What is an image, and what is a map? Just for the sake of introducing the term map, the term image is a general term, whereas map is more specific. That is, a map is an image, but an image is not necessarily a map ... which is a small distinction, and most analysts generally refer to maps as images. However, to be more exact, many refer to maps as images of flat surfaces. For example, because all EPMA projects are with flat samples, we generally refer to spatial distributions of elements as elemental maps ... and while many will reference a backscatter electron image (BSE image), it is more appropriately an atomic number map, or a backscatter map. Because so many simply refer to maps as images, we will also do so here.
Types of detectors
The images on the left and below compare optical microscopy with backscattered electrons. The specimen provides an example of metamorphic textures in a high-pressure metagabbro. Direct comparisons can made by rolling your mouse over the images.
Immediately apparent is the color offered by optical, and crossed pols further adds information important to mineral identification. However, what should be equally apparent, is the visual confusion caused by the size of many of the mineral grains being on the same scale as the thickness of the thinsection. Contrast this confusion with the BSE image, which simplifies the assemblage of minerals allowing their relationships to become readily apparent.
At this scale, BSE imaging with the scanning electron microscope is the logical extension of the optical microscope. Chemical micro-analysis is also an extension, however notice many characteristics of composition are already apparent, and there is no need to map the composition.
Brief acquisition of x-ray spectra quickly identifies the dark-gray blades of spinel (Sp) and light-gray garnet (Gt). A slightly longer acquisition, and the presence of potassium identifies the darkest gray as amphibole (Am), which also separates it from the clinopyroxene, which is apparently zoned relative to Ca (lighter) versus Na (darker).
Absorbed current Before I leave BS images, I should mention backscattered electrons can be considered simplistically to be incident electrons which have "bounced" out of the sample, and the remainder of the incident electrons having been absorbed by the sample. These absorbed electrons can also be detected and collected as pixel values ... which, as you might expect, would ultimately become an image related to the BS image but opposite (i.e., dark where the BS image was bright). BS images are easier to acquire and have better spatial resolution, but keep absorbed current detection in your imaging toolbox.