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The optical properties of daguerreotypes, the earliest types of photographs, invented in the nineteenth century, come from the plasmonic response of the metallic nanostructures on their surface. This new finding, from researchers at the University of New Mexico (UNM) and the Metropolitan Museum of Art (The Met) in New York will not only be important for developing protocols for preserving these valuable works of art, but also for inventing new plasmonic colour printing technologies in the future. Plasmons are the collective oscillations of electrons on the surface of metallic nanostructures that interact very strongly with light and produce vivid colours. They could thus be used for developing new colour printing technologies for use in applications such as sensing, imaging, actuation and displays. Western Approach to the Acropolis, Athens, 1842. Purchase, Philippe de Montebello Fund, Mr. and Mrs. John A. Moran Gift, in memory of Louise Chisholm Moran, Joyce F. Menschel and Annette de la Renta Gifts, and funds from various donors, 2016. (Credit: The Metropolitan Museum of Art)
For centuries, people searched for increasingly better ways to capture an image, he says. Images projected by a camera obscura, for example, were first used as guides for drawing and painting natural scenery and portraits from the seventeenth century onwards, but these images could not be permanently captured. Early photographers understood that permanent image capture would require light-matter interactions and identified suitable photoreactive materials, such as silver halides, that might be used to do this. The rest is history. Daguerreotypy was the first commercially successful technology to make use of the light sensitivity of silver halides. It was invented by the French painter Louis-Jacques-Mandé Daguerre (1787-1851) and the technique was able to capture an image from a camera with extraordinary resolution and clarity that rival even the high-resolution images possible today. In their new work, Manjavacas and colleagues analysed the morphology and material characteristics of the nanoparticles on the surface of several daguerreotypes from the study collection at The Met as well as modern plates made by Century Darkroom in Toronto to recreate effects seen in historical plates. They then used the information they obtained to model the response of an “average” nanostructure using a Maxwell’s equation numerical solver. “The results of this modelling allow us to explain optical effects that were empirically known but never explained from a scientific perspective until now,” says Manjavacas. “For example: the change of colour of the image as we vary the viewing angle; the intense and tuneable colour tone achieved though ‘solarisation’ and other exposure techniques used during daguerreotypy; and the warmer hue provided to the daguerreotype image by gilding which involves adding a nanoscale layer of gold to it.” And that is not all; the UNM-Met team also found that the scattering spectrum strongly depends on the particle shape and size but not on its composition. Indeed, decreasing the height of a nanoparticle creates a blueshift in the spectrum while increasing overall nanoparticle size causes a redshift and a broadening of the spectral peak (so reducing the viewing angle-dependent colour-change effect). “Our results provide invaluable insights for how to protect these valuable works of art, which are irreplaceable records of history and culture that must be preserved for posterity,” says Manjavacas. “They could also inspire new approaches for plasmonic colour printing in the future.” The researchers, reporting their work in PNAS 10.1073/pnas.1904331116, say they will now use the knowledge they have gained in this study to understand some of the degradation processes that daguerreotypes suffer. By Belle Dumé. Physics World. Posted: June 19, 2019. |
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