Terahertz sensor and digital signal image processing for moisture detection in terahertz transparent materials

This article proposes a system for moisture detection using highly sensitive terahertz sensors at room temperature and relatively high-resolution digital image processing of collected pixels. Moisture can lead to swelling, cracking, or chemical degradation in hygroscopic materials and is typically a time-dependent process. Such a system may help conduct noninvasive experiments with hydrophobic coatings or handle different challenges in appropriate material selection. It is helpful in applications involving recycled plastic, paper, ceramics, pharmaceuticals, and biological tissues. Even small amounts of absorbed moisture in a material can dramatically increase the terahertz attenuation. Besides different hydrophobic material analyses, it may also be helpful in plant health analysis. Pathogens, such as fungi, bacteria, viruses, and environmental stressors, usually cause plant infections and damage their leaves. They strongly influence plant health, yield, and overall aesthetic value. They are usually directly correlated with changes in the dielectric constant of leaf tissues and moisture, which is, again, known to have a strong absorption coefficient for terahertz radiation. This phenomenon leverages terahertz radiation's unique, noninvasive interaction with water molecules and plant tissues. We can repeat noninvasive moisture detection several times and analyze the amount of detected moisture in different areas. We developed and manufactured our low-cost, highly sensitive terahertz sensors and installed them in a terahertz camera. The article describes the properties of our terahertz sensor and some examples of practical usage of the terahertz camera in biology. We will show that the amount of leaf water content or internal leaf moisture for different leaf areas can be easily detected with such a camera and displayed as a relatively high-resolution gray-scale image using some digital signal processing. Combining optical and terahertz pictures of the leaf may mitigate the risk of infections and promote overall plant health.