Jonas Ferdinand Gabriel Lippmann, known as Gabriel Lippmann, accomplished a lot during his 75 years. Born in Luxembourg in 1845, Lippmann, a physicist, was a pioneer of color photography. His method for producing color photographs, which relied upon the interference phenomenon, earned Lippman the Nobel Prize in Physics in 1908.

According to a recent paper published in the Proceedings of the National Academy of Sciences (PNAS), Lippmann’s color photography technique distorted the colors in a scene. Physicists at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have determined the precise distortion recorded by Lippmann’s photographs. Further, the team has developed a way to reconstruct the original spectrum of light that originally exposed the color plates. The analysis shows, theoretically and experimentally, how the spectrum reflected in a Lippman plate is different from the spectrum that exposed the plate. By combining this analysis with an analysis of the color absorption properties of a plate, the original exposing color spectrum can be recovered using an algorithm.

Compared to a typical digital image sensor and most other photographic techniques, which records red, green and blue, EPFL’s physicists discovered that Lippmann’s approach typically captured 26 to 64 spectral samples in the visible region. Lippmann’s technique relies upon the ‘same interference principles that recently enabled gravitational waves to be detected and which is the foundation of holography and much of modern interferometric imaging.’

Gilles Baechler, a co-author of the new paper published in PNAS, said, ‘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, 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.’

‘Lippmann photography pipeline. (A) The original power spectrum incident on one spatial location of the plate, made of an emulsion layer on a glass sheet. (B) The silver density throughout that spatial location’s depth is assumed to be proportional to the exposing interference pattern. (C) After exposure, the plate is developed and the silver density is multiplied by a spatial window that models development effects. (D) When the plate is illuminated, the light is partially reflected on each infinitesimal layer of the recorded pattern. These partial waves interfere, creating the reflected wave, which is represented by a complex wavefunction (real part in color and imaginary part in gray). (E) The power spectrum of the reflected wave resembles the original incoming power spectrum.’ Credit: G. Baechler et al./Proc. Natl. Acad. Sci., 2021

Lippmann’s color photography process included projecting an image onto a photographic plate, which seems standard so far. However, what separated his technique from others is that the projection went through a glass plate that had been coated with a transparent emulsion of very fine silver halide. The fine-grain resulted in higher resolution and required much longer exposure times, limiting the practicality of Lippmann’s technique. There was also a liquid mercury mirror in contact with the emulsion, so the light went through the emulsion, hit the mirror and then reflected to the emulsion. This causes light to interfere and the pattern of interference exposed the emulsion differently across various depths. This resulted in the exposure being ‘encoded,’ so to speak, in the emulsion in an interference pattern.

‘Self-portrait of Gabriel Lippmann viewed under different illumination. (A) Diffuse illumination. (B) Directed light whose incoming direction is the mirror image of the viewing direction with respect to the plate’s surface.’ Credit: G. Baechler et al./Proc. Natl. Acad. Sci., 2021

Upon investigation, although the multi-spectral images reflected from a Lippman plate look accurate to the eye, they are inconsistent, an observation that has never before been documented. ‘We ended up modeling the full process from the multi-spectral image that you capture, all the way to recording it into the photograph. We were able to capture the light reflected back from it and measure how it differed from the original,’ Baechler said.

Baechler continued, ‘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. 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.’

‘Spectrum recovery. (A) The recovery algorithm is verified using self-made plates. (Left) Photographs of the color checker and of the Lippmann plate of the color checker. (Right) The original (i), measured (ii), and recovered (iii) spectra. (B) The recovery algorithm is applied to two historical plates. (Left) Photographs of the plates. (Right) The measured and recovered spectra.’ Credit: G. Baechler et al./Proc. Natl. Acad. Sci., 2021

It’s interesting enough to investigate Lippmann’s early 20th-century photographic technique using modern tools and science. However, the team believes that revisiting Lippmann’s technique can inspire new technological developments as well. Further, the team has already constructed a prototype of a digital Lippman camera. The team hopes that multi-spectral image synthesis and a multi-spectral camera could have intriguing benefits in the 21st century.