaccurate readings and possible detector damage may
result. Consult the detector manual for damage thresholds.
Individual detector responses change with time, espe-
cially in the ultraviolet. They should therefore be
calibrated at 1 year intervals 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.
2.5.2 Quantum Detector Temperature Effects
Photodiode characteristics are significantly affected by
temperature. At longer wavelengths, quantum detectors
typically lose sensitivity with increasing temperature.
However, the detector/amplifier combination noise
increases exponentially with temperature. For silicon
detectors, this is generally on the order of a few picoamps
at temperatures near room temperature. With uncooled
germanium detectors, however, this drift is on the order
of a few nanoamps, or typically 1,000 to 10,000 times
greater than silicon. These drifts can be nulled. (See
3.4.4) Since drift changes occur with time, the
BACK-
GROUND
should be set just prior to any measurements.
If the detector temperature is constant, sensitivity
changes are significantly reduced. In addition, if detec-
tors are cooled, noise will decrease. For the most
accurate measurements, particularly with germanium
detectors, the user should cool the detector to approxi-
mately 0
o
C, and to control the temperature of the detector
to approximate
±
1 degree centigrade.
Since different detectors differ significantly in their charac-
teristics, consult the appropriate detector manual for
information about the details of the detector you are using.
2.5.3 Ambient and Stray Light
Ambient and stray light striking the detector will be
measured by the Model 840-C, and should be considered
when making careful measurements. Ambient light can be
distinguished from dark current by either turning off or
blocking the source and covering the detector face with
opaque material, such as a piece of black metal.
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