  A serious barrier impedes the comparison between the theoretical prediction
and the experimental observation in field emission because there is no way to
measure the emission area. We introduce three dimensionless variables
$\mathcal{R}_{J}$, $\mathcal{S}$ and $\mathcal{D}$ to construct a formulation
for connecting directly the theoretical variables and experimental data without
measuring the emission area. Based on this formulation we can analyze that the
behaviors of $\mathcal{R}_{J}$ $\mathcal{S}$ and $\mathcal{D}$ with the
voltages between the anode and the emitter to reveals the characteristics of
current-voltage (I-V) curve and detect the physical properties of emitters.
This formulation provides a way to understand the fundamental physics of I-V
curve in field emission and to set up a map between the physical properties of
emitters and the experimental I-V curve.
