Very rapid and accurate linewidth measurements in selected subregions of an LSI mask or wafer are made by means of a low-cost apparatus. The apparatus embodies the recognition that an accurate linewidth determination can be made for any particular feature among a variety of features in a repeated array by a calibrated and normalized measurement of the average light transmission or reflection of a subregion that includes the feature. In turn, the measurement is automatically converted to a linewidth reading by analog computing circuitry.