= Solution
In <photoemission electron microscopy>, illuminate a surface with ultraviolet light or X-rays and form an electron-optical image of the emitted <electrons>. An extraction field accelerates them away from the sample; an immersion objective and projection lenses magnify their spatial distribution onto an <electron> detector. Unlike a scanned probe, this is a full-field image. Surface preparation and <vacuum> matter because the emitted low-energy <electrons> are strongly affected by contamination and local <electric fields>.
Contrast can come from work-function differences, composition, occupied electronic states, illumination and topography. Near threshold, a small work-function change can produce a large emission-yield change. Therefore brightness is not simply geometrical height. An <energy filter> can select an emitted-electron <energy> interval; scanning the selection yields spatially resolved spectra. Sweeping the <photon> <energy> across an <absorption edge> can add element and chemical-state sensitivity. Appropriate polarization-dependent measurements can probe magnetic domains.<Photo emission electron microscope by JEOL>[instrument definition and optional energy-filter imaging]{ref}
Resolution depends on electron-lens aberrations, the accepted angular and <energy> spread, extraction geometry, sample flatness, space charge, mechanical stability and detector sampling. Rejecting <electrons> improves some aberrations at a cost in signal. An unfiltered <photoelectron> image and a chemically selected image therefore answer different questions even when they use the same microscope.
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