the PLA1, the losses become significant. After the beam enters the specimen chamber, the electron losses increase exponentially at a rate depending on the prevailing pressure, the nature of gas and the acceleration voltage of the beam. The fraction of beam transmitted along the PLA1 axis can be seen by a set of characteristic curves for a given product p0D, where D is the aperture diameter. Eventually, the electron beam becomes totally scattered and lost, but before this happens, a useful amount of electrons is retained in the original focused spot over a finite distance, which can still be used for imaging. This is possible because the removed electrons are scattered and distributed over a broad area like a skirt (electron skirt) surrounding the focused spot. 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. As a rule of thumb, for a 5 kV beam in air, it is required that the product p0L = 1 Pa·m or less. By this second principle of electron beam transfer, the design and operation of an ESEM is centered on refining and miniaturizing all the devices controlling the specimen movement and manipulation, and signal detection. The problem then reduces to achieving sufficient engineering precision for the instrument to operate close to its physical limit, corresponding to optimum performance and range of capabilities. A figure of merit has been introduced to account for any deviation by a given machine from the optimum performance capability. Signal detection The electron beam impinges on the specimen and penetrates to a certain depth depending on the accelerating voltage and the specimen nature. From the ensuing interaction, signals are generated in the same way as in an SEM. Thus, we get secondary and backscattered electrons, X-rays and cathodoluminescence (light). All of these signals are detected also in the ESEM but with certain differences in the detector design