Spectral Instruments
Referencing now the right-hand side of Figure 1., since the CC illustrated
supports four-port readout there are two serial registers labeled SR1 and SR2.
Each serial register is divided into two halves, which is shown figuratively as a
black line in the parallel register. The parallel register can also be divided and
that is shows as a vertical black line. Neither black line exists in the CC nor in
the image that is read out four ports, the divisions are presented as black lines for
clarity in showing how the single sensor is effectively divided into quadrants for
four-port readout. The first pixel comes out A, B, C and at the same time.
They are combined into a single data stream with pixel data from A then pixel
data from B then C and finally . This pixel data stream has pixels from all of
the quadrants interleaved. Software sorts them out so they are presented properly.
The columns are still horizontal on the figure and rows are vertical in the image.
Spectral Instruments’ SI-Image software package displays the first pixel, the 0,0
pixel, at the lower left-hand side of your display. The pixels in each row are
displayed vertically. Row numbers increment from left to right in the display.
1.1.3 Multi-Port Readout And Sub-Arrays
It is possible to read only a portion of the entire image – a sub-region or sub-
array. For a one-port CC , setting the sub-array to read out is accomplished by
sending the camera a new set of readout and format parameters that include serial
and parallel offsets to the beginning of the sub-array and the serial and parallel
dimensions of the sub-array. The ensuing image readout will only include the
selected pixels. This ability to “home in” on a region of interest provides a quick
means of aligning or focusing a camera.
Multi-port CC cameras allow sub-array selection in the same fashion. The
difference is that a sub-array is provided for each active channel using the
submitted parameters as offsets from their respective readout corners. The
ensemble of sub-arrays are read out (and displayed in SI Image as a single image)
in the same time it would take for just one sub-array because the readout occurs
simultaneously for all active ports. So, it may require some imagination, but sub-
array readout, even in multiple port cameras, can be quite useful.
1.1.4 Cooling The CCD - Why/How - Implications O Temperature
SICC cameras are cooled to reduce the image contaminant called dark signal.
Images accrue this unwanted signal at a rate that decreases as the temperature of
the CC is lowered. Usually it is not the dark signal that is the problem (it could
be subtracted from the image), it is the noise associated with the dark signal. That
noise cannot be subtracted; it must be prevented.
A CC camera can be cooled too much. If the temperature of the CC is
lowered below about -120oC, the performance of the sensor starts to be adversely
affected.
Pt # 4501 9
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