3.2.1. Light Optical Microscope

Samples can be observed in both reflected and transmitted light. There is only one magnification (400x) available for observation on the MBX probe. This is much higher than used during standard petrographic examination and means that the field of view is only about 500 μm across; consequently, locating mineral grains of interest in a thin section is not a simple process. It is important that the sample be thoroughly characterized petrographically and grains of interest be carefully marked prior to microprobe analysis. Marking is best accomplished using thin lines of India ink leading to or circling grains of interest. Photographs taken in transmitted or (better) reflected light are valuable for marking analysis points. It is also possible to produce an image of an entire thin section using a high resolution optical scanner.

The optical system mirror is located inside the objective lens assembly about 50 mm above the sample surface (Figure 3.2.1). It is essentially series of mirrors with holes in them through which the electron beam passes. This system permits simultaneous examination of the electron beam and the specimen during operation. The beam position, size and shape can be determined by using a fluorescent sample prior to analysis of minerals that do not fluoresce. The optical system provides a resolving power of 0.55 µm (the smallest tick marks on the optical reticule are 1 µm apart) and a field of view 500 µm wide. The depth of focus is small (0.9 µm) insuring that the sample is in good focus for analysis.

Figure 3.2.1. Light optics system. The sample is observed (and illuminated) by a series of mirrors, the latter three of which are coaxial with the electron beam. The illumination/reflection path of the bottom two mirrors is shown schematically by a dashed line.

Copyright 1997-2003, James H. Wittke

Last update: 01/18/2006 01:47 PM.