Lippmann Process

(1) A method of color photography invented in 1891 by G. Lippmann.

In the process a layer of transparent, fine-grained photographic emulsion is deposited on a mirrorlike reflecting metal layer (for example, mercury amalgam). During exposure the objective lens projects an image of the object onto the emulsion, and each small area of the emulsion is exposed to rays of a certain color (that is, of one or more specific wavelengths λ). After reflection by the metal layer, the rays again pass through the emulsion. As a result of interference between the incident and reflected rays, standing light waves are generated within the emulsion layer.

In the developed photographic plate the silver forms a system of layers that are parallel to the surface of emulsion and that correspond to the antinodes of standing waves of the light used to expose the plate. For monochromatic light the optical distances between the layers (the planes of maximum silver concentration) are equal to λ/2. (A mixture of rays with various λ, which produces the sensation of a certain color, gives a more complex distribution of silver density.) If a developed and fixed Lippmann plate is illuminated by white light that is normally incident on the surface of the plate, each area ΔS (each element of the colored image) will reflect with maximum intensity the rays of the wavelengths that were interfering during exposure, that is, rays of the color that initially illuminated the area. Thus, the plate as a whole (a multiplicity of ΔS) reproduces the color image projected on it.

Because of technical difficulties the Lippmann process was not widely used in its original form. However, it paved the way for the present rapid development of holography, which is based on a similar principle.

(2) The integral Lippmann process (proposed by Lippmann in 1908) is a method that makes possible the production of three-dimensional images on a flat plate. The images can be viewed directly, without optical devices. The method is based on the use of an “integral plate,” which consists of a set of small biconvex lenses arranged in front of a layer of light-sensitive emulsion (Figure 1,a). An object may be photographed using an integral plate without any optical auxiliaries. Each lens element functions as an independent lens, producing on the emulsion a microscopic image of the object. The inversely developed and fixed integral plate is illuminated by diffuse light coming from the direction of the emulsion (Figure l,b). Thus, a three-dimensional (stereoscopic) image of the object is formed in the region of space occupied by the object during exposure. The image is the result of superimposition of the individual images generated during the reverse passage of light rays through each lens element. Such integral images are characterized by highly stereoscopic properties, by the play of light and shade and of light reflections, and by development of the image when viewed from different sides.

Figure 1. Integral photography according to the Lippmann process: (a) photography of an object O with an integral plate P; (b) reconstruction of the three-dimensional (integral) image O’; (R₁, L₁) and (R₂, L₂) positions of the right and left eyes of a person during exposure and viewing of the image, respectively

Very great technical difficulties encountered in preparing high-quality integral plates preclude large-scale use of the integral Lippmann process. However, wide areas of use have been found for methods that are simplifications of the process, such as raster stereophotography and three-dimensional motion pictures. In the latter case the projection of the image involves the use of a radial-raster stereoscopic screen consisting of conical lens elements.