Figure 27. Selecting The Clean Mode (Hardware Setup|Cleans Tab Page) - Princeton Instruments PhotonMAX System Manual

Chapter 5
For signal levels low enough to be readout-noise limited, longer exposure times, and
therefore longer signal accumulation in the CCD, 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 CCD by
the signal or the loss of dynamic range due to the buildup of dark charge in the pixels.
Note: Do not be concerned about the DC level of this background. What you see is not
noise. It is a fully subtractable bias pattern. Simply acquire and save a dark charge
"background image" under conditions identical to those used to acquire the "actual"
image. Subtracting the background image from the actual image will significantly reduce
dark-charge effects.
Caution
If you observe a sudden change in the baseline signal, there may be excessive humidity in
the camera vacuum enclosure. Turn off the camera and contact Princeton Instruments
Customer Support. See page 94 for contact information.
Offset (Bias)
CCD cameras are typically designed to produce a certain level of offset (also known as
bias) when no light is present and the exposure time is set to zero (0). Typically, the
offset (bias) is subtracted from the sample image for quantitative measurement. Since the
offset can change based on several factors such as multiplication gain, speed, etc., it is
recommended that a fresh offset (bias) image be taken with the same settings as the
sample image and then be subtracted from the sample image.
Cleaning
Cleaning removes charge from the CCD by
clocking the charge to the serial register and then
directly to ground. This process is much faster
than a readout, because the charge does not go
through the readout node or the amplifier. The
selected clean mode determines when to clean the
CCD array. Not all cleaning modes are available
for all cameras.
The clean modes are described below:
•
Clear Never: Never clean the CCD. Useful
for performing a readout after an exposure has
been aborted.
•
Clear Pre-Exposure: Before each exposure,
cleans the CCD by the number of cleans
selected. This mode can be used in a
sequence. It is most useful when there is a
considerable amount of time between
exposures.
•
Clear Pre-Sequence: Before each data acquisition sequence, cleans the CCD the
number of times specified by the number of cleans selected. If no sequence is set up,
this mode behaves as if the sequence has one exposure. The result is the same as
using Clear Pre-Exposure. For the PhotonMAX, this is the most useful method of
cleaning.
Operation
Figure 27. Selecting the Clean Mode
(Hardware Setup|Cleans tab page)
43