 Mass–thickness contrast

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.