Laser excited super resolution thermal imaging for nondestructive inspection of internal defects

Abstract

A photothermal super resolution technique is proposed for an improved inspection of internal defects. To evaluate the potential of the laser-based thermographic technique, an additively manufactured stainless steel specimen with closely spaced internal cavities is used. Four different experimental configurations in transmission, reflection, stepwise and continuous scanning are investigated. The applied image post-processing method is based on compressed sensing and makes use of the block sparsity from multiple measurement events. This concerted approach of experimental measurement strategy and numerical optimization enables the resolution of internal defects and outperforms conventional thermographic inspection techniques.

Thermographic detection of internal defects using 2D photothermal super resolution reconstruction with sequential laser heating

“…For a regular measurement grid of equilateral triangles of side length r d = 0.743 mm across the ROI with n m = 403 vertices/independent measurement positions arranged in 7 rows with 54 or 53 positions each and subsequent reconstruction using the sparse matrix stacking approach, the resulting reconstruction of a rec is shown in Fig. (6). At each grid point a single measurement has been recorded after an illumination with a laser spot with spot size d spot = 0.6 mm at Q = 15 W. To eliminate the time dimension, an evaluation time of t eval = 500 ms has been taken into account.…”

“…The reconstruction result for applying the sparse matrix stacking algorithm is shown in Fig. (6). For this reconstruction the ADMM-parameters λ 2,1 = 1570, λ 2 = 100, ρ = 16 and n iter = 400 have been used.…”

“…The thermal PSF resembles the kernel function in the Green'sfunction like representation of the thermal diffusion differential equation. The PSF can be calculated analytically for the special case of a defect-free thin plate with thickness L as follows 6,11 :…”

“…Currently well-established laser-based photothermal SR reconstruction methodology is restricted to reconstruction of defects along a single spatial dimension 6,12 . Here, the OuT is heated using line-shape laser spots or patterns consisting of several laser lines either by scanning continuously over the ROI or by step-wise scanning.…”

“…While geometrical SR techniques enhance the spatial resolution of modern detectors to a sub-pixel accuracy 1,2 , optical super resolution techniques allow to overcome the classical Abbe diffraction limit of optical imaging systems 3,4 . However, currently established photothermal SR reconstruction methods within the field of active thermographic materials testing are restricted to the reconstruction of one-dimensional defect structures [5][6][7] or only approximate two-dimensional SR by combining several one-dimensional illumination patterns 8 . Furthermore, in this context it should be noted, that in contrast to the aforementioned methods relying on the spatially structured heating of the OuT, there also exist reconstruction methods working with temporally structured illumination, a) https://www.bam.de/thermography which allow for a three-dimensional reconstruction of internal structures while also trying to eliminate thermal diffusion effects 9,10 .…”