A novel technique, frequency modulated thermal wave imaging (FMTWI) has recently been proposed and demonstrated for non-destructive subsurface defect detection in solids. Its theory and mathematical analysis is provided in this paper illustrating, how the technique simultaneously combines advantages of conventional pulse based thermography as well as lock-in thermography (LT) The specimen is heated for launching thermal waves into the sample, not at a single frequency (Lock-in) or at all frequencies (pulse), but in a desired range of frequencies. While peak power requirement is reduced, phase images obtained retain known advantages. Experimental results are also presented in support. © 2008 American Institute of Physics.