Physicists unlock multispectral secrets of earliest color photographs

French physicist Gabriel Lippmann pioneered color photography and snagged the 1908 Nobel Prize in Physics for his efforts. But according to a recent paper published in the Proceedings of the National Academy of Sciences (PNAS), Lippmann's technique distorted the colors of the scenes being photographed. Physicists at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland were able to determine the nature of that distortion and developed a means of reconstructing the original spectrum that created the plates.

"These are the earliest multi-spectral light measurements on record, so we wondered whether it would be possible to accurately recreate the original light of these historical scenes," said co-author Gilles Baechler. "But the way the photographs were constructed was very particular, so we were also really interested in whether we could create digital copies and understand how the technique worked."

A physics professor at the Sorbonne, Lippmann became interested in developing a means of fixing the colors of the solar spectrum onto a photographic plate in 1886, "whereby the image remains fixed and can remain in daylight without deterioration." He achieved that goal in 1891, producing color images of a stained-glass window, a bowl of oranges, and a colorful parrot, as well as landscapes and portraits—including a self-portrait. (Fun fact: Lippmann's laboratory protégés included a promising Polish physics student named Marie Skłodowska, who went on to marry Pierre Curie and win two Nobel Prizes of her own.)

Lippmann's color photography process involved projecting the optical image as usual onto a photographic plate. The projection was done through a glass plate coated with a transparent emulsion of very fine silver halide grains on the other side. There was also a liquid mercury mirror in contact with the emulsion, so the projected light traveled through the emulsion, hit the mirror, and was reflected back into the emulsion.

"This causes the light to interfere, and the resulting interference pattern exposes the emulsion differently at different depths," Baechler et al. wrote in their PNAS paper. The exposure was thus "encoded" within the emulsion in an interference pattern. After several minutes of exposure, the plate was removed from the liquid mercury and processed.

For viewing purposes, the finished plate would be turned upside down and a prism was attached to the surface, typically with a Canada-balsam adhesive. Then the plate would be lit from the front at a perpendicular angle with white light. At any point on the plate where the wavelength of light that had generated the laminae matched the wavelength of the incoming light, it would be reflected back toward the viewer; other wavelengths would be absorbed or scattered by the silver grains or just pass through the emulsion to be absorbed by a black anti-reflective coating on the back of the plate.

Lippmann's process never caught on commercially, largely because it required long exposure times and there was no way to make color prints. But it did inspire further advances in color photography. He foreshadowed Dennis Gabor's invention of the holographic method in the 1940s, as well as the development of optical laser holography in the 1960s.

Lippmann's technique was largely forgotten, and his photographic plates were locked away in museum vaults. When Baechler and his EPFL colleagues were offered access to some of those original plates, they jumped at the chance. Modern multispectral cameras capture hundreds of spectral samples in the visible range, but most photographic techniques simply take three measurements of red, green, and blue, according to the authors. The researchers found that Lippmann's process captured between 24 and 64 spectral samples, making it the earliest-known multispectral imaging technique.

Furthermore, "although the reproduced colors can look accurate to the eye, if we examine the full spectrum reflected from a Lippmann plate and compare it to the original, we notice a number of inconsistencies, many of which have never been documented even in modern studies," the authors wrote.

They wanted to better understand the nature of those inconsistencies, in order to determine whether it was possible to undo the distortions and reconstruct the original input spectrum. So they used Lippmann plates to photograph a full spectrum of light, and they discovered that using a layer of liquid mercury shifted the colors of light towards the red end of the spectrum. Using a reflective layer of air shifted the colors toward the blue end of the spectrum.

That proved to be the key. "With the historic plates, there are factors in the process that we just cannot know, but because we understood how the light differed, we could create an algorithm to get back the original light that was captured," said Baechler. "We were able to study invertibility, that is, given a spectrum produced by a Lippmann photograph, we know it is possible to undo the distortions and reconstruct the original input spectrum. When we got our hands dirty and made our own plates using the historical process, we were able to verify that the modeling was correct."

Baechler et al. believe that revisiting Lippmann's pioneering technique could one day lead to new multispectral cameras, printing, and display designs. In fact, the team has already built a prototype digital Lippmann camera. The researchers are currently looking into printing multispectral images on glass with femtosecond lasers.

"The principle is almost the same as Lippmann's except that, instead of relying on photochemistry, we use ultrafast lasers to locally modify the refractive index of substrates such as silica," they wrote. "Since refractive index changes lead to reflections, we can, at least in principle, print Lippmann-style multispectral images at will."

DOI: PNAS, 2021. 10.1073/pnas.2008819118  (About DOIs).

Listing image by G. Baechler et al./Proc. Natl. Acad. Sci., 2021