Finally, note that the above analysis is asymptotic in the limit L ≪ ≪ R , similarly to the standard Fowler–Nordheim theory using the SN barrier. However, the addition of the quadratic terms renders it significantly more accurate for emitters with radii of curvature in the range ~5–20 nm. For sharper emitters there is no general approximation for the current density. In order to obtain the current density, one has to calculate the electrostatic potential and evaluate the JWKB integral numerically. For this purpose, scientific computing software has been developed (see e.g. GETELEC).