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Defense & Security

Animal eyes for military and homeland security applications

Optical imaging systems based on the biological superposition eye are being developed for hard x-ray optics and video surveillance with telescopic resolution.
25 October 2007, SPIE Newsroom. DOI: 10.1117/2.1200710.0884

While cameras, telescopes, microscopes and other imaging sensors are all instruments supporting the human eye, biologically-inspired optical imaging systems are becoming increasingly attractive for military and homeland security applications.1 Since the development of classical optical systems, such as the Maxwell fish-eye2 and the Luneburg lens,3 with features reminiscent of fish eyes, other animal eyes have recently inspired new artificial vision applications. These include designs based on the eyes of the lobster, moth, fly, and mantis shrimp,4 the latter capable of producing generalized color vision, in which red, green, and blue (RGB) are combined in various ways to reproduce other colors.

Although the Maxwell fish-eye and the Luneburg lens provide good points of departure for design, they are challenging to implement because they are based on a 3D inhomogeneous gradient-index (GRIN) medium that is difficult to realize, especially in the visible/IR-region. One solution has been to develop catadioptric imaging systems based on both mirrors (catoptric) and lenses (dioptric) to retain the Maxwell fish-eye concept without introducing a GRIN medium.1 These systems combine the imaging properties of refractive and reflective optics and offer a 360° omnidirectional (panoramic) view, which is very useful for applications such as video surveillance, border patrol, periscopic vision, etc. Recently, an electronic zoom and pan-tilt-zoom capacity was added to one of these designs at Physical Optics Corporation, providing 360° instantaneous telescopic-resolution view and electronic frame stabilization without requiring camera motion.

Bug eyes, or fly eyes, are non-imaging systems with a detector located close to guiding channels. They are called apposition eyes, and have no transparent zone for imaging.4 Moth eyes are a type of nocturnal bug eyes that have antireflection coatings based on sub-wavelength relief gratings. In contrast, lobster eyes are true imaging systems due to the presence of a clear zone between the guiding channels and detector surface, allowing use of the lens principle for imaging. These eyes are called reflective superposition eyes.4 They consist of many tiny channels that are nearly perfectly square and completely cover the spherical surface of the eye, with their axes converging towards the center of the sphere. Lobster eyes produce images by reflection, unlike most other eyes that focus by refraction. The sides of the channels are very flat and shiny mirrors, and their precise geometrical arrangement allows the reflection of all parallel light rays to a common focus. The square shape of the channels is crucial. Only when the reflectors are at right angles can the system form an image from light rays coming from any direction. This provides the eye with a very large angle of view. Another advantage is that, since many channels contribute to image formation at any given point on the retina, the resulting image is very bright.5

In artificial analogs, the basic principle of lobster eye reflective imaging has recently been applied to the design of focusing devices for hard x-rays (∼ 100keV photon energy) and for the visible/IR-region.5,6 It should be emphasized that the design incorporates a "lobster eye" radially symmetric superposition of reflection channels. This creates an analog of the transmission panoramic lens, or, more strictly speaking, of the convex spherical mirror, except that virtual imaging rays are replaced by real ones, in spite of the use of reflection optics.6 As such, it represents a curious example of a catadioptric system.7

The lobster eye has interesting connections with Darwinian evolution, since two conditions must be fulfilled to be defined as a superposition (lens) system: (a) a clear zone must be present between lens and detectors, and (b) the cross-section of the reflection channels must be quadratic. However, bug eyes do not satisfy (a), while some animals do not satisfy (b). All crustaceans (shrimp, lobster, crayfish) satisfy both conditions. From an evolutionary point of view, it is interesting to note that the larval stages of some shrimp have apposition eyes with hexagonal facets, hence not satisfying (b), but they change into superposition eyes with square facets at metamorphosis, thus, satisfying (b).4

The most attractive application of lobster eye technology has been the creation of the first imaging system for hard x-ray optics that allows the detection of concealed objects, such as improvised explosive devices, at standoff distances approaching 50-100m.8 Another military application has been the development of hemispheric field-of-view IR-imaging sensors, that can be "dropped" from the air as "sensor rocks" in any terrain.5

Further work with animal eyes seeks to extend the number of color primaries from three, as in human RGB vision, to a larger number, such as the twelve of the mantis shrimp.9,10 Another idea is to apply the RGB concept of hyper-spectral imaging to the IR region. Coupled to the electronic zoom concept developed for catadioptric systems, the idea could be applied to developing omnidirectional (360°) video surveillance with telescopic resolution (<1mrad), instantaneous at any view, within a 360° full angle, and without moving parts, which represents the ultimate goal of imaging optics for military and homeland security surveillance. 5

Tomasz Jannson
Physical Optics Corporation
Torrance, CA

Tomasz Jannson, currently Chief Scientist at Physical Optics Corporation, developed the majority of the company's optoelectronic and communication products. His expertise resides mostly in communications, fiber optics, soft computing, holography, Fourier optics, photonics, robotics, and diffraction optics. He has authored more than 100 publications in international journals, and he is a SPIE Fellow.