12
Using the human hand to cover the detector is not
advised because it both emits a significant amount of
infrared radiation, and because it radiates at a tempera-
ture significantly 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.
The effects of ambient light are greatly reduced when
using a fiber-connectorized input with the Model 840-C. If
free space beam measurements are desired, using the
supplied attenuator will reduce stray light, and often
improve the ratio of source signal to that due to the
ambient conditions. Wavelength-specific filters, such as
optical cutoff, bandpass, or spike filters can also be used
if the signal wavelength spectrum permits. Other tech-
niques to reduce stray light include using apertures,
placing the detector in a box or other housing to shield
the surface from light which is not coming from the
source, and turning off room and other lights.
2.5.4
Environmental Measurement Conditions
All measurements should be made at ambient tempera-
ture within the range 0
°
C to 50
°
C, and with a relative
humidity of 0% to 80% up to 35
°
C. For instruments
operating above 35
°
C derate humidity specification 3%
per
°
C up to 50
°
C. If the instrument has been subjected to
extremes of temperature, allow sufficient time for internal
temperatures to reach environmental conditions. Typi-
cally, it takes 30 minutes to stabilize a unit that is 10
°
C
(18
°
F) out of specified temperature range.
The photodetector should be at or below the calibration
temperature.
NOTE
The photodetectors supplied with this instrument
have characteristics which are temperature depen-
dent. Dark noise and detector response change with
temperature. See the appropriate detector data
sheet for more information.
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