  In this paper we study the effect of actual, locally resolved, field emission
(FE) area on electron emission characteristics of uniform semimetallic
nitrogen-incorporated ultrananocrystalline diamond ((N)UNCD) field emitters. To
obtain the actual FE area, imaging experiments were carried out in a vacuum
system in a parallel-plate configuration with a specialty anode phosphor
screen. Electron emission micrographs were taken concurrently with $I$-$V$
characteristics measurements. It was found that in uniform (N)UNCD films the
field emitting site distribution is not uniform across the surface, and that
the actual FE area depends on the applied electric field.
  To quantify the actual FE area dependence on the applied electric field, a
novel automated image processing algorithm was developed. The algorithm
processes extensive imaging datasets and calculates emission area per image. By
doing so, it was determined that the emitting area was always significantly
smaller than the FE cathode surface area of 0.152 cm$^2$ available. Namely, the
actual FE area would change from $5\times10^{-3}$ \% to 1.5 \% of the total
cathode area with the applied electric field increased.
  Finally and most importantly, it was shown that when $I$-$E$ curves as
measured in the experiment were normalized by the field-dependent emission
area, the resulting $j$-$E$ curves demonstrated a strong kink and significant
deviation from Fowler-Nordheim (FN) law, and eventually saturated at a current
density of $\sim$100 mA/cm$^2$. This value was nearly identical for all studied
(N)UNCD films, regardless of the substrate.
