“This experiment demonstrates that our natural born vision can achieve exceptional tasks that we normally would only assign to expensive and sophisticated machinery.”Sandy Peterhänsel

New research has shown that the color vision of the human eye has the ability to see differences even down to the nanoscale. Scientists from the University of Stuttgart in Germany and the University of Eastern Finland have carried out tests that show the color-sensing abilities of the human eye allow it to distinguish between objects that differ in thickness by only a few nanometers.

The research, as reported in Optica [Peterhänsel et al. Optica (2015) DOI: 10.1364/OPTICA.2.000627], assessed the limits of unaided human vision for small variations using volunteers to identify subtle color differences in light passing through thin films of titanium dioxide under highly controlled lighting conditions. A series of these thin films, which are key to many commercial and manufacturing applications such as anti-reflective coatings on solar panels, were produced one layer at a time through atomic deposition, allowing careful control of their thickness and therefore how small a variation could be identified.

The team were able to determine the sample thickness from the observed color based on a color-matching experiment between the thin film samples and a simulated color field presented on an LCD monitor that displayed a pure white color, apart from 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 could then be changed by the test subject until it matched accurately the reference sample.

The human color observation was found to provide a very accurate evaluation, comparable to more sophisticated instrumental methods such as vapor deposition. As principal author Sandy Peterhänsel said, “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.”

The tests only took a couple of minutes, with some test subjects managing to estimate the thickness of the samples which differed by only one or two nanometers from the actual value measured through conventional methods. Compared to common automated methods of determining thickness, the approach also performed very favorably. However, it is doubtful if this method will replace automated methods in the near future as eyes tend to tire very easily, but could be seen as complementary if used in fabrication control as a quick check by an experienced technician, with any deviations detected needing further characterization through other techniques.