Optical linewidth measurements on patterned wafers are complicated by the wide variety of materials and correspondingly wide variation in optical parameters, complex refractive index and thickness, used in the manufacture of integrated circuits. It has been shown that in addition to linewidth, two key parameters, the normalized local reflectance R and the optical phase difference Φ at the line edge, determine the characteristics of the optical image and, therefore, affect the accuracy and precision of linewidth measurements. Both of these parameters, R and Φ, are dependent upon the illuminating wavelength or spectral bandpass and the coherence parameter of the optical system. To achieve the measurement precision and accuracy required for VLSI dimensions (e.g., 10% tolerance for 1-μm linewidths), it is necessary to control coherence, spectral bandpass, and image integrity as well as to achieve reproducible edge detection and focus criteria. When a system can be operated without further operator intervention despite changes in the materials being mea-sured, it is possible to calibrate the linewidth measurement system using a standard fabri-cated from only a few materials representing a range of image characteristics. The desirable characteristics of such a standard are discussed with respect to durability, edge definition, and equivalence of the image characteristics to materials used in the manufacture of ICs. A prototype design consisting of combinations of SiO2 and chromium layers on a silicon substrate is presented.