Abstract: System and method for detecting defects on a sample such as a lithography mask blank or a semiconductor substrate. The con-focal imaging system uses dual beam interference to enhance signal contrast from a light scattering defect on a sample surface. An incoming light beam is split into a probe beam and a reference beam. Destructive interference between the probe beam and the reference beam is established by moving a movable portion of a mirror system, to tune the system. The system is then used to detect defects on the surface of the sample, wherein intensity detected by a detector indicates the presence of a defect on the sample. Destructive interference is used to cancel out and eliminate the directly reflected light, without blocking out the scattered light, resulting in a detection signal that is more sensitive to scattered light than conventional con-focal microscopes.