Thermal techniques have matured for the nondestructive characterization of the internal structure of opaque solids in recent years. While CW-modulated thermal wave imaging techniques have proven applicable to the inspection of many types of structures, difficulties arise when the layers are thick or have low thermal diffusivity. The depth, ℓ, into the specimen which can be probed is approximately one thermal diffusion length, δ = (2α/ω)l/2 where α is the thermal diffusivity and ω is the angular modulation frequency. When ℓ/δ is large because of a low thermal diffusivity, the modulation frequency must be lowered to allow the full thickness of the structure to be examined. As the modulation frequency is decreased, the dwell time required at each point for construction of an image by a point-scanning technique increases as do the data acquisition times. For low diffusivity materials, the data acquisition times are too long for thermal wave imaging to be a feasible routine inspection technique.