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A relative imaging cmos image sensor for high dynamic range and high frame rate machine vision imaging applications
A Relative Imaging CMOS Image Sensor for High Dynamic
Range and High Frame-Rate Machine Vision Imaging
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
prototype design is fully scalable towards newer CMOS fabrication nodes and
higher sensor resolution.
High dynamic range (HDR) imaging is needed in many applications, such as
remote sensing , biomedical imaging , photography  and automotive
industry . 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
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.