Field ion microscopy (FIM) is the precursor to APT. In FIM, an intense electric field induces ionisation of rare gas atoms (imaging gas) in the vicinity of a needle shaped specimen. Beams of ions are generated near the sample surface and projected towards a screen (phosphorous or digital) forming an image of the topography of the sample surface at the atomic scale.
In the LEAP, FIM experiments can be digitally captured.
FIM can be useful as it provides complementary structural information about the specimen. FIM has higher spatial resolution than APT due to the fact that gas ion trajectories are less influenced by local bonding and local changes to the radius of curvature, so the relationship and size of small phases or precipitates may be more precisely imaged.

FIM schematics.