Review of Moisture and Liquid Detection and Mapping using Terahertz Imaging

Abstract

The relatively high permittivity of liquid water compared to other materials in the Terahertz (THz) range enables a contrast mechanism for the detection and imaging of moisture. In this paper, spatial mapping of moisture and liquid detection by THz imaging is reviewed. Analysis of the moisture content is discussed in terms of a double Debye model for liquid water and effective medium models for the permittivity of the dry and 'wet' materials of interest. Examples from medical applications, forestry products, agriculture/ food products, and polymers are reviewed. Extraction of diffusion rates and diffusion maps from THz images are discussed.

Non-destructive evaluation by terahertz spectroscopy for penetration of acid solutions into epoxy resin

“…The main advantages of THz-TDS for NDT studies are the high penetration of THz electromagnetic waves into polymers [13][14][15][16][17][18], high sensitivity to the inter/intra molecular vibrations (especially for water) [18][19][20][21][22][23][24][25][26], and ability to estimate the materials dielectric properties without direct contact [27][28][29]. The THz spectra of various thermoplastics [13][14][15] and thermosets [16,17] measured by THz-TDS were reported.…”

“…Although free and bound water was experimentally studied by FT-IR [10,30,31], NMR [32], thermomechanical analysis [31,32] etc., it is still a highly controversial subject. Since the interactions of the bound water and free water show slightly different dielectric properties [14,20,[22][23][24][25]29], THz-TDS could have an advantage to distinguish these two states of water. In fact, the THz dielectric spectra of bulk water is composed of several spectral peaks related to the different relaxation times and intermolecular stretching vibration of water [23,24].…”

“…In fact, the THz dielectric spectra of bulk water is composed of several spectral peaks related to the different relaxation times and intermolecular stretching vibration of water [23,24]. The detection and imaging of liquid water and moisture in materials by THz spectroscopy had been previously studied and various applications had been demonstrated on medical, agricultural, food, and other products [20,27]. Note that water molecules in material can also exist as both 'free' and 'bound' water and it is difficult to separate experimentally the contributions from bound and free water to the THz spectra [20].…”

“…The detection and imaging of liquid water and moisture in materials by THz spectroscopy had been previously studied and various applications had been demonstrated on medical, agricultural, food, and other products [20,27]. Note that water molecules in material can also exist as both 'free' and 'bound' water and it is difficult to separate experimentally the contributions from bound and free water to the THz spectra [20]. In certain cases, the assumption that only bound water exists in a material makes it possible to measure its content [18,20].…”

“…Note that water molecules in material can also exist as both 'free' and 'bound' water and it is difficult to separate experimentally the contributions from bound and free water to the THz spectra [20]. In certain cases, the assumption that only bound water exists in a material makes it possible to measure its content [18,20]. For example, the amounts of water in polyamide and wood plastic composites were estimated with THz-TDS by measuring the complex refractive index as a function of water content [18].…”