The solid-state field-controlled emitter (SSE) structure is an ultrathin large gap semiconductor (UTSC) layer on a,metallic surface, Quantum calculations show that the electron emissions from the SSE cold cathodes result from a serial two-step mechanism: first is the injection into the UTSC of a large concentration of electrons through the Schottky junction with the consequence of a significant lowering of the emission barrier, followed by the emission of the electrons from the UTSC surface, a current modulated by the applied voltage. The determination of the current densities versus the applied field from the experimental total current versus applied voltage data, measured for the SSE planar cathodes, used results obtained with electron optics numerical simulations of the field distributions across the planar area in front of a hemispherical probe. Discussions about this methodology are presented. This allows a comparison between the experimental results and the theoretical predictions. The specific behavior of the SSE cathodes was highlighted as well as the role of the different parameters (temperature, thickness,) in the control of the electron emission from these novel flat cold cathodes.