  In this work, it is shown that image reconstruction methods from ultrasonic
imaging can be employed for thermographic signals. Before using these imaging
methods, a virtual signal is calculated by applying a local transformation to
the temperature evolution measured on a sample surface. The introduced
transformation describes all the irreversibility of the heat diffusion process
and can be used for every sample shape. To date, one-dimensional methods have
been primarily used in thermographic imaging. The proposed two-stage algorithm
enables reconstruction in two and three dimensions. The feasibility of this
approach is demonstrated through simulations and experiments. For the latter,
small steel beads embedded in an epoxy resin are imaged. The resolution limit
is found to be proportional to the depth of the structures and to be inversely
proportional to the logarithm of the signal-to-noise ratio. Limited-view
artefacts can arise if the measurement is performed on a single planar
detection surface. These artifacts can be reduced by measuring the
thermographic signals from multiple planes, which is demonstrated by numerical
simulations and by experiments performed on an epoxy cube.
