ELECTRON SOURCE BRIGHTNESS AND DEGENERACY FROM FRESNEL FRINGES IN-FIELD EMISSION POINT PROJECTION MICROSCOPY

Fresnel edge fringes observed in a lensless point projection field-emi ssion electron microscope operating at 90 eV have been studied and fou nd to be formally equivalent to the fringes observed in transmission e lectron microscopy (TEM) under weak scattering conditions at the edge of an opaque object. The tip-to-spectrum distance z1 plays the role of the objective lens defocus setting DELTAf in conventional TEM. The im age magnification, effective source size, transverse coherence width, instrumental resolution, and source brightness are all obtained from a n analysis of the fringe spacings and intensity. The quantum mechanica l upper limit on source brightness, as well as relationships among bea m brightness, coherence parameters, and degeneracy, are discussed, and the degeneracy measured from experimental Fresnel fringes.