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Diffraction Brings Street Signs to Life

Photonics Spectra
Dec 1997
R. Winn Hardin

ROSKILDE, Denmark -- A new process that impresses holographic optical elements onto polymer rolls could significantly reduce the cost of keeping roads safe at night.
According to Ibsen Micro Structures developmental engineer Michael Rasmussen, street signs and reflectors embedded along roads depend on retroreflection as a passive light source. Retroreflection is the process of turning a light ray around and sending it back to its source -- in this case the headlights of an automobile. The results are highly reflective road signs, paint or clothing that keep both drivers and pedestrians safer at night.

By changing the size of holographic elements, designers can optimize nighttime reflection to highlight the red in street signs such as this.
The major devices used to achieve these results are microspheres or corner cube reflectors. Microspheres use refraction to focus the incoming light on a reflecting material coated onto the back of the sphere. The light then bounces back toward the entrance point, where the spherical surface recollimates the light for its return trip. Nonparaxial rays will not focus on the correct spot, reducing contrast. Also, a layer of microspheres has gaps, leaving about 30 percent of the surface without a retroreflective coating.
Corner cubes depend on three perpendicular surfaces inside the prismlike cube to redirect the incoming light. They offer excellent reflectance efficiencies.
Diffractive holographic elements use submicron relief patterns to create light sources at the face of the polymer surface. The light then focuses on a specularly reflective coating, diffuser coating or another holographic optical element on the back of the sheet.
Rasmussen said the retroreflective efficiency of the holographic element approach falls slightly below that of cube approaches and above the spherical material. For broadband visible reflection, prototypes exhibit 60 percent reflectance. By altering the holographic relief to match a certain color or designing the material for a certain angle of incidence, designers can improve the retroreflection efficiency, he said.

A cheaper solution
The greatest benefit, he said, is the cost savings associated with the production process. To create the prototype, Ibsen rolled a metallic master across the polymer or silica material, similar to a printing process. The result is a highly retroreflective material that offers significant cost savings over the casting/flatbed embossing process associated with corner cubes or metallized sphere production in a fluid polymer solution. G

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