 Because the electron skirt width is orders of magnitude greater than the spot width, with orders of magnitude less current density, the skirt contributes only background (signal) noise without partaking in the contrast generated by the central spot.The particular conditions of pressure, distance and beam voltage over which the electron beam remains useful for imaging purposes has been termed oligo-scattering regime in distinction from single-, plural- and multiple-scattering regimes used in prior literature.For a given beam accelerating voltage and gas, the distance L from PLA1, over which useful imaging is possible, is inversely proportional to the chamber pressure p0.