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 very large scale integrated design. A scalable and compact hardware implementation is proposed as a proof-of-concept toward relative image acquisition. The proposed photo-current-ratio-detecting pixels completely bypass the need of conventional photo-current integration which enables high frame-rate operation of up to 24 000 frames/s. The pulse-width-modulated output of the proposed pixel is captured by compact column-parallel readout circuits based on digital counters. The developed 32 x 32 pixel array prototype CMOS image sensor consumes 4 mW of power operating at a nominal 9765 frames/s frame rate, and 6.8 mW of power operating at a maximum 24 000 frames/s. The presented prototype design is fully scalable toward newer CMOS fabrication nodes and higher sensor resolution.
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