than the pixel dwell time, the useful signal intensity decreases together with an increase of signal background noise or smearing of image edges due to the ions lagging behind. As a consequence, the above derivations, which include the total electron and ion contributions must be modified accordingly with new equations for the case of fast scanning rates. The electrode geometry can be altered with a view to decrease the ion transit time as can be done with a needle or cylindrical geometry. This fundamental approach helps also understand the so-called “specimen absorbed current” mode of detection in the vacuum SEM, which is limited only to conductive specimens. Image formation of non-conductive specimens now possible in the ESEM, can be understood in terms of an induced displacement current in the external circuit via a capacitor-like action with the specimen being the dielectric between its surface and the underlying electrode. Therefore, the (misnomer) "specimen absorbed current" per se plays no part in any useful image formation except to dissipate the charge (in conductors), without which insulators cannot be generally imaged in vacuum (except in the rare case when the incident beam current equals the total emitted current). SE detector gain By use of a derivation for the Townsend coefficient given by von Engel, the gain factor G, in the case of SE with total current collection Itot (i.e. for R=1), is found by: where A and B are tabulated constants for various gases. In the diagram supplied, we plot the gain characteristics for nitrogen with A=9.0 and B=256.5 valid in the range 75–450 V/(Pa·m) for the ratio E/p. We should note that in ESEM work the product pd<3 Pa·m, since at higher values no useful beam is transmitted through the gas layer to the specimen surface. The gray-shaded area shows the region of GDD operation provided also that the γ processes are very low and do not trigger a breakdown of the proportional amplification. This area contains the maxima of the gain curves, which further re-enforces the successful application of this technology to ESEM. The curves outside the shaded area can be used with beam energy greater than 30 kV, and in future development of environmental or atmospheric transmission scanning electron microscopes employing very high beam energy. General implementations The diagram showing the principle of GDD constitutes a versatile implementation that includes not only the SE mode but also the BSE and