= Transmission electron microscopy contrast
<Transmission electron microscopy contrast> is a difference in recorded signal between specimen regions under specified illumination, angular acceptance and detector settings. For a positive reference signal $S_{\mathrm{ref}}$, one possible signed contrast convention is
$$
C=\frac{S-S_{\mathrm{ref}}}{S_{\mathrm{ref}}}.
$$
Negative $C$ means darker than the reference; it does not specify a surface height or <chemical composition>. The definition must accompany a numerical contrast value, since other conventions exist. A conspicuous feature need not establish fine <spatial resolution>: detecting a small signal difference and distinguishing two nearby features are different measurement questions.
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