Introduction to the historical process
In 1908, Gabriel Lippmann won the Nobel prize for his work on colour photography. Unlike modern approches, his remarkable technique does not capture and reproduce colours using a few `base’ colours (such as an RGB). Instead, it uses interference phenomena to capture and reproduce rich multi-spectral information. Therefore, Lippmann did not just develop a colour photography technique; he developed a multi-spectral camera and printing process.
The figure above summarises how his acquisition process works. Standard camera optics are used to focus a scene onto a photographic plate, consisting of a photo-sensitive emulsion on a sheet of glass. Furthermore, the emulsion is put in contact with liquid mercury to create a mirror at one side of the emulsion. When an image is captured, light from the scene passes through the camera optics, glass and emulsion and is reflected back by the mercury. This create an interference pattern in the photosensitive emulsion and, by standard photochemical principles, this results in metallic silver to build up, roughly proportional to the average intensity of the interference pattern at that point. Therefore, the interference pattern is stored in the emulsion and, since there is a one-to-one map between the spectrum and the interference pattern in the emulsion, so is the spectrum.
Digital Lippmann Camera
Inspired by the Lippmann photographic process, we are working on a digital spectral camera design based on interferences. Such a camera enables superior color rendition of Lippmann photography to be transposed into the world of digital photography, where storage, analysis, processing and storage of images are greatly facilitated.
We have already built a functional laboratory prototype. Contrarily to Lippmann photography, where the interference pattern appears in the thickness of the emulsion, our design uses a moving two-paths interferometer, which allows to scan this pattern using a standard bidimensional photographic sensor. The recorded data is then processed to recover the spectral content of the image.
In comparison with other spectral imaging principles, this one has the great advantage of using a significant part of the available light, thus improving greatly the efficiency of the image acquisition.
Due to their multispectral nature and sub-micron scale of interference patterns, Lippmann photographs are very difficult to replicate. We investigate a number of ways to print a full digital multispectral image, which, together with multispectral acquisition, will allow for copying Lippmann Photographs. Once developed, the technology may be used to make Lippmann works available for the general public while allowing museums like the Musée de l’Elysée to safely preserve the original photographs. It can be also used for security purposes, as such photographs are much easier to read than to record.
Collaboration with Galatea Laboratory
The goal of our collaboration with GALATEA is to print multispectral photographs using femto-second lasers. This kind of laser allows for very high power pulses, and thus for nonlinear effects in materials, such as a permanent change of the index of refraction. Small dots of different index of refraction can then be arranged to reflect different wavelengths differently.