Note: The best flat field calibration can be achieved by performing it at the mid
DN level of the working range used in the operation. Any flat field error associated
with residual non-linearity in the pixel will be halved as compared to performing a
calibration at the peak value of the operating range. A simple way of performing
this is to reduce exposure time to half what is used in the operation in order to get
the mid DN level for flat field calibration. Once complete, return the exposure time
to its original setting.