Some QSI cameras have a selectable gain setting (e.g. 620, 540, 583). In general the "high" gain setting (a lower number) is used with 1x1 binning. The "low" gain setting (a higher number) is used when binning higher than 1x1. "High" gain is the default setting. The gain can be changed in the camera settings dialog in most camera control applications.
Gain values are selected to maximize the dynamic range available from any given sensor. The idea is to convert the number of electrons read from the sensor for any given (possibly binned) pixel and convert that into a number that fits into 16 bits (0-65,535). For most CCDs, the Horizontal Shift Register (HSR) and Output Register have a higher capacity than an individual pixel.
For instance, the QSI 540 uses a Kodak KAI-04022 sensor. Kodak specifies the capacity of a single pixel as 40,000 electrons (e-). The actual capacity of most pixels is somewhat higher. The "high" gain setting for a QSI 540 is 0.8e-/ADU. The gain value is divided into the charge read from the pixel to produce the digital pixel value (ADU). This division isn't actually a purely mathematical operation. The charge read from the sensor is adjusted to a specific range (the gain) before being fed into the Analog to Digital Converter (ADC) which then produces a number between 0 and 64K. With a capacity of 40,000 electrons, a gain of 0.8e-/ADU will yield a number between 0 and 50,000.
The charge capacity of the HSR in the KAI-04022 is 100,000e-. The "low" gain setting for a 540 is 1.9e-/ADU. Dividing a charge of up to 100,000e- by 1.9 yields ADU counts between 0 and ~52,600. Both of these gain settings allow some head room for pixels with somewhat higher capacity than shown in the sensor specification.
Gain settings for all QSI camera models can be found on the
500 Series Comparison page or 600 Series Comparison page.
Full sensor specifications can be downloaded from a link under the "Specifications" tab for each QSI camera model.