We have recently developed a non-contact technique capable of detecting microscopic variations in the surface potential of charged dielectric films such as xerographic photoreceptors. The technique is based on measuring the charge induced on a small capacitive probe held at a constant distance from a charged sample surface. Distance control is achieved by aerodynamic floating, which is an inexpensive and simple passive feedback system capable of maintaining a constant probe-sample separation despite minor variations in sample morphology. We have used the technique to detect the presence of microscopic electrostatic defects in organic photoreceptors, such as charge deficient spots (CDSs), which are a source of image degradation in xerographic copiers and printers.