Ocean Optics SIR Series - Manual (Page 7)

07-December-2010
MIR Control (Detector Tab)
Detector
This is a direct readout from the AD system. This is a 16bit Analog to Digital
Converter. If the system is at ‘0’, the detector is under range. If the Analog value
is saturated, the value is possibly over range. Try reducing the amount of light
into the system, and possibly adjusting the dark offset.
Temperature:
If the SIR system has a TE Cooler, it will be necessary to turn this on. Note that
not all systems have a TE-cooler. Please consult the supplied calibration sheet to
verify the presence of a TE cooler, and the valid range of operation. It will take up
to one minute for the system to reach temperature and stabilize. Please verify
that the actual temperature reaches its “Set Temperature”. If the system is in too
warm of an environment, the detector may not be able to reach its selected
temperature. If this is the case, please reprogram the set temperature to a
warmer temperature and give the system time to stabilize. Due to slight
calibration errors there can be a difference of +/-3 degrees between the set and
actual temperature.
Dual Detector
If the spectrometer system does not have a dual detector system, this setting
should default to primary. Otherwise, this switch will toggle between the two
detectors.
Offset
To account for the large variation possible inherent with infrared detectors, the
SIR spectrometer system has a Dark offset Adjustment. This is accomplished
with a 12 bit DAC that connects directly to the Analog Preamplifier. Adjusting this
DAC value will offset the Dark current to increase the effective analog range of
the detector. As the voltage level of the DAC increases, the amount of negative
Dark current offset increases. If the AD is reading “65,535” the DAC is too low. If
the Analog to Digital Converter is reporting “0”, the DAC is too high and should
be adjusted down.
The offset level is set at the time of manufacture, to give optimum performance,
but can be adjusted by the user to account for ambient conditions.
After the system has stabilized, Please set the offset level. This is done by
1. Removing all light going into the system.
2. Monitoring the AD value at the top of the Spectrometer Window.
3. The range for AD values is from 0 to 65535. The target for the adjusting
the baseline depends on which wavelength region the system is operating
in. Systems working up to 2.6um should be set for approximately 1000 to
2000 counts. Systems measuring beyond 2.6um, should have a dark
baseline of 2500 to 5000 counts.
4. The correct offset level can be found by incrementally increasing the 12
bit number in the offset location between 0 and 4095 until an optimal
setting is found.