Dependence of the chromatic aberration spot size on the form of the energy distribution of the charged particles

A practical quantitative definition of the size of the image of a point blurred by chromatic aberration is the diameter of the circle containing 50% of the charged particles. The spot size is proportional to the width of the energy spread. The proportionality factor depends on the form of the energy distribution. If, instead of the usual full width half maximum (FWHM) measure of the width, the full width containing 50% of the particles (FW50) measure is used, the value of the proportionality factor is nearly constant. For example, for a Gaussian distribution the values are 0.34 and 0.60 for the FWHM and FW50 measures, respectively, while for a Lorentzian distribution they are 0.62 and 0.62.