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Oversampled binary image sensor
An oversampled binary image sensor is a new image sensor that is reminiscent of traditional photographic film.〔(L. Sbaiz, F. Yang, E. Charbon, S. Süsstrunk and M. Vetterli, The Gigavision Camera, ''Proceedings of IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP)'', pp. 1093 - 1096, 2009. )〕〔(F. Yang, Y.M. Lu, L. Saibz and M. Vetterli, Bits from Photons: Oversampled Image Acquisition Using Binary Poisson Statistics, ''IEEE Transaction on Image Processing'', vol. 21, issue 4, pp.1421-1436, 2012. )〕 Each pixel in the sensor has a binary response, giving only a one-bit quantized measurement of the local light intensity. The response function of the image sensor is non-linear and similar to a logarithmic function, which makes the sensor suitable for high dynamic range imaging.〔
==Introduction==
Before the advent of digital image sensors, photography, for the most part of its history, used film to record light information. At the heart of every photographic film are a large number of light-sensitive grains of silver-halide crystals.〔T. H. James, The Theory of The Photographic Process, 4th ed., New York: Macmillan Publishing Co., Inc., 1977.〕 During exposure, each micron-sized grain has a binary fate: Either it is struck by some incident photons and becomes "exposed", or it is missed by the photon bombardment and remains "unexposed". In the subsequent film development process, exposed grains, due to their altered chemical properties, are converted to silver metal, contributing to opaque spots on the film; unexposed grains are washed away in a chemical bath, leaving behind the transparent regions on the film. Thus, in essence, photographic film is a binary imaging medium, using local densities of opaque silver grains to encode the original light intensity information. Thanks to the small size and large number of these grains, one hardly notices this quantized nature of film when viewing it at a distance, observing only a continuous gray tone.
The oversampled binary image sensor is reminiscent of photographic film. Each pixel in the sensor has a binary response, giving only a one-bit quantized measurement of the local light intensity. At the start of the exposure period, all pixels are set to 0. A pixel is then set to 1 if the number of photons reaching it during the exposure is at least equal to a given threshold ''q''. One way to build such binary sensors is to modify standard memory chip technology, where each memory bit cell is designed to be sensitive to visible light.〔S. A. Ciarcia, A 64K-bit dynamic RAM chip is the visual sensor in this digital image camera, ''Byte Magazine'', pp.21-31, Sep. 1983.〕 With current CMOS technology, the level of integration of such systems can exceed 109~1010 (i.e., 1 giga to 10 giga) pixels per chip. In this case, the corresponding pixel sizes (around 50~nm 〔Y. K. Park, S. H. Lee, J. W. Lee et al., Fully integrated 56nm DRAM technology for 1Gb DRAM, in ''IEEE Symposium on VLSI Technology'', Kyoto, Japan, Jun. 2007.〕) are far below the diffraction limit of light, and thus the image sensor is ''oversampling'' the optical resolution of the light field. Intuitively, one can exploit this spatial redundancy to compensate for the information loss due to one-bit quantizations, as is classic in oversampling delta-sigma conversions.〔J. C. Candy and G. C. Temes, Oversamling Delta-Sigma Data Converters-Theory, Design and Simulation. New York, NY: IEEE Press, 1992.〕
Building a binary sensor that emulates the photographic film process was first envisioned by Fossum,〔E. R. Fossum, What to do with sub-diffraction-limit (SDL) pixels? - A proposal for a gigapixel digital film sensor (DFS), in ''IEEE Workshop on Charge-Coupled Devices and Advanced Image Sensors'', nAGANO, jUN. 2005, PP.214-217.〕 who coined the name ''digital film sensor''. The original motivation was mainly out of technical necessity. The miniaturization of camera systems calls for the continuous shrinking of pixel sizes. At a certain point, however, the limited full-well capacity (i.e., the maximum photon-electrons a pixel can hold) of small pixels becomes a bottleneck, yielding very low signal-to-noise ratios (SNRs) and poor dynamic ranges. In contrast, a binary sensor whose pixels only need to detect a few photon-electrons around a small threshold ''q'' has much less requirement for full-well capacities, allowing pixel sizes to shrink further.
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