Single frequency thermal wave radar: A next-generation dynamic thermography for quantitative non-destructive imaging over wide modulation frequency ranges

Abstract

Single-Frequency Thermal Wave Radar Imaging (SF-TWRI) was introduced and used to obtain quantitative thickness images of coatings on an aluminum block and on polyetherketone, and to image blind subsurface holes in a steel block. In SF-TWR, the starting and ending frequencies of a linear frequency modulation sweep are chosen to coincide. Using the highest available camera frame rate, SF-TWRI leads to a higher number of sampled points along the modulation waveform than conventional lock-in thermography imaging because it is not limited by conventional undersampling at high frequencies due to camera frame-rate limitations. This property leads to large reduction in measurement time, better quality of images, and higher signal-noise-ratio across wide frequency ranges. For quantitative thin-coating imaging applications, a two-layer photothermal model with lumped parameters was used to reconstruct the layer thickness from multi-frequency SF-TWR images. SF-TWRI represents a next-generation thermography method with superior features for imaging important classes of thin layers, materials, and components that require high-frequency thermal-wave probing well above today's available infrared camera technology frame rates.

Detection and monitoring of early dental caries and erosion using three-dimensional enhanced truncated-correlation photothermal coherence tomography imaging

“…3. As reported previously 18 and further explored in the Appendix, SF-TWR imaging contrast is similar to LIT at low modulation frequencies but significantly increases at higher frequencies with the added advantage of improved spatial resolution. Fig.…”

“…15 and 16 are calculated by averaging the pixel phase values on the tooth on positions along the red arrows, which mark the location of the demineralized treatment window. It must be noted that the similarity of the results at 0.5 Hz is also evident from the pixel intensity plots accompanying each image: the LIT and SF-TWR images have different phase range values as they are processed with different phase reconstruction formulas, 18 leading to different color contrast; however, the phase difference source data are the same for both. After 6 days of demineralization (Fig.…”

“…7 Appendix: Comparing LIT with SF-TWR LIT is the oldest and most established photothermal imaging method; it has been applied to dental imaging for detection of early caries in multiple studies. 9,10,28 The recently developed SF-TWR 18 operates on the same physical principles as LIT and provides similar output, but it employs a different signal processing algorithm based on the thermal wave radar concept, which can result in images with higher SNR compared with LIT. 29 In SF-TWR, the starting and ending frequencies of an LFM chirp are the same, making it directly comparable to (and technically an evolution of) LIT/TPLI.…”

“…SF-TWR uses the same hardware but has been shown to achieve higher SNR over a considerably wider range of modulation frequencies irrespective of camera frame-rate limitations. 18 This feature was tested in dental caries imaging applications in this study.…”

“…The current study reports this evaluation accomplished through longitudinal imaging of controlled, artificial demineralization using established protocols on multiple healthy tooth samples. These images are also compared with the output of single frequency-thermal wave radar (SF-TWR) imaging, 18 a more conventional depth-integrated 2D thermophonic imaging technology, which is also representative of the output achieved by LIT (see the Appendix). Furthermore, the 3D reconstruction capabilities of eTC-PCT for characterizing and visualizing the subsurface extent of early lesions were tested, including the demonstration of the ability of eTC-PCT to image early dental surface erosion.…”