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Microscopy Society of America

| Theory || Qualitative analysis || Quantitative analysis || Back-scattered and secondary electron imaging || Cathodoluminescence imaging |

The reflected-light optical microscope is used to focus on the sample being examined. The electron beam focus is variable. By adjusting the objective lens current, the electron beam can be focused on different planes as the working distance is changed by raising or lowering the sample stage. Since the optical microscope has a fixed focus, it is used to focus on a definite plane by moving the sample stage up or down, thus meeting the geometrical requirement for quantitative analysis by wavelength dispersive spectrometry.

Cathodoluminescence (CL) is light emitted by certain materials upon interaction with an electron beam. CL imaging may be done by replacing the ocular of the optical microscope with a photomultiplier. The secondary electron imaging system includes a photomultiplier that can be used for CL imaging. A CL image is thus acquired using the same electronic circuitry that processes the secondary electron signal. Hence, secondary electron imaging is not possible when the electron microprobe is set up for CL imaging. Also, focusing with reflected light is not possible during CL imaging. This is usually not a problem since a sharp focus is not a prerequisite for CL imaging in minerals like zircon. CL images of zircon showing fine-scale sector- (sz) and oscillatory- (oz) trace element zoning with overgrowth rim (og) are shown below:

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(Last Revised 11/29/2007 by Nilanjan Chatterjee)