Terahertz Femtophotography Reveals Ancient Documents
We already have light wave techniques that allow researchers to see faded writing and artwork in ancient documents, or to see “through” a more modern document to see the more ancient information underneath. But now, thanks to terahertz femtophotography, researchers can “read” old books and scrolls considered too fragile to open without even touching them.
So far, MIT researchers have been able to use the technique to read only a single letter each on a stack of nine sheets of paper, writes Larry Hardesty of MIT. That is an improvement over when the technology was first used ten years ago, when it could read just through an envelope.
The technique involves marrying femtophotography—which uses a high-speed camera that can capture nearly a trillion frames per second, allowing it to use scattered light to take pictures even around corners—with terahertz spectroscopy, which uses the band of electromagnetic radiation between microwaves and infrared light.
“Terahertz frequency profiles can distinguish between ink and blank paper, in a way that X-rays can’t,” Hardesty writes. “Terahertz radiation can also be emitted in such short bursts that the distance it has traveled can be gauged from the difference between its emission time and the time at which reflected radiation returns to a sensor. That gives it much better depth resolution than ultrasound.”
While other light spectrums can “see” through books, they typically can’t read text very well, according to MIT researchers. “So while you might be able to hardly detect pages of a closed book if you use a CT scan, you will not be able to see the text,” they explain. “Ultrasound does not have the resolution to detect 20 micron gaps in between the pages of a closed book—distinguishing the ink layers from the blank paper is out of the question for ultrasound.”
The X-rays that were used to read a scroll charred by Mount Vesuvius in Pompeii, for example, had trouble distinguishing the ink from the burned paper, because the ink itself was made of soot rather than having metallic elements. X-rays can also emit harmful radiation, writes Michael Casey of the Associated Press.
The terahertz system, which cost around $100,000, is so sensitive that it distinguishes the different pages of the document by recognizing the tiny air pockets between each page—a distance of only 20 micrometers, Hardesty writes. That’s about twice as big as a red blood cell, or one-fifth the thickness of a human hair.
“Time domain terahertz spectroscopy uses pulses much like radar and ultrasound, providing us with information about the depth and range of the pages by measuring the echo of terahertz pulses,” writes Ramesh Raskar, Director, Camera Culture Group, MIT Media Lab, which directs the project. “In addition, the variance of reflectivity between blank paper and inked paper enables us to recover the content of each page by mapping the distribution of pulses across the reconstructed surface of the page.”
There are three major impediments current to reading closed books, Hardesty writes:
- Spatial resolution, or knowing where the pages are
- Spectral contrast, or distinguishing between the paper and the ink on it
- Occlusion, or having the information from the earlier pages getting in the way of the information on the following pages.
Part of the technique also involved developing an algorithm that interprets the often distorted or incomplete images as individual letters—work that was developed by Georgia Institute of Technology, Hardesty writes. Moreover, that same technology could be used break into the “captcha” systems to help determine that a real human is using a web page, he warns.
While the system is being considered primarily in the context of reading ancient documents—as much as 90 percent of library archives are considered too fragile to be shared with the public—it could be used to analyze any materials organized in thin layers, such as coatings on machine parts, material under paint, or pharmaceuticals, Hardesty writes. Not to mention, it could also be used by spies who wanted to read documents without opening them, writes Charles Choi for LiveScience.
The next phase of the project involves developing a higher-powered system that can read beyond nine pages. In time, innumerable documents that have been too fragile to read could be scanned, giving people around the world access to this previously unknown knowledge.