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 contrast variation by gdd at high field contrast variation by gdd at low field the diagram showing the principle of gdd constitutes a versatile implementation that includes not only the se mode but also the bse and a combination of these. even if only the se signal is desirable to use alone, at least one additional concentric electrode is recommended to employ in order to help in the separation from interference of bse and also from other noise sources such as the skirt electrons scattered out of the primary beam by the gas. this addition may act as a “guard” electrode, and by varying its bias independently from the se electrode, the image contrast can be controlled purposefully. alternative control electrodes are used such as a mesh between anode and cathode. a multipurpose array of electrodes below and above the specimen and above the pressure limiting aperture of the esem has also been described elsewhere. the development of this detector has required devoted electronics circuitry, especially when the signal is picked up by the anode at high bias, because the floating current amplified must be coupled at full bandwidth to the ground amplifier and video display circuits (developed by electroscan). an alternative is to bias the cathode with a negative potential and pickup the signal from the anode at floating ground without the need for coupling between amplifier stages. however, this would require extra precaution to protect users from exposure to a high potential at the specimen stage. a further alternative that has been implemented at the laboratory stage is by the application of a high bias at the anode but by pickup of the signals from the cathode at floating ground, as shown in the accompanying diagram. concentric electrodes (e2, e3, e4) are made on a copper-coated fiberglass printed circuit board (pcb) and a copper wire (e1) is added at the center of the disk. the anode is made again from the same pcb with a conical hole (400 micrometres) to act as a pressure limiting aperture in the esem. the exposed fiberglass material inside the aperture cone together with its surface above are coated with silver paint in continuity with the copper material of the anode electrode (e0), which is held at high potential. the cathode electrodes are independently connected to ground amplifiers, which, in fact, can be biased with low voltage directly from the amplifier power supplies in the range of ±15 volts without any further coupling required. on account of the induction mechanism operating behind the gdd, this configuration is equivalent to the previous diagram, except for the inverted signal that is electronically restored. while electrode e0 is held at 250 v, meaningful imaging is done as shown by a series of images with composition of signals from various electrodes at two pressures of supplied air. all images show part of the central copper wire (e1), exposed fiber-glass (fg, middle), and copper (part of e2) with some silver paint used to attach the wire. the close resemblance of (a) with (b) at low pressure and (c) with (d) at high pressure is a manifestation of the principle of equivalence by induction. the purest se image is (e) and the purest bse is (h). image (f) has prevailing se characteristics, whilst (g) has a comparable contribution of both se and bse. images (a) and (b) are dominated by se with some bse contribution, whilst (c) and (d) have comparable contribution by both se and bse. the very bright areas on the fg material result from genuine high specimen signal yield and not from erratic charging or other artifacts familiar with plastics in vacuum sem. high yield of edges, oblique incidence, etc. can for the first time be studied from the true surfaces without obstruction in esem. mild charging, if present, may produce stable contrast characteristic of material properties and can be used as a means for studies of the physics of the surfaces. the images presented in this series are reproductions from photographic paper with limited bandwidth, on which attempting to bring up detail in dark areas results in saturating the bright areas and vice versa, whilst a lot more information is usually contained on the negative film. electronic manipulation of the signal together with modern computer graphics can overcome some old imaging limitations. an example of the gdd operating at low voltage is shown with four images of the same field of view of a polished mineral containing aluminum, iron, silicon and some unknown surface impurities. the anode electrode is a single thin wire placed on the side and below the specimen surface, several mm away from it. image (a) shows predominantly se contrast at low pressure, whilst (b) shows bse material contrast at higher pressure. image (c) shows cathodoluminescence (cl) from the specimen surface by use of water vapor (which does not scintillate), whilst (d) shows additional photon signal by changing the gas to air which scintillates by signal electrons originating from the specimen. the latter appears to be a mixture of cl with se, but it may also contain additional information from the surface contaminant charging to a varying degree with gas pressure. the gdd at high voltage has clear advantages over the low voltage mode, but the latter may be used easily with special applications such as at very high pressures where the bse produce a high ionization gain from their own high energy, or in cases when the electric field requires shaping to purposeful ends. in general, the detector should be designed to operate at both high and low bias levels including variable negative (electron retarding) bias with important contrast generation. further improvements have been envisaged, such as the use of special electrode materials, gas composition and shaping the trajectory of detection electrons by special electric and magnetic fields (page 91). further developments the above described principles have been further impemented in esem with detection of secondary and backscattered electrons for 3d imaging with multi-detector method by slowko et al. fitzek et al. have rectified some drawbacks of an available commercial esem by optimizing the pressure limiting system and the secondary electron detection resulting in high-quality imaging. commercial implementations the first commercial implementation of the gdd was carried out by electroscan corporation employing the acronym esd for “environmental secondary detector”, which was followed by an improved version termed “gaseous secondary electron detector” (gsed). the use of the magnetic field of the objective lens of the microscope has been incorporated in another commercial patent. leo company (now carl zeiss smt) has used the scintillation mode and the ionization (needle) mode of the gdd on its environmental sems at low and also extended pressure range.