Gabriel Lippmann

 ( 1845 Births ) 

In 1886, Lippmann's interest turned to a method of fixing the colours of the visible spectrum on a photographic plate. On 2 February 1891, he announced to the Academy of Sciences: "I have succeeded in obtaining the image of the spectrum with its colours on a photographic plate whereby the image remains fixed and can remain in daylight without deterioration."

The interference phenomenon in optics occurs as a result of the wave propagation of light. When light of a given wavelength is reflected back upon itself by a mirror, are generated, much as the ripples resulting from a stone dropped into still water create standing waves when reflected back by a surface such as the wall of a pool. In the case of ordinary incoherent light, the standing waves are distinct only within a microscopically thin volume of space next to the reflecting surface.

Lippmann made use of this phenomenon by projecting an image onto a special photographic plate capable of recording detail smaller than the of visible light. The light passes through the supporting glass sheet into a very thin and nearly transparent photographic emulsion containing sub microscopically small silver halide grains. A temporary mirror of liquid mercury in intimate contact with the emulsion reflects the light back through it, creating standing waves whose nodes has little effect while their antinodes create a latent image. After development, the result is a structure of lamellae, a very fine fringe pattern in distinct parallel layers composed of submicroscopic metallic silver grains, which is a permanent record of the standing waves. Throughout the emulsion, the spacing of the lamellae corresponds to the half-wavelengths of the light photographed; λ/(2n), λ being the wavelength of light in air and n is the refractive index of the emulsion. Thus colour information is stored locally. The larger the separation between the fringes, the longer was the wavelength recorded from the image colour, red being the longest.

The finished plate is illuminated from the front at a nearly perpendicular angle, using daylight or another source of white light containing the full range of wavelengths in the visible spectrum. At each point on the plate, light of approximately the same wavelength as the light which has generated the lamellae is strongly reflected back toward the viewer. Light of other wavelengths which was not absorbed or scattered by the silver grains is simply passed through the emulsion, usually to be absorbed by a black anti-reflection coating applied to the back of the plate after it had been developed. The wavelengths, and therefore the colours, of the light which had formed the original image are thus reconstituted and a full-colour image is seen.Bolas, T. et al: A Handbook of Photography in Colours, Marion & Co. (London, 1900):45–59 (Retrieved from archive.org on 11 February 2010)Wall, E. J.: Practical Color Photography, American Photographic Publishing Co. (Boston, 1922):185–199 (Retrieved from archive.org on 5 September 2010) Klaus Biedermann, "Lippmann's and Gabor's Revolutionary Approach to Imaging", Nobelprize.org. Retrieved 6 December 2010.

In practice, the Lippmann process is not easy to use. Extremely fine-grained high-resolution photographic emulsions are inherently much less light-sensitive than ordinary emulsions, so long exposure times are required. With a lens of large aperture and a very brightly sunlit subject, a camera exposure of less than one minute is sometimes possible, but exposures measured in minutes are typical. Pure spectral colours reproduced brilliantly, but the ill-defined broad bands of wavelengths reflected by real-world objects can be a problem. The process does not produce colour prints on paper and it proved impossible to make a good duplicate of a Lippmann colour photograph by rephotographing it, so each image is unique. A very shallow-angled prism was usually cemented to the front of the finished plate to deflect unwanted surface reflections, and this made plates of any substantial size impractical. The size of his early photographs are 4 cm by 4 cm, increased later to 6.5 cm by 9 cm. The lighting and viewing arrangement required to see the colours to best effect preclude casual use. Although the special plates and a plate holder with a built-in mercury reservoir were commercially available for a few years , even expert users found consistent good results elusive and the process never graduated from being a scientifically elegant laboratory curiosity. It did, however, stimulate interest in the further development of colour photography.

Lippmann's process foreshadowed laser holography, which is also based on recording standing waves in a photographic medium. Denisyuk reflection holograms, often referred to as Lippmann-Bragg holograms, have similar lamellar structures that preferentially reflect certain wavelengths. In the case of actual multiple-wavelength colour holograms of this type, the colour information is recorded and reproduced just as in the Lippmann process, except that the highly coherent laser light passing through the recording medium and reflected back from the subject generates the required distinct standing waves throughout a relatively large volume of space, eliminating the need for reflection to occur immediately adjacent to the recording medium. Unlike Lippmann colour photography, however, the lasers, the subject and the recording medium must all be kept stable to within one quarter of a wavelength during the exposure in order for the standing waves to be recorded adequately or at all.

When Lippmann presented the theoretical foundations of his "integral photography" in March 1908, it was impossible to accompany them with concrete results. At the time, the materials necessary for producing a lenticular screen with the proper optical qualities were lacking. In the 1920s, promising trials were made by Eugène Estanave, using glass Stanhope lenses, and by Louis Lumière, using celluloid.