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

Samim Ahmadi,Giuseppe Caire,Peter Burgholzer,Julien Lecompagnon,Mathias Ziegler,Peter Jung,Philipp Daniel Hirsch

doi:10.1038/s41598-020-77979-y

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.

Highlights

A photothermal super resolution technique is proposed for an improved inspection of internal defects
This paper focuses on the applicability of laser excited super resolution thermal imaging to an additively manufactured stainless steel sample with internal defects as test specimen
The most prominent peaks indicate the correct positions of the investigated internal defects, but sometimes some minor peaks occur as small artifacts as well

Summary

Introduction

A photothermal super resolution technique is proposed for an improved inspection of internal defects. 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. The diffuse nature of heat propagation in the material causes a degradation in spatial resolution and in reconstruction accuracy[7] To solve this problem, various measurement and thermal image processing strategies were applied such as pulsed-phase thermography[8] or lock-in thermography[9], making use of the relative amplitude or phase change to a reference area. Apart from that, so-called optical super resolution (SR) imaging—serving as an alternative measurement strategy to enhance the spatial resolution—gained attention in fields of structured illumination microscopy[13,14] These SR techniques rely on multiple measurements with a small position shift. While the latter approach is known[15,16] and already implemented in commercial IR camera systems, a method to overcome the diffusion limit of TT by means of an optical SR analogue was out of reach so far

Methods

Results

Conclusion