Ambient light affects the output voltages of the pixel capacitors equally, thus leaving the difference unchanged. However, sunlight may be 1,000 times brighter (50 times after optical filtering) than the LED light, thus causing the pixel to saturate quickly.
Unlike ordinary cameras, which in bright daylight can increase shutter speed or lens f/stop, time-of-flight systems like Canesta's must operate at full sensitivity to capture the maximum amount of infrared light. In the Canesta technology, however, the infrared illumination is stopped before the pixel can saturate. Then, by reversing the pixel capacitor leads and shorting them together, the common-mode signal caused by ambient light is discarded, and the useful differential signal is preserved. This operation is exactly the reverse of the differential-voltage cancellation that happens when two capacitors are shorted together without reversing the leads.
The SunShield operation is repeated about every 100 microseconds and thus never allows the pixel to saturate. By contrast, the small differential voltage is al-lowed to accumulate and build up over the entire frame time, of about 30 milliseconds, for about 1 million cycles of the light source.
At the end of each frame, a depth map is created from the accumulated differential charge at each pixel, the entire chip is reset and the process repeats.
Canesta is a fabless semiconductor house that outsources its manufacturing to Tower Semiconductor. Its 3-D CMOS image sensors are cast in a 180-nanometer process. Its business model calls for it to sell camera modules to OEMs and to license its technology to manufacturers that want to make their own cameras able to perceive in 3-D. So far it has sold about 100 development kits to OEMs and manufacturers for apps as varied as video games, industrial automation and automobiles.
Honda Motor Co. Ltd. has made investments in Canesta totaling more than $5 million over three years.
Canesta predicts that its first automobile design win will be for a "smart" automobile air bag that deploys with a force appropriate to the size of the passenger. Other pending automobile applications include adaptive cruise control, blind-spot detection, backup warning systems, pedestrian detection and parking assistance.