An area of a substrate is imaged with and without heating, to obtain a hot image and a cold image respectively. The hot and cold images are compared with one another to identify one or more locations as being defective, e.g. if the result of comparison at one location differs significantly relative to other locations. The comparison results in all locations form a differential image, and in several embodiments a number of differential images are obtained by repeatedly heating, imaging and comparing. In such embodiments, multiple differential images are averaged at each location, to improve the signal to noise ratio. Pump and probe lasers may be used for heating and for illumination respectively, or alternatively a single laser may be employed to generate both pump and probe beams.