Working distance is a distance between the specimen and the lower pole piece in SEM system. The final lens, focusing an electron beam on specimen, is operated with a relatively long working distance so that secondary electrons (SE) are easily collected with a lateral detector.

Figure 4586a shows schematics of the electron beam emerging from the final aperture in the objective lens and striking the specimen. A relatively large aperture and short working distance (W) in Figure 4586a (a) create a large convergence angle and thus a shallow depth-of-field. A small aperture diameter and long working distance (W) in Figure 4586a (b) create a small convergence angle and thus a large depth-of-field.

Figure 4586b shows the schematic illustration of EBSD setup in a SEM system. The working distance for EBSD is roughly between 8 and 45 mm.

Figure 4586b. The schematic illustration of EBSD setup in a SEM system.

For EDS measurements, the distance between the specimen and detector is critical, especially at long working distances in SEMs. The take-off angle is very large at such a working distance, therefore it is possible that only weak or no X-rays enter the detector. There is always an optimum where the maximum X-rays can be measured. Each microscope has a different geometry, and thus searching the optimum position of the detector is necessary at the installation of the EDS system.