Methods and systems for using common-path interferometry are described. In some embodiments, a common-path interferometry system for the detection of defects in a sample is described. An illumination source generates and directs coherent light toward the sample. An optical imaging system collects light reflected from the sample including a scattered component of that is predominantly scattered by the sample, and a specular component that is predominantly undiffracted by the sample. A variable phase controlling system is used to adjust the relative phase of the scattered component and the specular component so as to improve the ability to detect defects in the sample.