Bio-Inspired Electronic Eyes for Artificial Vision System

Recent technological developments in mobile electronics and machine vision have required advanced imaging recognition systems. However, conventional imaging devices using a flat sensor array have several limitations such as complex lens optics, optical aberration and large module size. To solve this problem, bio-inspired image sensors mimicking the structural and functional advantages of the of nature eye have been extensively studied. A curved image sensor that mimics hemispherical structure of the animal retina can effectively overcome the limitations of conventional imaging systems. It has a simple configuration and miniaturized module size while maintaining high optical performance such as wide field of view (FoV) and low optical aberration.<br/><br/>In addition, curved image sensors applying unique properties of organisms that evolved to survive in specific environments have also been reported. For example, artificial imaging systems that mimic the compound eyes found in insects have infinite depth of field (DoF) and a wide FoV. Imaging systems that mimic telescopic vision of avian eye and the visual system of xenos peckii for recognizing image depth have been demonstrated. By using various optical filters, artificial vision inspired by mantis shrimp can enhance the color sensitivity.<br/><br/>Recently, our group mimics the unique vision of feline eye. Feline-eye-inspired vision system mimics the variable pupil and biological reflector. Blurring the background to distinguish the prey from the complex environment is essential for targeting the prey. In the dim environment, the feline pupil fully dilates to make narrow DoFs in the both vertical and horizontal direction. In the bright environment, the feline pupil contracts to become a vertically elongated shape, resulting in a narrow vertical DoF and a deep horizontal DoF. In both cases, the focus is made on the overlapped region of the DoFs, and the background environment is blurred. Therefore, felines can target the prey at both night and day. Tapetum lucidum is a multi-layered tissue located between the retina and choroid. It consists of periodically repetitive cells including arrays of parallel crystal rods. Each array of rods (n = 1.83) and each intervening layer (n = 1.33) are approximated as alternating layers with a high and low refractive index. The transmitted light is reflected back to the photoreceptor via tapetum lucidum, which enhances the sensitive scotopic vision.<br/><br/>The device, inspired by the unique vision of the feline, consists of a hemispherical silicon photodiode array integrated with an artificial tapetum lucidum and a variable aperture. The artificial tapetum lucidum made of a patterned inverse opal structure is located behind the ultrathin silicon photodiode. It enhances the photo-absorption by reflecting the light back to the photodiode, which increases the sensitivity under a low light condition. The variable aperture mimics the feline’s pupillary movement, whose shape changes between the vertically-elongated slit-like shape and the fully-dilated circular shape. This variable aperture enables the targeted imaging with asymmetric depth-of-fields as well as the facile control of the incident light amount.