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Stop and Smell The Spectrum

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Sarina Tracy, [email protected]

When en route to a new destination, travelers are best served by focusing on the “now.” But that can be hard to do when one is moving amongst a flurry of ticket stubs, boarding time announcements and carry-on luggage. It’s far too easy to lose sight of the life that’s happening around us when we’re in flux.

Amsterdam Central train station, however, has taken matters into its own hands by displaying a commanding vista – using the light spectrum – that’s impossible to ignore.

Created by Dutch artist Daan Roosegaarde, “Rainbow Station” is a large-scale artwork that projects a vibrant rainbow onto the station’s 45-meter arch as it stretches over the platforms. Using technology developed by Dr. Michael Escuti at North Carolina State University, and by scientists at ImagineOptix Corp. and Dr. Frans Snik and Dr. Michiel Rodenhuis at Leiden University, the collaborative artwork takes advantage of a geometric phase hologram, or spectral filter, which disperses bright white light as a rainbow. Conventional techniques, such as a prism or regular diffraction grating, have trouble creating such a strong and targeted effect due to errant color dispersion or light leakage. Escuti’s technology, however, provides negligible light loss.

“That was particularly important for this installation, because Roosegaarde is using a powerful spotlight, and any leaked light could be unsafe for train operators or passengers,” Escuti said. “The work demonstrates quite dramatically the use of the geometric phase, instead of the traditional phase controlled by optical path length. The installation depends on a large-scale hologram built with novel patterned liquid crystal technology to precisely control the chromatic dispersion with very little loss.”

Photo courtesy of Studio Roosegaarde.

The modified polarization grating has roughly parallel lines in the form of concentric circles that provide the installation with a precise rainbow arch and ideal color dispersion. By manipulating the input light’s polarization, the element produces only the upper portion of a circle while suppressing the lower portion, creating an arch.

To provide the vibrant colors of Roosegaarde’s vision, the researchers used a two-dimensional pattern for the geometric phase hologram with a feature size of 1.5 μm. This separation allows the different wavelengths of light to appear distinct from one another. And to produce the full range of colors between deep red and violet, the scientists incorporated multiple layers of liquid crystal into the geometric phase hologram.

“It’s a structure we developed at NC State called a multitwist retarder, and in this case, we used it to reach an amazingly wide range of color,” Escuti said. “It actually includes many more colors than our typical smartphones and TVs can produce, or that our digital cameras can capture.”

And so every day, within an hour after sunset, Amsterdam Central Station’s east side will be home to quite the light show for the duration of 2015, the International Year of Light. For Roosegaarde, the project is one that combines art and science “to create an experience you cannot download.” As 50 million travelers yearly pass under the station’s expansive arches, Escuti hopes they will be reminded of our universal wonders.

“Admittedly, the professor in me hopes visitors are reminded that spectra are central to our understanding of the cosmos, both large and small, and central to our ability to engineer useful technology.”

Photonics Spectra
Feb 2015
FiltersLighter SidePrismsopticslight sourcesEuropeAmsterdam CentralDaan RoosegaardeRainbow StationDr. Michael EscutiNorth Carolina StateImagineOptixDr. Frans SnikDr. Michiel RodenhuisLeiden Universitygeometrix phase hologramlight spectrum artmulti-twist retarderStudio Roosegaardlight leakagelight art installation

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