Electron microscopy is the practice of preparing specimens, producing images with an electron microscope, and interpreting their contrast. A dark region can reflect electron deflection out of the admitted beam rather than complete absorption. A useful image therefore needs a stated preparation, detection signal and length calibration; brightness by itself does not identify a material. Transmission electron microscopy probes thin specimens, whereas a scanning electron microscope can examine the exposed surface of a bulk specimen.[ref]
Scanning Electron Microscopes (SEM) by JEOL. JEOL, Observation of the micro world using electrons and Various Function of SEM: transmitted versus surface-generated signals
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