Spectral Instruments Series 800, User Manual, Page: 28 / 70

Spectral Instruments
Pt # 2500- 28
readout, rows on the right hand side of the image were exposed to dark signal longer
than those on the left hand side – hence the ramp.
The image shows dark bands at the bottom and top. These bands are from pixels that
were digitized but that did not come from the active area of the sensor. The band at the
bottom is the 8-pixel prescan extension of the serial register. The wider band at the top
is the post scan extension of the serial register. The pre scan and post scan are
combined into the general term overscan.
The image shown in Figure 7 includes overscan in the parallel direction as well as in
the serial direction. It is not too easy to tell but the last 100 rows in the image have all
about the same signal level since all of those rows experienced the same exposure to
dark signal during readout. This is because they do not exist on the sensor and are sort
of “created” at the far (from the readout) edge and transit the sensor just like the last
real row did.
The best image for showing all of the components of overscan readout is a light-
exposure from a cold camera. This circumstance results in the light-sensitive pixels
being differentiated from the masked pixels and those from the “imaginary” bias pixels
that do not exist on the CCD sensor.
Dark signal only arises in pixels that exist within the parallel register of the CCD.
Note that the parallel overscan, while representing a region that is not on the imaging
area of the CCD does show dark signal. This signal is flat because all of the parallel
overscan rows are exposed to dark charge for some time (exactly the same time) while
passing over the active area during readout (which is when dark signal is experienced.)
Reference Figure A1 in Appendix A for an illustration of overscan readout
components.
2.6.3 Dark Image
In a dark image, the signal from those pixels that are exposed to the same dark signal
integration time all have the same brightness. With overscan, darker pixels show up
along one edge of the image. These are readout pixels that did not integrate dark signal
for the exposure-time duration - they are the “imaginary” bias pixels that do not exist
on the CCD sensor. Adjust the windowing of the software to show low pixel values as
gray levels and you will see several distinctly different levels along the sides of the
image. The brighter level corresponds to physical pixels on the sensor, some of which
may be masked to incoming light but can still detect dark signal. The other pixels are
not physically on the sensor but are generated by reading past the physical extent of the
edges of the CCD sensor. Depending upon the camera temperature and the amount of
the exposure, it may be possible to "see" the difference between masked pixels and
illuminated pixels on the parallel register. Finally, the dark signal that accrues during
readout is a ramp that starts near the bias level on the side of the serial register and
increases to a constant level in the parallel overscan.
Insure that all external illumination is extinguished and obtain a ½ second dark image.
The result is a dark image from a moderately cooled camera - a gradient in brightness