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A relative imaging cmos image sensor for high dynamic range and high frame rate machine vision imaging applications

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A relative imaging cmos image sensor for high dynamic range and high frame rate machine vision imaging applications

  1. 1. A Relative Imaging CMOS Image Sensor for High Dynamic Range and High Frame-Rate Machine Vision Imaging Applications ABSTRACT: This paper proposes an unconventional image acquisition scheme for machine vision applications, based on detecting ratios of illumination (pixel) intensities. Detecting relative ratios enables capturing the scene features and patterns almost independently from the local scene illumination resulting in potentially extremely high dynamic range. Moreover, detecting signal ratios using a fully differential circuit optimally suits the intrinsic nature of VLSI design. A scalable and compact hardware implementation is proposed as a proof-of-concept towards relative image acquisition. The proposed photo-currentratio- detecting pixels completely bypass the need of conventional photo-current integration which enables high frame-rate operation of up to 24000 frames-per-second (fps). The pulse width modulated output of the proposed pixel is captured by compact column-parallel readout circuits based on digital counters. The developed 32_32 pixel array prototype CMOS image sensor consumes 4mW of power operating at a nominal 9765 fps frame rate, and 6.8mW of power operating at a maximum 24000fps. The presented
  2. 2. prototype design is fully scalable towards newer CMOS fabrication nodes and higher sensor resolution. EXISTING SYSTEM: High dynamic range (HDR) imaging is needed in many applications, such as remote sensing [1], biomedical imaging [2], photography [3] and automotive industry [4]. The possibility of having high dynamic range enables resolving both highly illuminated and poorly illuminated areas of the captured scene. For automotive (machine vision) applications, HDR often represents a crucial requirement. Conventional CMOS image sensors suffer from several drawbacks that limit their dynamic range. The photo-generated signal is read out as a voltage value which is derived by integrating the photo-current over the photo-diode junction capacitance. The output voltage has a maximum value determined by the pixel and readout circuits. Therefore, the useful signal has a physical saturation level which limits the maximum signal amplitude, hence limiting the dynamic range. Moreover, since the photo-current is integrated over the junction capacitance, the amplitude of the useful signal also depends on the photo-diode integration time.
  3. 3. PROPOSED SYSTEM: In this paper, spatial pixel-relative acquisition is proposed instead of spatial pixel- gradient acquisition. Precisely, the proposed sensor is able to directly detect ratios between adjacent photo-diode output intensities both in horizontal and vertical directions. This is performed without acquiring the absolute signal values. Theoretically, if the spatial information of the captured scene is represented in pixel ratios, the reconstructed scene is independent from the actual scene illumination. Moreover, detecting ratios corresponds to the intrinsic nature of VLSI design much better than the detection of absolute values.