# Measuring Maritime Paint Thickness under Water Using THz Cross-Correlation Spectroscopy
## 1 Introduction
All these methods cannot measure individual layers in a full stack, which is relevant since ship paint usually consists of three parts: an anticorrosive layer, a binding layer, and the antifouling paint as the outermost layer [19]. Current technology, therefore, fails to measure the antifouling layer separately. It has been demonstrated that terahertz (THz) lightwaves can be used to detect and quantify individual layers in a multilayer paint stack [20,21]. This is typically done using either THz time-domain spectroscopy (THz-TDS) or THz frequency domain spectroscopy (THz-FDS), whilst the more recent modality, THz ellipsometry, is also gaining interest [22]. The common trait for all THz-based measurement techniques is that the broader the frequency spectrum, the more precise thickness measurement one can obtain. In addition, the thinnest measurable layer thickness is normally determined by the highest available THz frequency in the spectrum. However, THz technologies typically have a decreasing output power with increasing THz frequency. Higher THz frequencies additionally tend to have higher absorption coefficients in most materials, which further exaggerates the challenge of being able to measure layer thicknesses with high THz frequencies--particularly in noncontrolled environments in the field. Current commercially available THz technologies address these issues by employing very large bandwidth systems at the expense of price and footprint. This in turn leads to a very limited field use of THz technologies in general [23]. The issue of THz absorption in materials is particularly problematic when it comes to water. At 1 THz, the absorption coefficient is above 200 cm\({}^{-1}\)[24], leading to a 98% signal loss through a 100 \(\upmu\)m water layer as the beam would have to travel through the water layer twice. In practice, this means that measuring paint layer thicknesses through water is not feasible. In the case of container ships, however, the relevant part of the hull for paint monitoring resides below the water line [25,26]. In this work, we show that it is possible to measure a single paint layer underwater using a THz cross-correlation spectroscopy (THz-CCS) system. The THz-CCS system is sufficiently small and robust to be envisioned for field applications. In addition, the system is designed to operate mainly with low THz frequencies to minimize absorption loss in materials that are inspected. Figure 1 shows a conceptual implementation. The THz-CCS system sits in a dry environment and is coupled to a water-proof measuring head that can be operated below the water line. The head mainly contains a THz transmitter (Tx) and THz receiver (Rx), as well as a contact patch to the ship hull. The contact patch is pressed onto the ship hull to create a watertight seal. The THz radiation passes through the contact patch, interacts with the ship paint, and information is reflected back to the receiver. The primary components, which are developed and demonstrated in this work, are the underwater contact patch and data analysis algorithm for extracting paint thickness through the patch underwater.