Gain; Clearing Mode - Q Imaging OptiMOS Series User Manual

optiMOS USER MANUAL

Gain

Gain (with regards to cameras) is defined as the conversion factor of captured electrons to a digital signal,
often referred to as a grey value or ADU (Analogue to Digital Unit) and has units of electrons per ADU (e/ADU).
Knowing the gain of a camera allows users to directly compare an ADU value as measured from their software
to the physical number of electrons actually captured by the camera's sensor. Gain plays a critical role in many
of the camera's parameters including dynamic range and read noise.
While there are often several different gain settings that may be desirable for different sensors, the gain setting
predominately used in scientific imaging is one that maps the bit depth of the camera to the maximum full well
capacity of the sensor. This provides the largest single pixel dynamic range and allows an easy comparison of
the relative available well based on the measured ADU value. This gain is often referred to as a 1x gain.
The way this gain and bit depth is achieved in CMOS sensors however, is different than CCDs. With CCDs,
the ADC and gain are designed and defined by the analogue electronics of the camera. On CMOS sensors
however, the ADC design is inherent to the chip and is defined by the sensor manufacturer. In the sCMOS
sensors for example, each pixel is digitized by two gains: Low Gain (~21e-/ADU) and High Gain (~0.5e-/ADU).
Each individual gain has an 11-bit digitizer. In order to achieve a single gain and maximize the dynamic range
of the camera, the optiMOS combines these two 11-bit gains to a single 16-bit gain that is mapped to the
30,000e- full well of the camera. The result is a single 1x gain of approximately 0.5e/ADU.
With this combined gain, the original high gain represents the first 2,096 ADUs (~1,000e-) while the original
low gain is extrapolated out to represent the remaining 63,439 ADUs (~29,000e-). This extrapolation linearizes
the low and the high gains and makes it possible to combine them on a single image and preserve the
quantitative nature of the camera. It is important to be aware that this process does yield image histogram
gaps of 32 counts at light levels above 1,000e-. These gaps however are smaller than the shot noise at any
given measurement and are considered statistically insignificant for single frame measurements.

Clearing Mode

In order to capture the highest signal to noise ratio possible, it is important for any scientific camera to
minimize miscellaneous signal that's not derived from the sample. One contribution to this miscellaneous signal
is the buildup of charge prior to an exposure, which includes stray light and dark current. To eliminate this pre-
acquisition charge, most CCD and CMOS sensors support a sensor clearing mode which allows an electronic
discharge of all accumulated signal without digitization. This ensures that the captured signal is strictly the
charge accumulated during the experimental exposure and not miscellaneous signal accumulated prior to the
exposure.
The sensor clearing mode supported by the optiMOS is called "Clear Pre-Sequence". When the optiMOS is
set to "Clear Pre-Sequence", one clear occurs prior to acquiring an image sequence (size 1 to "n" number
of frames). No global clear occurs between frames which allows the camera to expose and read out images
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