Abstract: The method to discuss the geometric and chromatic aberrations of strongexcited einzel lenses by means of the equiangular and equiheight curves in phasespace is described. An apparatus of chromatic aberration type for measuring theenergy spread of ion sources is designed on the basis of this method, it can dis-play the differential spectrum of the energy spread immediately. The design va-lue of the energy resolution is as high as 20,000.