Tobi Delbruck
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Adaptive Photoreceptor Circuit

The original adaptive photoreceptor circuit is described in a long, rather tutorial version of about 25 pages, and a greatly shortened extract prepared for ISCAS 94. The most recent photoreceptor paper in ISCAS2004 is about an extremely low power self-biasing version of the circuit.

Simulating photoreceptors with SPICE

If you are using SPICE to simulate the photoreceptor, you may want to have a look at this PDF note about simulation of junction diodes.

Radiometry and photometry units for image and vision sensor developers

(relating chip illumination to scene illumination), see these Notes on practical photometry, rewritten from the Pulsed-Bipolar imager project. There is a magic formula relating chip illumination to scene illumination explained (albeit very concisely) and the error that existed previously is now corrected (Feb 2006). These notes also explain photometry units: nits, candela, lumens, lux, and they relate incident illumination to luminance of a surface.

An adaptive photoreceptor circuit can be used in massively parallel analog VLSI silicon chips that compute vision problems in continuous time.

The receptor provides a continuous-time output that has low gain for static signals (including circuit mismatches), and high gain for transient signals that are centered around the adaptation point. The response is logarithmic with illumination, which makes the response to a fixed image contrast invariant to absolute light intensity.

The 5-transistor receptor can be fabricated in an area of about 60 by 60 µm2 in a 2µm single-poly CMOS technology. It has a dynamic range of 1 to 2 decades at a single adaptation level, and a total dynamic range of more than 6 decades. Several technical improvements in the circuit yield an additional 1 to 2 decades dynamic range over previous designs without sacrificing signal quality. The lower limit of the dynamic range, defined arbitrarily as the illuminance at which the bandwidth of the receptor is 60 Hz, is at approximately 1 lux, which is the border between rod and cone vision and also the limit of current consumer video cameras.

The layout looks like this:

We describe an adaptive element that is resistant to excess minority carrier diffusion. We show measurements of the effectiveness of guard structures, and of the spectral sensitivities of devices that can be built in a BiCMOS process. The logarithmic transduction process makes the time constant scale inversely with intensity. As a result, the total A.C. RMS receptor noise is constant, independent of intensity. The spectral density of the noise is within a factor of two of photon shot noise and varies inversely with intensity. The connection between shot and thermal noise is beautifully illustrated.

The noise properties of subthreshold transistors is discussed much more fully in a paper by Rahul Sharpeskar, Tobi Delbruck, and Carver Mead.
Tobi Delbruck

September 13, 2007
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