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Mass–thickness contrast is image contrast caused by different amounts or strengths of electron scattering along the beam path. In bright-field transmission electron microscopy, a region that scatters more electrons outside the accepted transmitted beam generally appears darker.

A thicker part of one material and an equally thick region containing more strongly scattering atoms can therefore produce similar darkening. The image alone does not separate thickness from composition. Changing the collection aperture changes which scattered electrons are rejected, while electron diffraction in a crystal can add orientation-dependent image contrast that is not explained by mass and thickness alone.

This is a conventional transmitted-beam bright-field transmission electron microscopy image containing high-resolution electron phase contrast and visible lattice fringes, rather than an element-specific chemical map. Relative to the scale bar, the smaller particle has an outer projected diameter of roughly $40\mathrm{nm}$. It is approximately rounded and contains a denser outer rim with several lighter internal regions separated by material. A porous or internally hollow particle is a plausible morphological interpretation; the image is not a simple featureless solid sphere.

Greater projected thickness generally removes more intensity from the transmitted beam, producing darker mass–thickness contrast. Different crystallite orientations also scatter intensity differently and give diffraction contrast. Local lattice fringes reveal crystalline material and changes of fringe orientation indicate more than one coherent domain. Bright/dark edge fringes also contain defocus-dependent Fresnel contrast; their width is not a direct shell-thickness measurement. Neither an exact alumina polymorph nor three-dimensional closed porosity can be uniquely established from this projection alone. Use calibrated fringe spacings and electron diffraction for phase identification, tilt-series imaging for void geometry, and a through-focus or dose series to test contrast artefacts.