
Finnish startup develops 1mm thin optics for augmented reality
Although the use of diffractive gratings isn’t new per se to bring images in front of the wearer’s eyes, the researchers managed to eliminate several issues typically associated with this approach, namely rainbow effects and diffraction patterns due to transmissive diffraction.
It took Dispelix Oy’s CEO Antti Sunnari and his team a year and a half of development to come up with an optical solution that can be as thin as 1mm in glass, or a bit thicker in optical-grade plastic (for better mechanical resistance), something that would make smart glasses lighter and more aesthetic.
The whole optical path consists of a nanometre-scale coupling grating receiving the image from a display engine on one end, a single waveguide that routes the light beams via total internal reflection through the lens, and on the other end, an out-coupling nanometre-scale grating that expands the virtual image directly onto the wearer’s pupil.
This yields a field of view of 30º, supporting a virtual image equivalent to a 60-inch TV viewed from a distance of three metres, claims Sunnari.
Sunnari didn’t want to say much more about the physical features of the nanometre-scale coupling gratings, except that all the know-how is in the grating profile, which may not necessarily be homogeneous across the whole optical coupling area.
"We must adjust the gratings’ features for the optical materials being used", he explained.
"We could use plastic injection moulding for monochrome displays, or nano-imprint technology from a master designed with an e-beam to cast the lenses from high refractive index glass. The waveguide supports 450 to 650nm light sources and is not polarization-dependant" Sunnari commented.
The out-coupling gratings only show up as a greyish area on the lenses, yet because the features are so small, they hardly affect transparency. According to the CEO, the company could achieve a transparency of up to 70 or 75% using materials whose transparency is inherently limited to 80%. Typically, for stereoscopic views, two display engines and two separate optical paths could serve the left and right lenses separately.
Dispelix is in the process of raising funds to finalize development and bring its first products to market next year. The company intends to run a fabless operation, designing the gratings and prototyping the optics in its own facilities and then transferring the grating master or stamp to partnering mass manufacturers.
"Each smart glass manufacturer has its own specifications, with distinct requirements on transparency, thickness, optical efficiency, layout, and all these parameters affect how we design the gratings", Sunnari justified, "the optical path can also be curved to some extent" he concluded, hinting that corrections on the out-coupling grating may be necessary on a case-by-case design basis.
Visit Dispelix Oy at www.dispelix.com
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