Apparatus for determining the energy of charged particles

In a device for measuring the energy of charged particles separated from a sample by bombardment with ionizing radiation, which device includes an energy analyzer and an electron-optical system disposed between the sample location and the energy analyzer and containing means defining a deceleration path for the charged particles to be analyzed, the improvement wherein said electron-optical system comprises means defining two individual lenses between which an intermediate image is formed, and grids with high transmission disposed at the boundary between each said lens and said deceleration path, and wherein said deceleration path is disposed between said lenses at the location of such intermediate image, and said two lenses are constructed so that the image plane of the lens closer to the sample lies in the deceleration path, this image plane coincides with the object plane of the lens closer to the analyzer and the image plane of the lens closer to the analyzer lies in the entrance slit of the analyzer.