Lippmann Color Photography
Lippmann in 1881 made use of the phenomenon of stationary waves in light in colour photography.
A photographic plate having an extremely fine grain emulsion is covered with mercury on the emulsion side. The emulsion is in between glass and mercury. Here mercury acts as a reflecting surface. This photographic plate with glass side towards the light is exposed in a camera. Let light of various wavelengths (colours) be incident on the photographic plate. The wavelength of violet light is the least. Therefore the distance between two adjacent antinodal planes for violet light will be less. The wavelength of red light is maximum. Therefore the distance between the two adjacent antinodal planes for red light will be large. Each colour forms its own set of antinodal planes in the emulsion. When such an exposed plate is developed, it contains a number of layers of silver located at the antinodes. These layers are not uniformly separated but the layers for one particular wavelength (colour) are uniformly separated.
When such a developed plate is illuminated with white light, each layer will reflect strongly only that particular colour by which it was originally formed. Here, each layer will reflect only a small amount of light. As white light passes through each layer, it gets reflected. All radiations of the same wavelength (coherent) reflected from different layers due to which the layers were formed, are in phase with each other. Therefore they reinforce and the reflected light of that particular colour is of large intensity. Therefore a particular set of layers for which the path difference is not one wavelength (not coherent) will not reinforce and their intensity will be much less. If n waves of amplitude A combine, their resultant intensity is (nA)2 in the case of coherent waves but it is much less in the case of coherent waves. In the case of non-coherent waves the resultant intensity is nA2. To conclude, a particular set of layers for which the path difference between reflected light is λ will reinforce and that particular colour is strongly reflected by which these layers were originally exposed.
The colour photographs due to Lippmann process are extremely brilliant. But no prints can be taken from it and the process to prepare the plates is tedious.
Note: Lippmann process is not used commercially. The modern process is based on three colour process. The two methods used are (i) additive and (ii) subtractive.
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