Hidden painting: disclosing the secrets of Vermeer through new technology. From the article:

…“Girl With a Flute” shares stylistic similarities with “Girl With the Red Hat” and other Vermeer paintings. On the other hand, if “Girl With a Flute” is not an authentic Vermeer, perhaps “Girl With the Red Hat” is not, either.

“There have been doubts about the attribution for many years,” Dr. [Melanie] Gifford said.

Art experts, aided by a scientist who used to design cameras for reconnaissance planes, are increasingly taking advantage of a technique that is also used to study Mars to help answer questions like this….

In the past, all that art curators and conservators had to work with was what they could see on the surface of the artwork and whatever they could unearth in historical documents. Occasionally, they might remove a speck of paint to analyze an artwork’s layers.

X-rays provided some of the first looks at what could lie beneath the visible top layers. Through a technique called X-ray fluorescence, the same high-energy particles of light can also be used to identify elements like zinc, lead and copper that are found in certain paint pigments. These elements absorb X-rays and re-emit the energy at characteristic wavelengths, a sort of atomic fingerprint.

Dr. [John K.] Delaney’s specialty, reflectance imaging spectroscopy, is one of the newer methods, taking advantage of the fact that different molecules absorb light at different wavelengths. By analyzing the brightness of colors bouncing off something, scientists can often identify what that object is made of. That is of great use to geologists studying minerals on the surfaces of landscapes. The technology helps pharmaceutical companies ensure purity of their medicines, and intelligence experts use similar images taken by satellites and aircraft to find hidden enemy targets.

“You can distinguish between, well, I won’t say too much, but you can distinguish between some different types of painted objects and natural objects,” said Dr. Delaney, who worked for a company that designed cameras for U-2 reconnaissance planes before joining the National Gallery….

Marcello Picollo, a researcher at the Nello Carrara Institute of Applied Physics in Florence, Italy, was part of the team that was the first to apply the technique to the study of artwork. Trained as a geologist, he realized that many pigments are essentially crushed minerals. Reflectance imaging spectroscopy can also identify organic molecules like those found in cochineal insects that have been pulverized to produce a deep red pigment….

The early devices Dr. Delaney used could take images at several wavelengths, so they were called multispectral cameras. Over time, the devices became more sophisticated, able to differentiate between many more wavelengths. They are now described as hyperspectral instead of merely multispectral.