A new crack imaging technique is presented that is based on second derivative image processing of thermal images of laser heated spots. Experimental results are shown that compare well with those obtained by the dye penetrant inspection method. A 3D simulation has been developed to simulate heat flow from a laser heated spot in the proximity of a crack. A 'ghost point' method has been used to deal efficiently with cracks having openings in the micometre range. Results are presented showing the effects of crack geometry and system parameters on thermal images of laser heated spots.