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Chapter 7 Exposure and Signal 67
7.7 Remove Sensor Blemishes
Blemish correction is used to correct for bad sensor pixels, rows, columns, and/or clusters
either at the time of acquisition (Online Correction) or after acquisition (Post-Process
Blemish Correction.)
When identifying defective rows or columns, the coordinates of the starting point must be
defined, as well as the length of the defective row or column. (Column and row numbering
start at zero.) Pixel defects are defined by the coordinates of the defective point. When
identifying cluster defects, the defective rows, pixels, and columns in cluster must all have
the same Cluster ID and must be identified as to Type, Column, Row, and Length (a pixel
defect has a length of 1 when defined as part of a cluster). Note that all pixels in a cluster
must be touching (even if only at a corner).
Hardware binning is performed before blemish correction, and binned pixels are corrected if
they contain any pixels identified as defective.
REFERENCES:
For complete information about Blemish Correction, refer to
the User Manual supplied with the image acquisition software
being used.
7.8 Dark Signal
Dark signal is a buildup of charge on the array over time. Even with light into the detector
completely blocked, the array will collect a dark signal that is dependent on the exposure
time, detector temperature, and a variety of other factors. The longer the exposure time and
the warmer the detector, the larger and less uniform this background will appear. Thus, to
minimize dark signal effects, the detector temperature should be set for the lowest
temperature within the recommended range for the detector. For NIRvana-LN, a default
temperature of -190ºC provides the best performance.
Dark signal values vary widely from one array to another and are temperature dependent.
NOTE:
Dark signals are fully subtractable readout patterns. Refer to
Section 7.5, Background Subtraction , on page 64 for complete
information.
CAUTION!
!
If a sudden change in the baseline signal is observed, there may
be excessive humidity in the vacuum enclosure of the detector.
Immediately turn off the controller, and if using an LN-cooled
camera, remove the liquid nitrogen.
Next, contact Princeton Instruments Customer Support for
further instructions. Refer to
Contact Information on page 120
for complete contact information.