The accurate measurement of micrometer-size iinewidths on integrated-circuit photo casks and wafers requires more accurate edge detection tecnniques than traditional optical measuring systems can achieve. A scanning pnotometric optical microscope has the capability of determining edge location using an optical threshold with a resulting linewidth error much smaller than the Airy disc radius of the imaging oujective. However, the threshold corresponding to edge location is dependent upon the spatial coherence of the illumination. Analysis using tne theory of partial coherence has led to threshold equations witn corrections for contrast and optical phase difference at the line edge. Both the tneory and instrumentation which have been developed for measurements on both pnotomasks and wafers are described. Linewidths as small as 0.5 Âµm can be measured with a sensitivity of 0.01 µm and an estimated uncertainty of 0.05 µm.