Clock Induced Charge (Cic); Saturation; Table 1. Major Differences Between Dark Current And Clock-Induced Charge - Princeton Instruments ProEM+ EMCCD User Manual

Chapter 5
reference values be above zero. This allows the reference to be above zero so that the
variation (read noise) can be measured. When quantifying input light level, the bias value
must be subtracted from the real signal frame.
In EMCCDs, due to complex nature of the sensor and its drive electronics, the bias can
vary frame to frame depending on parameters including but not limited to the
temperature, speed, and EM gain. To counter this, ProEM+ has a built-in bias active
stabilization engine or BASE™. The camera reads "overscan" pixels — the pixels outside
the region of the CCD to account for any change in bias — and "actively" correct the bias
frames. As a result, each bias frame is self-corrected irrespective of the camera settings
and the bias value remains stable over extended sequences.
Since active bias stabilization is ON by default, no user input is required.

Clock Induced Charge (CIC)

Clock-induced charge (CIC) is a noise source that must be taken into account when
operating EMCCDs at single-photon levels. As charge is shifted from pixel to pixel
during readout, a random electron may be generated in the pixel purely due to clock
transitions. Once an electron is generated in the pixel, it undergoes the same
multiplication process as a photon-induced electron. Since this noise is generated during
readout, it is independent of exposure time. Empirical tests show that CIC is only weakly
dependent on the temperature of the sensor. Dark current, meanwhile, is a function of
exposure time and is dependent on temperature.
The presence of CIC creates an error in photon estimation. The state-of-the-art ProEM+
minimizes spurious charge by optimizing clock voltages and timing edges, down to 0.005
e-/pixel/frame (ProEM+: 512B).

Saturation

ProEM+ uses a special EMCCD to amplify input signal (electrons) to achieve low read
noise. Though, unlike intensified CCD cameras, EMCCDs can withstand bright light
sources, care must be taken not to (1) overexpose (2) use excessive EM gain. If the
camera is used in high light conditions and with excessive EM gain, the EM gain rapidly
degrades over time.
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.
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
Source of noise
Function of exposure time no
Temperature dependent
Units of measure

Table 1. Major differences between dark current and clock-induced charge

Operation
CIC
electronic
no (or weakly) yes
e-/pixel/frame
Dark current
thermal
yes
e-/pixel/second
65
65