A model is described for the formation of diffraction-limited optical images of chrome on glass photomasks using an optical microscope. The effect of the imaging transducer is also considered, and a Gaussian model for a television camera is presented. National photomask calibration standards were examined using bright- and dark-field illumination, and the results are compared with the images predicted by the models. From the results it is concluded that the physical edge and the material properties of the chrome coating cannot be ignored when making precise linewidth measurements on photomasks. A dark-ground measurement technique is described that makes use of the physical edge of the chrome in order to determine linewidths. It is demonstrated how this technique may be used with bright-field illumination in order to measure linewidths on semiconductor wafers precisely. Both the photomask and wafer linewidth measurements may be automated, and a method of adopting the technique for automation is described.