Measuring Visual Degradation with Light

Sarah Michaud

Scatterings image

Self-portrait of Leonardo da Vinci, circa 1510-1515

Leonardo da Vinci’s celebrated red-chalk-on-paper self-portrait shows obvious signs of aging, the result of hundreds of years of heat, humidity, and other insults. In particular, the severe yellowing of the paper has reduced the contrast with the chalk, leading to degradation of the image. But just how damaged is this work of art? A team of scientists from Italy and Poland developed a non-destructive way to measure the optical degradation of da Vinci’s self-portrait using reflectance spectroscopy. This method could eventually give art conservationists a way to determine how effective they are in preserving other valuable works (Appl. Phys. Lett., DOI: 10.1063/1.4879838).

Paper is mostly made of cellulose fibers. New paper doesn’t absorb light above 200 nm, which gives it a white appearance. Over time, cellulose is exposed to water, UV radiation and pollutants, which cause it to oxidize. This oxidation creates light-absorbing molecules called chromophores. Chromophores absorb light in the violet and blue range and scatter yellow and red, resulting in the yellowed appearance characteristic of old paper. By determining the concentration of chromophores in a piece of paper, and the relative concentration of chemically different types, you can ascertain the degree of decay and infer the conditions that might have led to it.

The researchers’ non-destructive method for measuring chromophore concentration uses an updated version of the Kubelka-Munk model for light propagation in turbid media, combined with time-dependent density functional theory. With a mix of reflectance spectroscopy and computer simulations, the team was able to correlate the optical reflectance spectra of da Vinci’s self-portrait with the absolute concentration of chromophores in the paper, revealing the level of optical degradation. They also modeled the relative concentration of chemically different types of chromophores in the paper.

The team compared the da Vinci measurements with a series of reference samples in different states of optical degradation, including modern samples that had been artificially aged under different conditions. They found the degree of degradation and the modeled mixture of chromophores in the da Vinci self-portrait similar to other ancient documents (modeled and real) stored in similar environments, thereby confirming the known history of the portrait. Repeating these measurements will help establish a more accurate chromophore formation rate for da Vinci’s work. The study authors hope their method will help preserve this and other priceless artifacts so that future generations can appreciate them.

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