Quantitative thermal emission microscope studies of cesium surface ionization on porous tungsten emitters show under clean surface conditions a surface migration length in the 2‐&mgr; range. The migration length from the pore exit is under this condition fairly independent of the flow rate per pore and the emitter temperature. It is therefore concluded that 107pores/cm2(traverse counting technique) yield maximum ionization efficiency on porous tungsten.Clean surface conditions were checked by Richardson work function measurements with the built‐in Faraday cage. Also, the ion‐current transition pattern in the threshold region was used for judgement of the surface condition. In case of a contaminated surface, a smooth transition from high to low ion current with decreasing temperature was observed in contrast to the steep ion‐current change‐over in the threshold region under clean surface conditions. Also the surface migration length increased on the contaminated surface.Molybdenum, sputtered onto the porous tungsten substrate, showed a uniform work function over the entire emitter area with &phgr;=4.20 eV.