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Chapter 7 Exposure and Signal 63
7.4 Cooling the Array
The detector in the NIRvana-LN is cryogenically-cooled. Although the detector can be
operated at temperatures in the range of -175°C to -194°C, pixel gain and offset corrections
are optimal when the camera is at its default temperature of -190°C. Generally speaking, the
lower the array temperature, the lower the dark current and, therefore, the greater the
sensitivity.
Cryogenically-cooled detectors use liquid nitrogen to reduce the temperature of the array. The
liquid nitrogen is stored in a Dewar that is enclosed in a vacuum jacket for minimal external
thermal losses. The chip temperature is regulated by a heating element driven by closed-loop
proportional control circuitry. A thermal sensing diode attached to the cooling block of the
detector monitors the chip temperature.
7.4.1 Array Temperature Control
Lowering the temperature of an array generally enhances the quality of the acquired signal.
•
When Light Field is being used, temperature control is done on the Sensor expander.
•
When WinSpec/32 is the controlling software, temperature control is achieved using the
Setup —> Detector Temperature
dialog shown in Figure 7-2 .
Figure 7-2: Typical WinSpec/32 Detector Temperature Dialog
Once the desired target array temperature has been configured, the software controls the
detector's cooling circuits to reach set array temperature. On reaching that temperature, the
control loop locks to that temperature for stable and reproducible performance. When the
Target Temperature
{
Temperature Setpoint
} has been reached, the current temperature
is
Locked
and an on-screen indication allows easy verification of temperature lock.
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