After the Columbia disaster in 2003, NASA grounded the space shuttles for more than a year as it worked on new safety protocols to ensure that such a tragedy would not happen again. As part of the preparations for the Return to Flight mission, the Agency required a method for detecting potentially hazardous defects in the external tank’s sprayed-on insulating foam prior to launch. The solution NASA Langley Research Center scientists suspected was a new imaging technology called terahertz imaging that had the potential to accurately find flaws in the foam on the external tank. Terahertz sensors are used to see through many materials and reveal defects like cracks, voids, and density variations. They can be used for imaging or as an anomaly detector, or both at the same time. Hidden objects or defects inside opaque structures can be seen using Terahertz waves; for example, a hidden glue gel inside a plastic enclosure is observed to change through absorption the Terahertz wave generated and detected by Terahertz sensors. As seen in this video, the hidden gel that is behind an opaque barrier that cannot be seen by a visible nor IR camera becomes visible in the Terahertz frequency range.
There has been intense interest in the use of millimetre wave and terahertz technology for the detection of concealed weapons, hidden metallic as well as non-metallic objects, explosives and other threats. Radiation at these frequencies is safe, penetrates barriers and has short enough wavelengths to allow discrimination between objects. In addition, many solids including explosives have characteristic spectroscopic signatures at terahertz wavelengths which can be used to identify them.
In many cases, real-time or video rate images with sub-millimetre resolutions are required to see small defects or hidden objects. Strong Terahertz sources are required for these imaging applications, which are currently expensive and bulky, while low-cost and room temperature detectors are required for imaging and sensing. These challenges have to be overcome in order for Terahertz technology to be widely adapted by industry.