Super-resolution thermagraphic imaging using blind structured illumination

Burgholzer, Peter; RECENDT; Austria

Burgholzer, P.; RECENDT - Research Center for Non-Destructive Testing GmbH; Austria
Berer, T.; RECENDT - Research Center for Non-Destructive Testing GmbH; Austria
Gruber, J.; School of Engineering, University of Applied Sciences Upper Austria; Austria
Mayr, G.; University of Applied Sciences Upper Austria; Austria
Hendorfer, G.; University of Applied Sciences Upper Austria; Austria

ID: ECNDT-0145-2018
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Session: Thermography and Thermosonics 1
Room: H1
Date: 2018-06-14
Time: 10:00 - 10:20

In non-destructive imaging of opaque or turbid samples, the spatial information about subsurface structures, such as defects, can be extracted from measured surface data, such as acoustic pressure for ultrasonic imaging or temperature for thermographic imaging. The information about the defects is transferred from the samples interior to its surface by acoustic waves or heat diffusion, respectively. Recently, we showed that the entropy production of these propagation processes limits the transferred information. This poses a principle resolution limit, similar to the Abbe limit in optics. The reason for the entropy production, which reduces the available information about the subsurface structures in the measured surface data, are the thermodynamic fluctuations. They are extremely small for macroscopic samples, but are highly amplified in the ill-posed problem of image reconstruction. For macroscopic samples, the resolution limit depends only on the amplitude of these fluctuations.

Diffusive processes show a high entropy production with increasing diffusion length. Therefore, the resolution limit for thermographic imaging is found to be proportional to the subsurface depth and is described by a depth-dependent thermal point-spread-function (PSF). Like in optics or acoustics, the blurring of imaged structures for thermographic imaging is modelled by convolution with this thermal PSF. Structures smaller than the width of the PSF cannot be resolved in conventional thermographic imaging. Circumventing such a principle resolution limit is called super-resolution. We discuss how blind (unknown) structured illumination combined with non-linear reconstruction algorithms using sparsity of the imaged structures allows thermographic imaging without high degradation of spatial resolution for deeper lying structures.

In turbid media, a coherent light from an infrared excitation laser produces unknown speckle-patterns inside the sample. This blind structured illumination was used for pulsed photothermal radiometry in an epoxy sample to demonstrate a resolution better than the principle limit given by the thermal PSF.