In the case that the illumination is highly defocused (over tens of microns), the specimen is homogeneous over tens of microns, and the size of the illuminated area on the viewing screen is much larger than the spectrometer entrance aperture, the electrons that miss the spectrometer entrance aperture due to chromatic aberration will be replaced by a similar number of electrons that enter the aperture in error also due to chromatic aberration. Operating under these conditions is sometimes useful on contamination-prone specimens, or when one needs to spread the illumination over a large are to minimize radiation damage, while maintaining some spatial resolution or a high collection angle.
Note that by using an entrance aperture (or collection angle) that is small enough, the effect of aberrations on energy resolution of the EEL spectrometers will be minimized, although a portion of the signal is lost.
Therefore, there is a trade-off between the EELS signal strength and energy resolution in the selection of collector aperture (also called entrance aperture) sizes, because of the following reasons:
Aperture sizes in the range of 0.5 to 5 m are typically provided by the manufacturers and applied in various analyses.
i) The aberrations are worse, and thus the energy resolution is degraded as the width of the electron beam entering the prism increases.
ii) The collector aperture limits the number of electrons entering the prism more significantly, and thus the efficiency of the spectrum detection decreases as the width of the electron beam entering the prism decreases.