Recent advances in the mobile electronics have dramatically increased the demand for the miniaturized and lightweight cameras with a capability of the wide field-of-view (FoV) imaging. In addition, such cameras are extremely crucial for the next-generation mobile devices including drones and flying robots to avoid obstacles and track objects efficiently. However, the conventional wide FoV cameras require a huge number of bulky and heavy lenses (known as ‘fish-eye lens’) to focus the panoramic scenes onto a flat image sensor to reduce optical aberrations caused by strong mismatch between focal plane (curved) and image sensor plane (flat). Therefore, technological breakthroughs in optics as well as devices have been required.
While we were trying to achieve the overarching goal of the miniaturized and lightweight wide FoV cameras, the eyes of aquatic animals in nature attracted our attention (Fig. 1a). Fish eye has been evolved to detect objects in the wide field with a single lens and a curved retina. Figure 1b exhibits the anatomy of the aquatic eye. It consists of a spherical monocentric lens with the parabolic refractive index (RI) profile, a hemispherically curved retina with the light-sensitive rod cells, an iris to block the stray lights, and retractor/protractor muscles for the lens movement. The protruding monocentric lens forms a hemispherical focal plane, which perfectly matches a hemispherical shape of the retina. Due to these structural and functional characteristics, the aquatic vision features the wide FoV of 160°, minimal optical aberrations, high visual acuity, deep depth-of-field (DoF), and facile visual accommodation even with its extremely small form factor. And, these features of the natural aquatic eye accord with the key requirements of the next-generation wide FoV camera.
In our recent publication, we have reported a novel wide FoV imaging device inspired by the natural aquatic eye. The developed device features the FoV of 120°, a miniaturized form factor, minimal optical aberrations, the DoF between 20 cm and infinity, and the facile visual accommodation. Specifically, we developed a core-shell-type monocentric lens with two distinct RI values for the core and shell respectively (Fig. 2a). It shows comparable optical characteristics to the natural protruding monocentric lens. We also developed a hemispherical silicon nanorod photodiode array, whose shape matches the hemispherical focal plane formed by the monocentric lens (Fig. 2b). It shows outstanding imaging performances even under vignetting due to its sensitive photodetection owing to the nanorod-type texturing and surface passivation. These optical and device components including a built-in aperture were integrated in a customized housing (Fig. 3a). The aquatic-vision-inspired camera successfully captured the images from wide angular directions and at various distances (Fig. 3b).
This work has brought significant progresses with regard to the development of state-of-the-art imaging devices, which offers new opportunities for the next-generation mobile electronics. For more details, please see our recent article published in Nature Electronics: An aquatic vision inspired camera based on a monocentric lens and a silicon nanorod photodiode array.