A photometric field-emission electron microscopy technique is described by which the spatial structure of the surface electronic tunneling barrier can be mapped with nanometer resolution. The technique involves performing a Fowler-Nordheim analysis on luminosity data extracted from a set of digitized field-emission images taken over a range of voltages. This approach is equivalent to older probe-hole methods, but with greatly improved spatial resolution and data accumulation rate. Virtual probe holes of arbitrary size and shape can be constructed by integrating over subregions in the field-emission images. Performance of a system utilizing this technique is demonstrated by measuring the work functions of the (111) and (100) crystallographic planes of a clean tungsten field emitter. Applications of this technique to adsorption phenomena and field-emission display technology are also discussed.