The specimen is in the form of a sharp tip. A positive potential is applied to the tip such that a very large electric field is present at the tip. The ambient gas surrounding the tip is usually Helium or Neon at a pressure of 1-3 x 10 to the minus 3 millibar. The gas atoms move towards the tip and strike it. The gas atoms may strike the surface many times, before an electron from the gas atom tunnels into the metal tip leaving the gas atom positively ionised. The gas atom is then accelerated away from the tip where it strikes a fluorescent screen. The net effect of many gas atoms is to create a pattern on the flourescent screen showing spots of light which correspond to individual atoms on the tip surface. The technique was invented by Erwin Müller in 1951.

The atom probe is a related technique whereby a sudden voltage pulse is applied to the tip. This causes atoms on the surface of the tip to be ejected. The atoms travel down a drift tube where their time of arrival can be measured. The time taken for the atom to arrive at the detector is a measure of the mass of that atom. Thus compositional analysis of the sample can be carried out on a layer by layer basis.