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University, Company Boost LC Projector Energy Efficiency

Picoprojector (ImagineOptix Corp.)

Picoprojector with polarization grating-polarization conversion technology (ImagineOptix Corp.)

Engineers at North Carolina State University and ImagineOptix Corporation, both in Raleigh, have created a technology to convert unpolarized light into polarized light, making liquid crystal (LC) projectors — the kind often used in classrooms and conferences — almost twice as energy efficient. Their research appeared last week in the journal Applied Optics (paid subscription required).

LC projectors use polarized light, which refers to the single-plane orientation of the light wave’s electric fields to the wave’s travel direction. Efficient light sources such as light-emitting diodes or LEDs, however, produce unpolarized light, which requires the light generated by LEDs to be converted into polarized light for LC projectors. Unpolarized light waves are transmitted on planes in multiple directions.

The most common method of converting unpolarized to polarized light is through a special polarizing filter. This method, however, wastes about half of the light generated by the source, and converts the wasted light to heat. As a result, many LC projectors also have large and noisy fans to dissipate the built-up heat.

The research team, sponsored by ImagineOptix, developed an alternative process for polarizing LED light that converts some 90 percent of the unpolarized light, thus reducing the wasted light to 10 percent and generating much less heat. The process is demonstrated in a device made of a small, four-part assembly in a single unit, called a picoprojector (pictured at top) that can be embedded into portable devices, such as smart phones and tablets.

The process, called polarization grating-polarization conversion, takes a beam of unpolarized light and passes it through an array of lenses, which focus the light on a grid of spots. The light then passes through a polarization grating, which consists of a thin layer of liquid crystal material on a glass plate.

The polarization grating separates the spots of light into pairs, which have opposite polarizations, and then passes through a collection of clear, patterned plates that gives the beams of light the same polarization. A second array of lenses finishes the process by focusing the spots of light back into a single, uniform beam of light.

NC State engineering professor Michael Escuti, one of the authors of the paper, says the commercial implications are far reaching. “Projectors that rely on batteries will be able to run for almost twice as long,” says Escuti. “And LC projectors of all kinds can be made twice as bright but use the same amount of power that they do now.”

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