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68 NIRvana
™
-LN System Manual Issue 4
7.9 Saturation
When signal levels in some part of an image are very high, charge generated in one pixel
may exceed the well capacity of the pixel, spilling over into adjacent pixels in a process
called
blooming
. In this case, a shorter exposure time is recommended with signal
averaging to enhance S/N (Signal-to-Noise ratio) accomplished through the software.
For signal levels low enough to be read-noise limited, longer exposure times, and therefore
longer signal integration on the array, will improve the S/N ratio approximately linearly
with the length of exposure time. There is, however, a maximum time limit for on-chip
accumulation, determined by either the saturation of the array by the signal or the loss of
dynamic range due to the buildup of dark signal.
7.10 Readout
Two factors to be considered with respect to array readout are:
•
Region of Interest (ROI);
ROI selection allows a section of an array to be specified and its data read out and
saved. All data not included in the specified ROI are read out and discarded.
•
Analog gain control.
Analog gain control determines how many photons are required to generate an
analog-to-digital unit. The gain choice should be based on the signal intensity.
7.10.1 Region of Interest (ROI)
NIRvana-LN supports Region of Interest (ROI) or area selection. While there will be no
increase in readout speed, there will be a reduction in the size of the data set.
Restrictions on ROI selection are:
•
An ROI must contain either:
—
4 pixels, or
—
A multiple of 4 pixels.
•
The starting pixel in an ROI must lie on a four-pixel boundary.
For example:
—
Pixel 1, pixel 5, pixel 9, pixel 13, etc., are valid starting pixels.
—
Pixel 2, pixel 4, pixel 8, and pixel 15 are all invalid starting pixels.
7.10.2 Controller Gain {Analog Gain}
Analog gain is the number of electrons required to generate an Analog-to-Digital Unit
(ADU) which is also known as a
count
.
The NIRvana-LN camera has fixed gain and is not configurable. The analog gain is
typically 7e
-
/ADU.