Rambus prototypes 2x2mm lens-less eye-tracker for headmount displays

June 21, 2016 // By Julien Happich
At last week's VLSI Symposia, Rambus presented a poster titled "Lensless Smart Sensors: Optical and Thermal Sensing for the Internet of Things" in which the company not only detailed the underlying technology but also demonstrated a working sensor prototype.

The Lensless Smart Sensors (LSS) rely on a phase anti-symmetric diffraction grating (either tuned for optical or IR thermal sensing) mounted directly on top of a conventional imaging array and co-designed with computational algorithms that extract the relevant information from the scene to be imaged. The grating is very thin and boasts a wide field of view, up to 120º, and the resulting imaging sensor is almost flat (only a few hundred micrometres separate the grating from the image sensor).

Phase anti-symmetric grating structure of the Lensless Smart Sensor.

The raw sensed image is encoded by the grating structure, calling for dedicated reconstruction algorithms and image processing, but in some applications such as range-finding or eye-tracking, it may not even be necessary to reconstruct a full image. Instead, extracting distance measurements may suffice and the particular phase anti-symmetric diffraction structure makes it very simple, explains the poster.

Light from left and right of the anti-symmetric boundary (at the centre of the grating) cancels in a curtain under the boundary. And the stereoscopic shift of the Point Spread Functions (PSF) of a point light source viewed through two gratings can be used to determine the distance of that light source with a much greater accuracy than would be feasible with stereoscopy using lenses, the authors write.

To put their theory into practice, the researchers designed an ultra-low power 2x2mm2 image sensor, with a 128x128 pixel array and integrated image change detection circuitry on the same die. They then used two identical phase gratings mounted in apertures on a shared pixel array. The gratings were only 1.86mm apart, each within apertures only 55μm in diameter. This setup was enough to measure distances up to 50cm with an error of less than 8%, they reported.

Rambus' image change detection sensor in 180nm
technology, with attached metal aperture and
phase grating.

The wide field of view and the very compact and flat form factor of such a stereoscopic lens-free sensor make it very well suited for wearable eye-tracking applications within smart goggles or head-mount displays. Here the light sources could be near-IR emitters integrated within the periphery of the glass frame.