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Top Row: Composed photo of all sample color fields. Bottom Row: Defects in those samples - seen at the edges of some - caused by thickness variation. Image Credit: Institut für Technische Optik and Research Center SCoPE, University of Stuttgart, Germany

How good is your eyesight?

The human eye can accurately distinguish between the most subtle differences in color. Where human vision falls short is in perceiving minuscule detail. Researchers in Finland have now captured that ability in us - using color.

In a paper published by The Optical Society's new journal Optica, a research team from the University of Stuttgart, Germany and the University of Eastern Finland in Joensuu, have created a test. Harnessing our color-sensing strengths, the team increased observers ability to distinguish objects differing in thickness by no more than a few nanometers — or the thickness of a cell membrane, or single virus.

This ability to go beyond the diffraction limit of the human eye was demonstrated by teaching a small group of volunteers to identify subtle color differences in light that had passed through thin films of titanium dioxide under highly controlled and precise lighting conditions. Their results revealed an untapped human potential rivaling sophisticated optic tools used to measure such minute thicknesses.

"We were able to demonstrate that the unaided human eye ican determine the thickness of a thin film — materials only a few nanometers thick — by simply observing the color it created under specific lighting conditions."

Sandy Peterhänsel PhD, Institut for Optical Techniques and Research Center, University of Stuttgart, Germany, 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. Thin 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 over a surface. Highly accurate if time consuming, this procedure, and techniques such as vapor deposits, are used throughout industry.

The optical properties of thin films influences light shining through their surfaces. It is the same process we observe that creates scintillating colors in soap bubbles and oil films floating on water. The colors produced depend strongly on the composition and thickness of the film material, as well as the properties of the incoming light. Skilled engineers can quickly estimate the thickness of surface films to within 10 to 20 nanometers.

Their observational abilities inspired researchers to test the limits of the general population's vision to measure how small a variation in thickness can be detected under normal, if ideal conditions.

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

Sandy Peterhänsel PhD

Experimental Setup

The setup for this experiment was simple. A series of thin films of titanium dioxide were manufactured and placed one layer at a time using atomic depositors. While time consuming, this enabled researchers to carefully control the thickness of each sample in order to test how small a thickness variation research subjects could identify.

Samples were displayed on a LCD monitor set to a pure white background, while colored areas were calibrated to match the apparent surface color of thin films of varying thicknesses.

The color of the reference field could be changed by the test subject until it perfectly matched the reference sample. Correctly identifying a color meant the observer also correctly determined film thickness. This could be done in as little as two minutes, and for some test subjects their estimation of thickness differed only by one-to-three nanometers from the actual value as measured by conventional mechanical means.

This level of precision was previously thought to be 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 with some techniques, the performance of the human eye compared very favorably. But, as human eyes tire very easily, this experimental process is unlikely to replace automated methods. It can, however, serve to support quick checks by experienced technicians.

"The intention of our study never was solely to compare human color vision to more sophisticated calibrations. Just finding out how precise human color vision can be was our main motivation."

Sandy Peterhänsel PhD

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. Tasks that we normally would only assign to expensive and sophisticated machinery," concludes Peterhänsel.

Abstract
We study how accurately a naked human eye can determine the thickness of thin films from the observed color. Our approach is based on a color-matching experiment between thin-film samples and a simulated color field shown on an LCD monitor. We found that the human color observation provides an extremely accurate evaluation of the film thickness, and is comparable to sophisticated instrumental methods. The remaining color differences for the matched color pairs are close to the literature value for the smallest visually perceivable color difference.

Paper: S. Peterhansel, H. Laamanen, J. Lehtolahti, M. Kuittinen, W. Osten, J. Tervo, "Human color vision provides nanoscale accuracy in thin-film thickness characterization," Optica, 2, 7, 627 (2015). doi: 10.1364/OPTICA.2.000627

About Optica
Optica is an open-access, online-only journal dedicated to the rapid dissemination of high-impact peer-reviewed research across the entire spectrum of optics and photonics. Published monthly by The Optical Society (OSA), Optica provides a forum for pioneering research to be swiftly accessed by the international community, whether that research is theoretical or experimental, fundamental or applied. Optica maintains a distinguished editorial board of more than 20 associate editors from around the world and is overseen by Editor-in-Chief Alex Gaeta of Cornell University. For more information, visit http://optica.osa.org.

About The Optical Society
Founded in 1916, The Optical Society (OSA) is the leading professional organization for scientists, engineers, students and entrepreneurs who fuel discoveries, shape real-life applications and accelerate achievements in the science of light. Through world-renowned publications, meetings and membership initiatives, OSA provides quality research, inspired interactions and dedicated resources for its extensive global network of optics and photonics experts. OSA is a founding partner of the National Photonics Initiative and the 2015 International Year of Light. For more information, visit http://www.osa.org.

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