This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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When the current of condenser lens is changed, e.g. with BRIGHTNESS knob to change the second condenser lens (CL3) current in commercial TEMs, the illuminating spot size and intensity on the viewing screen will be changed as shown in Figure 2701. Therefore, the focal point of the condenser lens can be above, on, or below the viewing screen. Note that the intensity becomes weaker when the illuminating spot size becomes bigger.