Systems and techniques for characterizing samples using optical techniques. Coherent light may be incident on a sample, and a diffraction pattern detected. Information indicative of diffraction pattern intensity may be used to determine one or more sample characteristics and/or one or more pattern characteristics. For example, sample characteristics such as stress, warpage, curvature, and contamination may be determined. The coherent light may be light of a single wavelength, or may be light of multiple wavelengths.