Diffractive vs Reflective Waveguides

The stakes are high in the race for lightweight, energy efficient waveguides for smart glasses. In this article adapted from ZDNet ¹, Israeli augmented reality optics company Lumus explains why it believes its range of reflective 2D waveguide technology beats the diffractive competition.

Augmented Reality (AR), where digital information is integrated with your view of the real world to deliver an enhanced overall experience, has yet to make a significant impact in the consumer market. The reason, says Lumus CEO Ari Grobman, is that AR glasses ‘need to be impressive both functionally and aesthetically’ – which is something he says the new Lumus technology addresses. ‘With Z-Lens, we’re aligning form and function, eliminating barriers-of-entry for the industry and paving the way for widespread consumer adoption,’ he said.

According to Lumus, its new 2D Z Lens technology will enable ‘smaller, lighter AR eyeglasses with high-resolution image quality, outdoor-compatible brightness and seamless Rx [prescription lens] integration.’

Founded in 2000, Lumus has built an IP portfolio around its reflective waveguide technology, which comprises a micro-projector (microLED, liquid-crystal-on-silicon (LCoS) or laser) and a series of transflective partial mirrors that expand the image across the X and Y axes (hence the ‘2D’ nomenclature). This enables a usable field of view while accommodating the tiny projector in the temple of the glass’s frame.

An alternative approach – employed by WaveOptics, among others (see HN May 2021) – uses diffractive waveguides, which normally inject light from the micro-projector perpendicular to the waveguide.

According to Lumus, the brightness and battery efficiency of reflective systems are 3-10x higher than their diffractive counterparts.

This means that not only are reflective waveguides brighter, but they require less battery power to achieve their brightness.

Another advantage of reflective waveguides is colour uniformity: this means that unlike other types of waveguides, reflective waveguides do not need to break up and then reassemble colour. Lumus single waveguides, apparently, use mirrors to reflect the true colour directly into the wearer’s eyes. Most of the competing waveguides need 2-3 waveguides for each eye to achieve RGB (red, green, blue) for the full colour spectrum.

Lumus’s flagship technology is Maximus, which uses an LCoS micro-projector, offers a 50° field of view, a 1:1 aspect ratio, 2048-by-2048 pixel resolution, full colour, and over 4,000 nits ² per watt of LED illumination. This is bright enough for outdoor use and so doesn’t require the lenses to be tinted, which can cause practical and social issues indoors.

The new Z-Lens architecture also offers 2K x 2K resolution and full colour, but shrinks the optical engine by 50%, providing glasses manufacturers with more flexibility over entrance aperture placement and allowing for lighter, less bulky and more natural-looking AR glasses.

Also, like Maximus, Z-Lens technology minimises light leakage, meaning that third parties are unable to see what the AR glasses wearer is viewing. The first Z-Lens prototypes will feature the same 50° field of view as Maximus, but the company’s roadmap has this reaching over 80° in due course.

One of the key criteria for consumer acceptance of smart glasses in public is that they don’t stand out in a crowd. These developments should go some way towards enabling ‘natural-looking glasses with augmented reality functionality that will unlock the consumer market and propel the industry forward,’ says Ari Grobman.

1 - www.zdnet.com/article/this-new-optical-tech-could-make-ar-glasses-look-much-cooler/

2 - The nit is a non-SI name used to specify the brightness of a display (1 nit = 1 candela/m2)