 Electromagnetic quadrupole electron lenses are used, the number of which depends on how much resolution and focusing flexibility the designer wishes.However, the ultimate resolution of LEEM is usually determined by that of the objective lens.Illumination beam aperture allows researchers to control the area of the specimen which is illuminated (LEEM's version of electron microscopy's “selected area diffraction”, termed microdiffraction) and is located in the beam separator on the illumination side.