avalanche also accompanies the electron avalanche and, by detection of the light produced with a photo-multiplier, corresponding SE images can be routinely made. The frequency response of this mode has allowed the use of true TV scanning rates. This mode of the detector has been employed by a latest generation of commercial instruments. The novel GDD has become possible first in the ESEM and has produced a practically 100% SE collection efficiency not previously possible with the Everhart-Thornley SE detector where the free trajectories of electrons in vacuum cannot all be bent towards the detector. As is further explained below, backscattered electrons can also be detected by the signal-gas interactions, so that various parameters of this generalized gaseous detector must be controlled to separate the BSE component out of the SE image. Therefore, care has been taken to produce nearly pure SE images with these detectors, then called ESD (environmental secondary detector) and GSED (gaseous secondary electron detector). Backscattered electrons Backscattered electrons (BSE) are those emitted back out from the specimen due to beam-specimen interactions where the electrons undergo elastic and inelastic scattering. They have energies from 50 eV up to the energy of the primary beam by conventional definition. For the detection and imaging with these electrons, scintillating and solid state materials have been used in the SEM. These materials have been adapted and used also in ESEM in addition to the use of the GDD for BSE detection and imaging. BSE pass through the gaseous volume between the electrodes of the GDD and generate additional ionization and avalanche amplification. There is an inner volume where the secondary electrons dominate with small or negligible BSE contribution, whilst the outer gaseous volume is acted upon mainly by the BSE. It is possible to separate the corresponding detection volumes so that near pure BSE images can be made with the GDD. The relationship of relative strength of the two signals, SE and BSE, has been worked out by detailed equations of charge distribution in the ESEM. The analysis of plane electrodes is essential in understanding the principles and requirements involved and by no means indicate the best choice of electrode configuration, as discussed in the published theory of the GDD. Adapted detectors Despite the above developments, devoted BSE detectors in the ESEM have played an important role, since the BSE remain a most useful detection mode yielding information not possible to