Princeton Instruments PhotonMAX System Manual page 42

42 PhotonMAX System Manual Version 1.B
Note: As the on-chip multiplication introduces additional noise, it is recommended that
the multiplication be used only as required. For more information, refer to the "On-Chip
Multiplication Gain" technical note. This technical note can be accessed by going to the
Princeton Instruments web site at www.piacton.com.
CCD Temperature
As stated before, lowering the temperature of the
CCD will generally enhance the quality of the
acquired signal. In EMCCD cameras, lower
temperatures result in lower dark current and
higher avalanche gain (multiplication gain). When
WinView or WinSpec is the controlling software,
temperature control is done via the Detector
Temperature dialog box (see Figure 26) accessed
Figure 26. Setting the Temperature
from the Setup menu. Once the target array
(Detector Temperature dialog box)
temperature has been set, the software controls the
camera's cooling circuits to reach set array temperature. On reaching that temperature, the
control loop locks to that temperature for stable and reproducible performance. When
temperature lock has been reached (temperature within 0.05°C of set value) the Detector
Temperature dialog box reports that the current temperature is Locked . The on-screen
indication allows easy verification of temperature lock.
The time required to achieve lock can vary over a considerable range, depending on such
factors as the camera type, CCD array type, ambient temperature, etc. Once lock occurs,
it is okay to begin focusing. However, you should wait an additional twenty minutes
before taking quantitative data so that the system has time to achieve optimum thermal
stability.
The deepest operating temperature for a system depends on the CCD array size and
packaging. Refer to Table 3, on page 83, for typical deepest cooling temperatures.
Note: If the CCD is cooled to low temperatures (below -50°C), exposure to ambient
light will over-saturate it. This may increase dark charge significantly. If the camera
remains saturated after all light sources are removed, you may have to bring the camera
back to room temperature to restore dark charge to its original level.
Dark Charge
Dark charge (or dark current) is the thermally induced buildup of charge in the CCD over
time. The statistical noise associated with this charge is known as dark noise. Dark charge
values vary widely from one CCD array to another and are temperature dependent.
With the light into the camera completely blocked, the CCD will collect a dark charge
pattern, dependent on the exposure time and camera temperature. The longer the
exposure time and the warmer the camera, the less uniform this background will appear.
Thus, to minimize dark-charge effects, you should operate at the default CCD
temperature.
Saturation
When signal levels in some part of the 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 is advisable, with signal averaging to
enhance S/N (Signal-to-Noise ratio) accomplished through the software.