Time derivative silicon retina

• T. Delbrück and C.A. Mead. (1991). Time-derivative adaptive silicon photoreceptor array. In T.S. Jay Jayadev (ed.) SPIE Vol. 1541, Infrared Sensors: Detectors, Electronics, and Signal Processing, pp. 92-99.
• MPEG movie of the scanned chip output. (Subjects are Manny and Tobi Delbruck)

We designed and tested a two-dimensional silicon receptor array constructed from pixels that temporally high-pass filter the incident image. There are no surround interactions in the array; all pixels operate independently except for their correlation due to the input image. The pixels compute the temporal derivative of the log intensity; thus, if the illumination varies over the image, only the derivatives of the reflectance are output, not the derivatives of the absolute intensities.

The high-pass output signal is computed by sampling the output of an adaptive, high-gain, logarithmic photoreceptor during the scanout of the array. After a pixel is sampled, the output of the pixel is reset to a fixed value. An interesting capacitive coupling mechanism results in a controllable high-pass filtering operation. The pixel looks like this:


The resulting array has very low offsets. The computation that the array performs may be useful for time-domain image processing, for example, motion computation.

In the movie, note how the low contrast features like the lines on the back of hand stand out sharply. These features would have been invisible against a salt and pepper built-in static noise if the receptors were not adaptive high pass, high gain devices.

See the 2006 work on the asychronous transient silicon retina for other work on time-domain vision.