= 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.
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