Pulsed THz imaging for thickness characterization of plastic sheets

 Zhai, Min, Alexandre Locquet, and D. S. Citrin. "Pulsed THz imaging for thickness characterization of plastic sheets." NDT & E International 116 (2020): 102338.

Abstract

The thickness of 2, 4, 6, 8, 10, and 12 mm thick polycarbonate (PC) and poly-methyl methacrylate (PMMA) sheets were characterized nondestructively by terahertz time-of-flight tomography in reflection. Due to terahertz attenuation and dispersion in the material, first characterized in transmission, features in the reflected signal associated with structure deep in the sample (i.e., the back interface) may be difficult to identify, and therefore signal processing is employed, namely frequency-wavelet domain deconvolution and a cross-correlation approach in which we account for dispersion. The refractive indices are found to be 

$n\left(\nu \right)=1.67-0.0096v$

for PC and 

$n\left(\nu \right)=1.61-0.0085v$

for PMMA, while the absorption coefficients, which follow the form of the universal dielectric response, are 

$\alpha \left(\nu \right)=0.564+9.075v^{1.669}$

 for PC and 

$\alpha \left(\nu \right)=0.0406+13.66v^{1.776}$

 for PMMA, where ν is measured in THz and the absorption coefficient is in units of cm−1. We show that straightforward analysis techniques that neglect dispersion are limited to measuring layer thicknesses in these materials of less than ~17 mm, while accounting for dispersion results in a factor-of-two improvement, allowing us to measure samples as thick as ~36 mm.

for full paper  and the missing lines see 

https://www.sciencedirect.com/science/article/abs/pii/S0963869519305079

… Tensile strength, ISO 527-2, 60 MPa, 65 MPa. Elongation at break, ISO 527-2, 6%, 4%. THz reflection and transmission spectra were measured with a Teraview TPS Spectra 3000, a broadband THz time-domain system, shown schematically in Fig …