  An accurate estimation of the experimental field-emission area remains a
great challenge in vacuum electronics. The lack of convenient means, which can
be used to measure this parameter, creates a critical knowledge gap, making it
impossible to compare theory to experiment. In this work, a fast pattern
recognition algorithm was developed to complement a field emission microscopy,
together creating a methodology to obtain and analyze electron emission
micrographs in order to quantitatively estimate the field emission area. The
algorithm is easy to use and made available to the community as a freeware, and
therefore is described in detail. Three examples of dc emission are given to
demonstrate the applicability of this algorithm to determine spatial
distribution of emitters, calculate emission area, and finally obtain
experimental current density as a function of the electric field for two
technologically important field emitter materials, namely an
ultrananocrystalline diamond film and a carbon nanotube fiber. Unambiguous
results, demonstrating the current density saturation and once again proving
that conventional Fowler-Nordheim theory, its Murphy-Good extension, and the
vacuum space-charge effect fail to describe such behaviour, are presented and
discussed. We also show that the transit-time limited charge resupply captures
the current density saturation behaviour observed in experiments and provides
good quantitative agreement with experimental data for all cases studied in
this work.
