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Scanning tunneling microscopy uses a scanning tunneling microscope to map a surface from the electric current across a tunnel junction. In constant-current operation, feedback control adjusts the tip height to maintain a chosen electric current. The current-distance test of vacuum tunnelling explains the underlying distance sensitivity and its measurement limits.

The image records the response of a tunnel junction, rather than photographing chemical atoms. Its apparent height can reveal both surface structure and the local density of states; distinguishing them is part of interpreting the measurement.^[ref]