The individual diodes in photodiode array (PDA) in GIF and EELS measurements have slightly
different sensitivities to radiation. Such variations are supposedly no more than a few
percent for a high-quality photodiode array, but some imperfections
such as those in the scintillator and in its optical coupling to the array, contribute further to the
‘gain variations’ between channels.
In EELS measurements, the correction of CCD channel-to-channel gain variation is important in order to obtain high detective quantum efficiency (DQE), to eliminate artifacts, and to measure an EEL spectrum without loss of signal-to-noise ratio, because each EEL spectrum can be shifted horizontally due to the energy instabilities. The correction of some energy instabilities (especially for low frequency instabilities) can be performed with scripts in Digital Micrograph.
Each spectrum should be corrected for dark current and gain variations between the detector elements of the CCD and for multiple scattering applying Fourier–log deconvolution.
Note that occurrence of degradation in the energy spread can be due to the substantial broadening in the detector for EELS if there is cross-talking between neighbouring channels.
The channel to channel gain variation in EELS measurements can be minimized by normalizing the experimental spectrum with independently obtained gain spectrum of the spectrometer. On the other hand, in modern GIF system, this normalization is done in EFTEM mode.