A new camera can distinguish layers of ink hidden by several layers of ink and paper, and could also be used to analyze paintings:
MIT researchers and their colleagues are designing an imaging system that can read closed books. In the latest issue of Nature Communications, the researchers describe a prototype of the system, which they tested on a stack of papers, each with one letter printed on it. The system was able to correctly identify the letters on the top nine sheets. "The Metropolitan Museum in New York showed a lot of interest in this, because they want to, for example, look into some antique books that they don't even want to touch," says Barmak Heshmat, a research scientist at the MIT Media Lab and corresponding author on the new paper. He adds that the system could be used to analyze any materials organized in thin layers, such as coatings on machine parts or pharmaceuticals.
Also at TechCrunch.
Terahertz time-gated spectral imaging for content extraction through layered structures (open, DOI: 10.1038/ncomms12665) (DX)
(Score: 2) by bd on Tuesday September 13 2016, @10:14AM
No, you are measuring the difference of thickness of the pages. The air just needs to be there so that you get individual reflections from the pages. Otherwise, what you would measure would be the (not very useful) overall thickness of the book. It could also work if pages are fixed together with glue that has high refractive index.
A more fundamental limitation is the THz wavelength (in the mm range), which limits the axial resolution of the image. This technology will be fundamentally unusable to produce sharp images of smaller font sizes.
Another fundamental limitation is absorption. It may work for paper, but that does not necessarily mean it will work for parchment or the charred papyrus from Herculaneum. (Of course, with the papyrus, there may not be sufficient gaps). Metal based ink will propably work like a mirror and occlude text beneath it.