Secondary electrons are low energy electrons (<50eV) ejected from the conduction band of a sample following interaction with the high energy primary beam.  Generally considered to be emitted from the upper  <~100 nm  of the analytical volume, they are thus well suited to image fine scale, three dimensional structures.

Comparison of BSE and SE imaging

Note that the SEI image (below) has a 3-D look but there is no  tonal differentiation between pyrrhotite (Fe, S) and clinopyroxene (Ca, Al, Ti, Mg, Fe, Si, O).  A clear compositional distinction is seen in the BSE image, but the image is ‘flat’.  These signals can be mixed on the microprobe CRT screen to display a 3-D compositional image.

Smelter Glass Bead

Groove In Brass

pollen (from web)

SEI image of pyrrhotite-bearing vug in clinopyroxene

BSE image of pyrrhotite-bearing vug  in clinopyroxene

Mixed SEI + BSE image