more compact, more reliable and easier to use than its home- made laboratory alternatives and predecessors. Progress in form factor, weight, and data rates are ...
Compact, portable Terahertz systems for on-site inspection applications a
Albert Redo-Sancheza, Norman Lamana, Brian Schulkina, and Thomas Tonguea Zomega Terahertz Corporation, 15 Tech Valley Dr., Suite 102, East Greenbush, NY 12061, USA
Abstract— This paper describes an example of using a compact Terahertz (THz) system to analyze the layered structure of a composite plastic samples in a non-destructive manner. Timedomain data is analyzed to measure the thickness of each layer of the sample and determine the presence or absence of adhesive bonding the plastic parts. The presence and position of the adhesive is clearly visible in the THz images and the measured thickness shows an excellent agreement with nominal thickness.
I. INTRODUCTION AND BACKGROUND
T
echnological progress in Terahertz (THz) instrumentation in recent year has produced commercial THz systems that offer excellent performance and that are more compact, more reliable and easier to use than its homemade laboratory alternatives and predecessors. Progress in form factor, weight, and data rates are, perhaps, the parameters that have shown the highest improvements. These parameters also have a major impact in deploying THz systems outside a laboratory environment. For example, Zomega’s Micro-Z THz time-domain spectrometer is a handheld, battery-operated, THz-wave broadband spectrometer with a weight of less than five pounds, a the form factor similar to a large cordless drill, and capable to operate at waveforms rates up to 500 Hz. The increasing performance and integration level of THz systems will reduce the gap between technological capabilities and high demanding requirements in applications in quality control and in-line monitoring in manufacturing environments. THz time-domain systems have been extensively used in spectroscopy applications to analyze the spectral fingerprints of compounds. However, similar to ultrasound systems, the waveform provided by THz time-domain systems can also be analyzed by measuring the position, amplitude, and polarity of the echoes of the main pulse reflected by a sample[1]. This analysis of the waveform provides depth and layer structure information of a sample, including thickness measurements. In this paper, we show an example of using a compact THz timedomain system to analyze the layered structure of two plastic slabs bonded with adhesive. Data was acquired using a Mini-Z THz time-domain system in normal incidence reflection geometry. THz data can determine the presence or absence of adhesive in cases in which plastic is not optically transparent[2]. II. RESULTS Fig. 1 shows an amplitude image (Fig. 1c) of a sample in which the position and shape of the adhesive thread can be clearly identified. Fig. 1d, 1e, and 1f show different depth profiles (b-scans) in which the position of each layer can be seen and also the presence/absence of adhesive. In these b-
scans, we can see that if adhesive is absent, the layer corresponding to the gap interface and back slab appears flat. Otherwise, the deflection of the layer is related to the thickness and index of refraction of the adhesive. If index of refraction of the sample is known, the real thickness of the layer can be measured from the depth profiles. The thickness measurement of the front and back slab and the gap is reported in Table 1. Agreement between nominal and measured thicknesses is excellent.
Fig. 1. Table 1. Front slab Measured thickness (mm) 1.279 Nominal thickness (mm) 1.320 Discrepancy -3%
Back slab 1.294 1.320 -2%
Gap 0.294 0.290 1%
The structural information capabilities that a time domain THz systems can provide (as shown in this example) can be extended to other type of samples and applications. For example, a similar inspection could be done to measure coating thicknesses and check the integrity of a seal. Other applications have been proposed to investigate the structure of art paintings[3]. Furthermore, the compact size, high speed, and high integration level of systems such as the Mini-Z and Micro-Z enables the deployment of THz technology as a quality control tool at the production site. REFERENCES [1] [2] [3]
K. Kawase, T. Shibuya, et al, “THz imaging techniques for nondestructive inspections,” C. R. Phys., 11(7-8), 510–518 (2010). S. Wietzke, C. Joerdens, et al, “Terahertz imaging: a new nondestructive technique for the quality control of plastic weld joints,” JEOS:RP, 2 (2007). E. Abraham, A. Younus, et al, “Non-invasive investigation of art paintings by terahertz imaging,” Appl. Phys. A, 100(3), 585–590 (2010).
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