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. References Microscopes Microscopy Scientific techniques