  Measured field electron emission (FE) current-voltage Im(Vm) data are
traditionally analysed via Fowler-Nordheim (FN) plots, as ln{Im/(Vm)**2} vs
1/Vm. These have been used since 1929, because in 1928 FN predicted they would
be linear. In the 1950s, a mistake in FN's thinking was found. Corrected theory
by Murphy and Good (MG) made theoretical FN plots slightly curved. This causes
difficulties when attempting to extract precise values of emission
characterization parameters from straight lines fitted to experimental FN
plots. Improved mathematical understanding, from 2006 onwards, has now enabled
a new FE data-plot form, the "Murphy-Good plot". This plots
ln{Im/(Vm)**(2-({\eta}/6)} vs 1/Vm, where {\eta} depends only on local work
function. Modern ("21st century") MG theory predicts that a theoretical MG plot
should be "almost exactly" straight. This makes precise extraction of
well-defined characterization parameters from ideal I_m(V_m) data much easier.
This article gives the theory needed to extract characterization parameters
from MG plots, setting it within the framework of wider difficulties in
interpreting FE Im(Vm) data (among them, use of the "planar emission
approximation"). Careful use of MG plots could also help remedy other problems
in FE technological literature. It is argued MG plots should now supersede FN
plots.
