The reflected-light optical microscope is used to focus on the sample being examined. The electron beam focus is adjustable. 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 on the JXA-733 is 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.
On the JXA-8200, the light from the sample is split such that both the optical microscope and a cathodoluminescence spectrometer are able to intercept the light. The CL spectrometer resolves light wavelengths to provide semiquantitative estimates of the CL-producing component in the specimen. The optical microscope and the secondary electron detector may be used simultaneously.
CL images of zircon showing fine-scale sector- (sz) and oscillatory- (oz) trace element zoning with overgrowth rim (og) are shown below: