Princeton Instruments NIRvana-LN, User Manual, Page: 64 / 122

64 NIRvana
™
-LN System Manual Issue 4
The deepest operating temperature for a system depends on the array size and packaging;
and the time required to achieve lock can vary over a considerable range, depending on such
factors as the detector type, array type, ambient temperature, etc. Once lock occurs, it is
okay to begin focusing. However, you should wait an additional thirty minutes before taking
quantitative data so that the system has time to achieve optimum thermal stability.
NOTE:
The
Detector Temperature
dialog does not display
temperature information during data acquisition.
7.5 Background Subtraction
Background subtraction removes a fixed noise pattern from the integrated signal.
In the case of the InGaAs FPA, the fixed pattern noise is due to material variations that cause
pixel input offset voltages to vary. The input offset voltage of a photodiode may be:
• Zero Bias;
No dark current.
• Reverse Bias;
Dark current with the same bias as the photo current.
• Forward Bias;
Dark current opposite to the photocurrent.
The pixel-pixel variation is repeatable from one readout of the array to the next provided the
array is maintained at the same temperature, the same experiment settings are being applied, and
ambient conditions do not change. Because the pixel-to-pixel variation is not random electrical
noise, it can be removed by background subtraction.
When acquiring a background to be subtracted, the identical experiment settings are used
but the experiment’s signal source remains OFF. After the background has been saved,
background subtraction can be set up to occur automatically as part of the data acquisition
process, or the background data can be subtracted from the experiment signal as a post-
acquisition process. Subtracting the background removes the dark signal (ambient signal
and dark current component) of the signal data.
Figure 7-3 illustrates the configuration of Automatic Background Subtraction in
WinSpec/32.