Terahertz (THz) Imaging: Seeing Through the Opaque
By Eva Cai
Ever dreamt of having X-ray vision like your childhood superhero? That vision may soon turn into a reality. However, rather than X-rays, it is X-rays’ longer-wavelength cousin—terahertz (THz) waves—that could take seeing through opaque objects outside the realm of science fiction. Recently, researchers at MIT developed novel terahertz imaging technology to analyze opaque layered objects—such as books—without physically opening the objects, marrying the impact of technology with the preservation of knowledge.
X-rays and THz waves both belong to the electromagnetic (EM) spectrum, the range of all electromagnetic radiation. Electromagnetic radiation, as its name suggests, consists of the propagation of both electric and magnetic fields, creating oscillating waves that make up the electromagnetic spectrum. From microwaving food to transmitting radio signals to creating visible light that we perceive with our eyes, electromagnetic radiation plays a fundamental role in our universe. Depending on their wavelength and frequency, electromagnetic waves occupy a specific niche on the electromagnetic spectrum, giving them specific properties and uses. Among the many types of waves in the electromagnetic spectrum, terahertz waves are undoubtedly one of the most emerging and fascinating.
Terahertz imaging uses terahertz waves to characterize opaque objects, making the technique suitable for tasks ranging from security screening2 to analyzing material quality. In 2016, Redo-Sanchez and his team at the Camera Culture group of the MIT Media Lab took these usages one step further, as they used THz imaging to scan and extract page-by-page information from closed books. But how exactly does THz imaging work?
With a wavelength in between those of microwave and infrared waves, terahertz waves pose various advantages over traditional imaging wavelengths. Prior to this application, THz imaging was used extensively for security purposes, taking advantage of the fact that “different chemicals absorb different frequencies of terahertz radiation to different degrees”2, 3. Similarly, ink and paper absorb and reflect terahertz waves to different extents, allowing researchers to identify and differentiate between them when reading through a book; such contrast between paper and ink cannot be achieved with X-ray or ultrasound imaging, providing THz waves with a unique property and advantage in imaging.4
When light propagates from one medium to another, its path changes course due to the change in the medium’s refractive index, which is a measure of a particular medium’s ability to bend light. In the case of books, the air gap between pages and the pages themselves are considered two different mediums; at the air-page interface, terahertz waves are both reflected and refracted.3 Recording these reflected wavelengths and their time of arrival, researchers are able to gauge the depth of a particular page in the book, thereby determining the order of information presented in those books.2 Beyond simply obtaining images using this technique, researchers have even developed algorithms to “read” these images, identifying individual letters on the page. Because THz imaging can be used to examine the contents of an opaque object non-invasively, one can imagine using THz imaging in the screening of delicate objects.
The development of terahertz imaging, much like any novel technology, faces challenges and limitations. Currently, researchers are able to “read” through nine pages of a closed book.1,3,4 As the number of pages, and thus the book’s thickness, increases, it becomes increasingly difficult to differentiate between background noise and actual signals reflected from the pages in the book.3 Additionally, as terahertz waves dive deeper into the book, the lack of contrast between ink and page makes the interpretation of letters difficult.4 Thus, future directions will likely be focused on overcoming such challenges of THz imaging, as well as considering the cost implications of making a complex imaging system that utilizes terahertz waves.
Novel “layer extraction methods” developed by the MIT Media Lab extend the traditional inspection usages of terahertz waves to archaeological applications, especially the examination of archival manuscripts—which may be particularly fragile and thus challenging to open physically.4 Additionally, the increased resolution and identification power demonstrated by THz imaging are applicable in fields of “material inspection”, in which layered objects such as books, paintings, and industrial coatings are analyzed to uncover their interior structures.4 As THz imaging delves into areas previously inaccessible to other imaging techniques, it in turn enables both the preservation and acquisition of knowledge. With new terahertz imaging technology, judging a book by its cover may soon be a reality.
- Redo-Sanchez, A. et al. Terahertz time-gated spectral imaging for content extraction through layered structures. Nat. Commun. 7:12665 doi: 10.1038/ncomms12665 (2016).
- Armstrong, Carter M. “The Truth About Terahertz.” IEEE Spectrum: Technology, Engineering, and Science News. IEEE Spectrum, 17 Aug. 2012. Web. 02 July 2017. <http://spectrum.ieee.org/aerospace/military/the-truth-about-terahertz>.
- Hardesty, Larry. “Judging a book through its cover.” MIT News. MIT News Office, 09 Sept. 2016. Web. 09 July 2017. <http://news.mit.edu/2016/computational-imaging-method-reads-closed-books-0909>.
- MIT Media Lab. “Reading Through a Closed Book.” MIT Media Lab. MIT, n.d. Web. 10 July 2017. <https://www.media.mit.edu/projects/reading-through-a-closed-book/>.