Pulsed THz imaging for thickness characterization of plastic sheets

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(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.