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EETE DEC 2015

imaging Rice University reports no-lens camera By Peter Clarke Researchers from Rice University (Houston, Texas) have prepared a research paper on Flatcam, a camera system that replaces lenses with a coded mask and computation. The Flatcam has similarities to the lensless image sensor being developed by Rambus, which replaces the lens in a conventional camera with a diffraction grating and computation. However, the Flatcam does provide a complete optical reconstruction of the given scene, while the Rambus system does not as it is intended for use by machines. The advantage of the Flatcam over a conventional camera is its extremely thin profile. As the coded mask is positioned almost directly on top of the sensor the assembly can measure as little as 0.5mm. The disadvantage is the energy cost of computation required to recreate the image. The mask consists of a pattern of opaque and transparent areas and each transparent region can be considered as a pin-hole. Light gets diffracted and modulated by the mask such that light from each point in the scene casts a complex shadow on the sensor and this mapping can be represented by a linear operator. A computational algorithm can be deduced and used to recover the original scene from the sensor measurements. The potential of the architecture has been demonstrated using a prototype camera made using a commercially available sensor at 512 by 512 pixel resolution and a mask. The monolithic integration and extreme thin-ness could make such cameras useful for mobile and wearable Every point in a scene casts an image on the sensor, but this data must then be processed using the reverse function to that represented by the mask to produce the image. Source: Rice University. applications and the Internet of Things, the authors claim. 1mm thin optics for augmented reality By Julien Happich Founded a month ago as a spin-off from the VTT Technical Research Centre of Finland, startup Dispelix Oy has developed special gratings and optical waveguides that can route any image from a display engine located in the frame of smart spectacles, directly to the wearer’s retina. Although the use of diffractive gratings isn’t new per se to bring images in front of the wearer’s eyes, the researchers ma- naged 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 opticalgrade 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 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 caseby case design basis. 16 Electronic Engineering Times Europe December 2015 www.electronics-eetimes.com


EETE DEC 2015
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