Teledyne ProEM Series, System Manual

Page: 73 / 162

Chapter 5 Operation 73
5.3.6 Bias Active Stabilization Engine (BASE)
All CCDs and EMCCDs produce a baseline output signal even when there is no incident
light and the exposure is zero. Camera electronics process this information to produce
what is known as a bias image. For quantitative applications, it is critical that the bias
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 corrects 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.
5.3.7 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).
Table 5-1: Major Differences: Dark Current versus Clock-Induced Charge
CIC Dark current
Source of noise electronic thermal
Function of exposure time no yes
Temperature dependent no (or weakly) yes
Units of measure
e
-
/pixel/frame e
-
/pixel/second