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298 Biomimetics: Biologically Inspired Technologies

Figure 11.8 Superpositional compound eye: (a) electron micrograph and (b) schematic.

array is then superimposed upon this substrate, allowing for each lenslet to be facing a different
radial direction. Subsequent exposure to light through the lenslets provides a method for defining
the corresponding waveguides (Figure 11.10). The individual lenslets have been shown to have
comparable fields of view to natural compound eyes.
Researchers in Israel have recently been developing an artificial version of a subset of the
appositional compound eye. To biologists, it is known somewhat confusingly, as the neural super-
positional eye. While the ommatidia are optically isolated from one another, the information from
each one is superimposed through neural processing (Figure 11.11). In this way, the images
obtained from multiple facets are compared in order to increase sensitivity. The structure is slightly
different than the pure appositional eye described previously.
Rosen and Abookasis (2003) have taken this approach to develop a sensor capable of imaging
through scattering media such as biological tissue. Coherent light is projected through a biological
sample to a microlens array. Each microlens detects a unique speckle pattern and summation of
these patterns reveal an outline of interior structures of the sample. Rosen et al. have demonstrated
the ability to image bony arrangements placed between two slices of muscle tissue (Figure 11.12).
While still in its initial stages, this technology has the potential for low-cost, safe, and noninvasive
medical diagnoses.

11.3.2 Superpositional Compound Eyes

One of the most famous examples of biologically inspired optics comes from a paper written in
1979, entitled ‘‘Lobster eyes as x-ray telescopes’’ (Angel, 1979). As described in various papers of
the time, lobster eyes consist of tapered tubes having rectangular cross-sections and reflective
interior surfaces (Figure 11.13). These tubes are arranged spherically, and light reflecting twice
from the interior surfaces behaves identically to a spherical mirror. The tubes share a common
retina making the lobster eye a superpositional compound eye (Figure 11.14).
Roger Angel theorized that such a mechanism could be adapted for x-ray telescopes if each
cell could be made with an aspect ratio of approximately 100. Conventional x-ray telescopes