I’ve wondered how Japanese artists of the 16th to 18th centuries were able to beat gold down to the nanoscale for application to screens. How could they see what they were doing? I may have an answer at last. According to some new research, it seems that the human eye can detect colour at the nanoscale.

Before getting to the research, here’s the Namban screen story.

This image is from an Indiana University at Bloomington website featuring a page titled, Portuguese-Speaking Diaspora,

A detail from one of four large folding screens on display in the Museu de Arte Antiga in Lisbon. Namban was the word used to refer to Portuguese traders who, in this scene, are dressed in colorful pantaloons and accompanied by African slaves. Jesuits appear in black robes, while the Japanese observe the newcomers from inside their home. The screen materials included gold-covered copper and paper, tempera paint, silk, and lacquer.

Copyright © 2015 The Trustees of Indiana University

Getting back to the Japanese artists, here’s how their work was described in a July 2, 2014 Springer press release on EurekAlert,

Ancient Japanese gold leaf artists were truly masters of their craft. An analysis of six ancient Namban paper screens show that these artifacts are gilded with gold leaf that was hand-beaten to the nanometer scale. [emphasis mine] Study leader Sofia Pessanha of the Atomic Physics Center of the University of Lisbon in Portugal believes that the X-ray fluorescence technique her team used in the analysis could also be used to date other artworks without causing any damage to them. The results are published in Springer’s journal Applied Physics A: Materials Science & Processing.

Gold leaf refers to a very thin sheet made from a combination of gold and other metals. It has almost no weight and can only be handled by specially designed tools. Even though the ancient Egyptians were probably the first to gild artwork with it, the Japanese have long been credited as being able to produce the thinnest gold leaf in the world. In Japanese traditional painting, decorating with gold leaf is named Kin-haku, and the finest examples of this craft are the Namban folding screens, or byobu. These were made during the late Momoyama (around 1573 to 1603) and early Edo (around 1603 to 1868) periods.

Pessanha’s team examined six screens that are currently either part of a museum collection or in a private collection in Portugal. Four screens belong to the Momoyama period, and two others were decorated during the early Edo period. The researchers used various X-ray fluorescence spectroscopy techniques to test the thickness and characteristics of the gold layers. The method is completely non-invasive, no samples needed to be taken, and therefore the artwork was not damaged in any way. Also, the apparatus needed to perform these tests is portable and can be done outside of a laboratory.

The gilding was evaluated by taking the attenuation or weakening of the different characteristic lines of gold leaf layers into account. The methodology was tested to be suitable for high grade gold alloys with a maximum of 5 percent influence of silver, which is considered negligible.

The two screens from the early Edo period were initially thought to be of the same age. However, Pessanha’s team found that gold leaf on a screen kept at Museu Oriente in Lisbon was thinner, hence was made more recently. This is in line with the continued development of the gold beating techniques carried out in an effort to obtain ever thinner gold leaf.

So, how did these artists beat gold leaf down to the nanoscale and then use the sheets in their art work? This July 10, 2015 news item on Azonano may help to answer that question,

The human eye is an amazing instrument and can accurately distinguish between the tiniest, most subtle differences in color. Where human vision excels in one area, it seems to fall short in others, such as perceiving minuscule details because of the natural limitations of human optics.

In a paper published today in The Optical Society’s new, high-impact journal Optica, a research team from the University of Stuttgart, Germany and the University of Eastern Finland, Joensuu, Finland, has harnessed the human eye’s color-sensing strengths to give the eye the ability to distinguish between objects that differ in thickness by no more than a few nanometers — about the thickness of a cell membrane or an individual virus.

A July 9, 2015 Optical Society news release (also on EurkeAlert), which originated the news item, provides more details,

This ability to go beyond the diffraction limit of the human eye was demonstrated by teaching a small group of volunteers to identify the remarkably subtle color differences in light that has passed through thin films of titanium dioxide under highly controlled and precise lighting conditions. The result was a remarkably consistent series of tests that revealed a hitherto untapped potential, one that rivals sophisticated optics tools that can measure such minute thicknesses, such as ellipsometry.

“We were able to demonstrate that the unaided human eye is able to determine the thickness of a thin film — materials only a few nanometers thick — by simply observing the color it presents under specific lighting conditions,” said Sandy Peterhänsel, University of Stuttgart, Germany and principal author on the paper. The actual testing was conducted at the University of Eastern Finland.

The Color and Thickness of Thin Films

Thin films are essential for a variety of commercial and manufacturing applications, including anti-reflective coatings on solar panels. These films can be as small as a few to tens of nanometers thick. The thin films used in this experiment were created by applying layer after layer of single atoms on a surface. Though highly accurate, this is a time-consuming procedure and other techniques like vapor deposition are used in industry.

The optical properties of thin films mean that when light interacts with their surfaces it produces a wide range of colors. This is the same phenomenon that produces scintillating colors in soap bubble and oil films on water.

The specific colors produced by this process depend strongly on the composition of the material, its thickness, and the properties of the incoming light. This high sensitivity to both the material and thickness has sometimes been used by skilled engineers to quickly estimate the thickness of films down to a level of approximately 10-20 nanometers.

This observation inspired the research team to test the limits of human vision to see how small of a variation could be detected under ideal conditions.

“Although the spatial resolving power of the human eye is orders of magnitude too weak to directly characterize film thicknesses, the interference colors are well known to be very sensitive to variations in the film,” said Peterhänsel.

Experimental Setup

The setup for this experiment was remarkably simple. A series of thin films of titanium dioxide were manufactured one layer at a time by atomic deposition. While time consuming, this method enabled the researchers to carefully control the thickness of the samples to test the limitations of how small a variation the research subjects could identify.

The samples were then placed on a LCD monitor that was set to display a pure white color, with the exception of a colored reference area that could be calibrated to match the apparent surface colors of the thin films with various thicknesses.

The color of the reference field was then changed by the test subject until it perfectly matched the reference sample: correctly identifying the color meant they also correctly determined its thickness. This could be done in as little as two minutes, and for some samples and test subjects their estimated thickness differed only by one-to-three nanometers from the actual value measured by conventional means. This level of precision is far beyond normal human vision.

Compared to traditional automated methods of determining the thickness of a thin film, which can take five to ten minutes per sample using some techniques, the human eye performance compared very favorably.

Since human eyes tire very easily, this process is unlikely to replace automated methods. It can, however, serve as a quick check by an experienced technician. “The intention of our study never was solely to compare the human color vision to much more sophisticated methods,” noted Peterhänsel. “Finding out how precise this approach can be was the main motivation for our work.”

The researchers speculate that it may be possible to detect even finer variations if other control factors are put in place. “People often underestimate human senses and their value in engineering and science. This experiment demonstrates that our natural born vision can achieve exceptional tasks that we normally would only assign to expensive and sophisticated machinery,” concludes Peterhänsel.

Here’s a link to and a citation for the paper,

Human color vision provides nanoscale accuracy in thin-film thickness characterization by Sandy Peterhänsel, Hannu Laamanen, Joonas Lehtolahti, Markku Kuittinen, Wolfgang Osten, and Jani Tervo. Optica Vol. 2, Issue 7, pp. 627-630 (2015) •doi: 10.1364/OPTICA.2.000627

This article appears to be open access.

It would seem that the artists creating the Namban screens exploited the ability to see at the nanoscale, which leads me to  wonder how many people who work with color/colour all the time such as visual artists, interior designers, graphic designers, printers, and more can perceive at the nanoscale. These German and Finnish researchers may want to work with some of these professionals in their next study.