Pulsed THz imaging for thickness characterization of plastic sheets

• Published in: NDT & E international : independent nondestructive testing and evaluation 2020-12, Vol.116, p.102338, Article 102338
• Main Authors: Zhai, Min, Locquet, Alexandre, Citrin, D.S.
• Format: Article
• Language: English
• Subjects: Engineering Sciences; Nondestructive testing; Optical constants; PMMA; Polycarbonate; Terahertz imaging
• ISSN: 0963-8695; 1879-1174
• DOI: 10.1016/j.ndteint.2020.102338

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(ν)=1.67−0.0096vfor PC and n(ν)=1.61−0.0085vfor PMMA, while the absorption coefficients, which follow the form of the universal dielectric response, are αν=0.564+9.075v1.669 for PC and αν=0.0406+13.66v1.776 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. •Terahertz reflection and transmission has been applied to measure the thickness of PC and PMMA sheets.•Dispersion for both PC and PMMA broadens the width of THz reflected signal, degrading the depth resolution.•Cross-correlation accounting for dispersion is an effective approach of extrapolating the structure of thicker samples.•A factor-of-two improvement enables commercial pulsed THz systems to measure thicknesses of plastics in the few -cm range.