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How good is your eyesight?
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.
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.
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.
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 researchers speculate that it may be possible to detect even finer variations if other control factors are put in place.
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 The Optical Society