TECHNOLOGY FOR CHANGE

Smarter Silicon Retina Aims to SeeBetter

R. Colin Johnson | Date: 04-20-11 | Comments

The European Union is aiming to revolutionize artificial vision for both man and machine with an imager that mimics the real-time spatial and temporal processing of biological retinas.

The E.U. SeeBetter project combines the latest understanding of image processing in the biological retina with semiconductors structures that realize the same functions, enabling smarter imaging sensors that filter raw data with feature detection.

Vision sensors today are mainly just pixel arrays that measure the incident light, but the cells in real retinas have a complex wiring structure that preprocesses the raw data for easier feature detection and pattern recognition--cleaning the raw data before it even gets transmitted to the brain.

The SeeBetter project aims to mimic these actions in retinal ganglion cells using photodetector and packaging technology from the Interuniversity MicroElectronics Center (Imec). Other participants in the project include the Friedrich Miescher Institute for Biomedical Research, the Imperial College of London and the University of Zurich.

Imec will manufacture the novel image chip on its 8-inch complementary metal oxide semiconductor (CMOS) line using a hybrid mix of analog and digital circuitry.

The biggest pitfall of conventional image sensors, in comparison to biological retinas, is the stream of redundant data that gets read-out frame-by-frame even if there are no changes in the scene. Biological retinas, on the other hand, use a variable pulse-density encoding that only transmits changes in the visual data. As a result, output activity spikes during fast changes in the visual scene, then descends to a slow firing rate during times when the scene is not changing much--instead of just chugging along at a fixed frame-rate such as conventional semiconductor imagers.

The magic of biological retinas is in their hierarchical architecture using layers of cells that preprocess visual data before summarizing it for transmission to the brain. For instance, what is called on-center/off-surround circuitry wires retinal sensors to ganglion cells that respond strongly to light in the center of their field while simultaneously suppressing the output of surrounding receptors--in essence outlining anything that moves. As a result, fast motion elicits high frequency firing, whereas weak signals result in firing at a low frequency firing--an automatic mechanism that works regardless of the changes in illumination that plague traditional imagers.

In addition to on-center/off-surround spatial processing, the biological eye also uses temporal processing circuitry that takes into account the short-term persistent-memory of retinal cells. In all, the SeeBetter program will emulate six different kinds of retinal ganglion cells with an eye to both machine vision applications and prosthetic-eye replacements. The image chips will use state-of-the-art backside illumination for its heterogeneous array of pixels wired to simultaneously perform both spatial and temporal image enhancement.

Imec plans to manufacture the novel photodetector on its 8-inch (200 millimeter) complementary metal oxide semiconductor (CMOS) line using a mix of analog and digital circuits to perform spatio- and temporal-processing on a single chip.