diffraction pattern: the spots indicate the presence of single microcrystals.
The apertures (red circles) are localized around the direct beam for
recording the bright field (BF) image and
around a few diffracted beams for the dark field (DF)
image. The intense direct beam is blocked by a metal rod (black shadow
on the left center) to avoid overexposure.
BF and DF images of the same area of microcrystalline ZrO2. In the BF
image (left), some crystals appear with dark contrast since they
are oriented (almost) parallel to a zone axis (Bragg contrast).
In addition, thickness contrast occurs: areas close to the edge
are thinner and thus appear brighter (lower right side) than those
far of the edge (upper left side). In the DF image (right), some
of the microcrystals appear with bright contrast, namely such
whose diffracted beams partly pass the objective aperture.