We have recently replaced the electron energy filter of the Zeiss EM 902 with an improved PMP filter. The design, incorporating concave curved entrance and exit faces and convex curves on the side facing the electrostatic mirror was based on the theoretical calculations of Jiang and Ottensmeyer and Barfels et al. Care was taken in the design of the filter to allow its introduction into the existing geometry of the microscope and its use of the existing electronics. As a result, dismantling the existing prism housing and support (which were used as the base for the new prism), reinstallation, and adjustment required only three working days. The resolution in tjie energy dispersive plane of the new filter was measured using the 15.0 eV plasmon peak of aluminum as an internal standard. Such measurements yielded a FWHM of 1.1 eV over the full image plane (fig. 1) with an optimum resolution of approximately 0.8 eV possible with corrections to the stability of the voltage supply and further small alignment adjustments.

This improved energy resolution permits the detection and imaging of signals in the low loss region previously inaccessible to us in the EM902.