Inspiration from nature

Electro-optical imaging systems have seen significant advances over the past several decades. These advances were due in large part to the progress in focal plane arrays (FPAs), driven by the revolution in microelectronics. In contrast, no revolutionary changes have occurred in optical elements and lens systems. Although there have been many dramatic improvements in manufacturing methods, optical coatings, and computer-aided design, most lens systems still consist of multiple elements made by grinding and polishing spherical surfaces on homogeneous glasses.

Unlike synthetic optics, biological optics found in animal eyes can be amazingly simple in structure yet support excellent visual acuity (see oemagazine, February 2002, page 26). An obvious example is the lens of the human eye, which is a single-lens system that has minimal spherical and chromatic aberration. Other single-lens systems in biology feature performance no less remarkable, such as the wide field-of-view (FOV) of fish lenses and the hyperacuity of vision in birds of prey.

Aside from single-lens biological systems, there exists a diverse array of compound lens systems in nature. In all, there are eight distinct types of biological optical systems.¹ The unique capabilities of many biological optics arise from sophisticated features such as gradient index (GRIN) lenses and dynamic shape change. In a biological optic, GRIN lenses comprise thousands of layers that vary in protein concentration. The refractive index of a spherical lens in a fish eye, for example, varies from 1.54 in the center to 1.37 at the periphery.

The aim of some recent research is to develop new materials that can support bio-inspired designs, specifically, simple optical systems that provide high performance but at the same time are compact and low cost—for example, based on molded polymers. The human eye comprises approximately 22,000 layers and can change shape to accommodate near and far focus. As an alternative to changing shape, researchers envision artificial systems in which an applied electric field would dynamically control the gradient of an electro-optical lens material. Although synthetic materials can be deformed to change focus just as biological systems are deformed using a concentric muscle structure, an alternative is to fabricate lenses with electrodes controlling electro-optical materials such that changes in applied voltage cause proportional changes in refractive index. Developing such optics will be challenging in terms of both materials engineering and computational design.

Many synthetic materials can be used as glasses in lens systems; some are particularly good candidates for incorporating new functionality such as variable index of refraction or deformability. Liquid crystals is an example of a material system that has a fairly large change in index of refraction as a function of electric field (Δn greater than 0.6).

applications

System design is not the only challenge in bio-inspired optics. A key part of the success of this field will be finding those applications in which bio-inspired optics can make a contribution from either a cost or performance perspective. Military systems for reconnaissance, surveillance, pilotage/navigation, and missile identification typically contain sophisticated high-cost, multi-element optical systems. Although the designs achieve the required high performance, multi-element optical systems are inherently costly as well as bulky and heavy. Considering the push toward a new generation of extremely small, lightweight, unmanned aerial vehicles, an initial niche for bio-inspired optics might be those applications for which they provide a significant size and weight reduction with no appreciable loss in performance. Similarly, in the commercial camera business, bio-inspired optics could introduce meaningful reductions in size, weight, and cost that would revolutionize lens production—for example, lenses capable of auto-focusing or zoom functions with no moving parts.

Another application is the use of compound optics to improve IR imaging systems. In this case the use of large arrays of micro-optics would allow for improved photon-collection efficiency and cold-shield efficiency. The ability to do this is quite limited by the need for materials with good transmission in the 3- to 5-µm and 8- to 12-µm spectral regions.

A specific example is the use of adaptive optics to perform foveated imaging. A foveated imaging system creates images with spatially varying resolution, much like the human visual system, which provides high resolution in the center of the visual field with gradually reduced resolution in the radial direction, reaching a minimum at the outermost peripheral field.² To mimic this capability in a conventional imaging system, we start with a lightweight, wide FOV optic with a poor modulation transfer function (MTF) and integrate a small, dynamic lens element such that a limited area on the focal plane has an excellent MTF. We can select the exact location of this area, or sweet spot, by adjusting the small, adaptive lens element.

The design offers twofold advantages. First, it allows wide FOV coverage, albeit with poor resolution, while simultaneously providing a small, selectable, high-resolution foveated region. Both capabilities are achieved using the small optic. In essence, the system can provide the wide FOV, low-resolution image, and the narrow FOV high-resolution image simultaneously without requiring heavy, complex, expensive optics. Second, when matched to a foveated FPA, the full system provides a method for greatly reducing data transmission requirements.

Each potential application for bio-inspired optics will have difficult specifications to meet. Biological imaging systems typically have curved focal planes, whereas most imaging systems designed by humans—for example, camera systems using film or microelectronic imaging arrays—typically have flat focal planes. In the past, some scanning IR imaging systems formed linear arrays using discrete detectors, but these are more the exception than the rule. Designing optics to give good performance over a large flat field is always challenging; successful fabrication of a workable design requires lens tolerances on the order of 0.1λ to 0.25λ. The requirements for the use of inorganic glasses as lens materials will also present challenges because of their sensitivity to environmental changes such as temperature, humidity, and light exposure. oe

References

1. M. Land, D. Nilsson, Animal Eyes, Oxford University Press, Oxford 2002.

2. T. Martinex, D.V. Wick, et al., Optics Express 8[10], p. 555 (2001); D. Wick, T. Martinex, et al., Optics Express 10[1] p. 60.