Every night, for just a short time after sunset, Amsterdam Central Station becomes Rainbow Station. A four-kilowatt spotlight projects a stunning rainbow through a custom-designed liquid crystal spectral filter onto one of the station's 45 by 25 meter (148 by 82 feet) roof arches, just above platform 2b. This liquid crystal optics technology is being developed for research on exoplanets, but it will light up Amsterdam Central – and the lives of thousands of travelers – every night for a year.
The work comes courtesy of artist Daan Roosegaarde and NC State University researcher and ImagineOptix Corporation founder Michael Escuti, in celebration of the 125th anniversary of the station's opening and the start of the UNESCO International Year of Light 2015.
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It couldn't have worked using conventional methods of rainbow projection, such as shining light through a prism or diffraction grating, because these would have resulted in either a washed-out look or a high percentage of light "leaking" into the surrounding environment, the latter being a particular issue because the light could temporarily blind train operators or passengers. Escuti and his team at ImagineOptix instead looked to polarization gratings (patterned parallel vertical lines that polarize and split beams of light) as a starting point to craft something new.
The concentric circles of the geometric phase hologram, as seen through a polarizing optical microscope (Image: ImagineOptix Corporation)
They developed what's known as a geometric phase hologram with a modified polarization grating that has its parallel lines curving around in concentric circles. These concentric circles are a mere 1.5 microns apart so that they can more clearly separate the different wavelengths of light. The geometric phase hologram, meanwhile, incorporates multiple layers of liquid crystal.
"It’s a structure we developed at NC State called a multi-twist retarder and in this case we used it to reach an amazingly wide range of color," Escuti explains. "It actually includes many more colors than our typical smartphones and TVs can produce, or that our digital cameras can capture."
All of this technology would result in a full-circle rainbow, but the input light's polarization is manipulated to suppress the lower portion of the circle and light up nothing more than the arch. It's so effective, in fact, that around 99 percent of the light goes into the rainbow, while the remaining fraction leaks harmlessly out into the surrounding environment.
You can see a video showcasing the Rainbow Station below.