A non-destructive, non-contact method is described for imaging resistance variations along electrical conductors such as interconnects. In its basic implementation, an intensity modulated laser beam is scanned over the circuit carrying a d.c. current and the voltage induced in the circuit at the modulation frequency is used to vary the intensity of a synchronously scanned display. Resistance variations, such as caused by defects, produce variations in the image thereby identifying their spatial location. The voltage signal depends on a number of parameters including the temperature coefficient of resistivity of the conductor, the power of the incident laser, the absorption coefficient of the conductor and the rate of thermal diffusion to the surroundings. One advantage of the method is that it is possible to produce images of conductors below the surface. Whilst there is a degradation in contrast when imaging sub-surface features, it is still possible to identify and distinguish, for instance, tungsten vias and aluminum interconnects passivated by several microns of dielectric, as well as electromigration-induced damage. Under certain circumstances, other features in the immediate vicinity of the conductors, such as remanent photoresist and decohered interfaces, can also be imaged.