Dynamic Range determines the ability to see detail in scenes with varying illumination intensities, whether the viewing element is an eye, conventional film, or a digital image sensor. There are two distinct measures of dynamic range: interscene and intrascene. The first is the absolute range viewable, where the viewing mechanism has time to adapt to the full range of sensitivity to incident illumination and the second is the range achievable in just a single setting and time. The research reported in this Thesis investigates how to combine linear and logarithmic modes of circuit operation to improve CMOS imager intrascene dynamic range. A single chip, 360x288 pixel image sensor has been designed, fabricated and characterized to demonstrate the ideas developed during the research. The pixel circuits are switchable between linear and logarithmic modes: after the set exposure period the linear result is readout then the logarithmic mode is switched in and read-out. Single or two parameter calibration can be performed to reduce the relatively high level of FPN in the raw logarithmic data. The settling time of the logarithmic mode of operation is identified as an important constraint on this approach and is optimized by the inclusion of an amplifier. To permit a pixel pitch of 5.6µm in a 0.18µm technology and achieve a 33% fill-factor, circuit and layout architectures have been devised to place the majority of the amplifier in the column in a way that allows it to be switchable between rows. The combining of linear and logarithmic data in a single image provides an intrascene dynamic range in excess of 120dB. The sensor can operate at 26 frames per second when employing single parameter calibration of the logarithmic mode. Comprehensive characterization of both modes and the overall performance of the sensor is also outlined. Critical discussion and suggestions on further research conclude the Thesis.
|Storm, Graeme George
|University of Edinburgh
|Electronic Thesis or Dissertation
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