A description is given of an optical probe, consisting of an optical microscope with modified internal illumination, a dual chopper with its two edges straddling the image plane, and a dual area sensor mounted behind the chopper. An analysis shows that the sensitivity of this proximity probe is essentially the same as that of the Simon probe, while its linearity is much better. Also, its construction is much simpler. A prototype has shown resolution of 0.02 &mgr;m and linearity errors of 0.5 &mgr;m (at 100‐&mgr;m range), respectively.