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Biologically Inspired Optical Systems 297

Figure 11.7 Appositional compound eye: (a) schematic, (b) electron micrograph, and (c) image projection.
very fast image recognition and motion detection. However, this comes at the expense of the
brightness of the image, as only a limited amount of light is captured.
By contrast, the facets of superpositional compound eyes are not optically isolated, and, as their
name would indicate, form images from the superposition of their respective light rays (see Figure
11.8). The light rays fall from multiple lenses onto a shared retina, with a concomitant increase in
photosensitivity. Surprisingly, these eyes form a single erect image, which can be accomplished by
cleverly designed refractive index gradients or mirrors.
In terms of biomimetic engineering, it seems that the appositional arrangement is vastly more
popular than the superpositional. This may be due to the relative conceptual simplicity in setting up
multiple cameras versus coordinating multiple lenses on a shared retina. Still, as will be discussed
later, the superpositional compound eye is well adapted for certain fields and successes there may
spur increased interest.

11.3.1 Appositional Compound Eyes

In the early 1990s, Ogata et al. (1994) fabricated an artificial compound eye and integrated electronic
retina by taking advantage of integrated circuit technologies at the time. Their design used planar
arrays (16 16) of gradient index of refraction rods (GRIN rods) which focused light through
pinholes onto a photodetector array (with eight shades of gray scale). The pinholes acted as lens-
isolating elements and the arrangement of the arrays in a circle increased the available field of view.
The image output was decidedly pixelated, but this modest beginning has led to modern versions with
increased resolution and color capabilities (Figure 11.9) (Tanida et al., 2003; Duparre et al., 2004).
Continuing with the theme of nonflat surfaces from their cephalopod eye design, Jeong et al.
(2005) have also developed a number of appositional compound eyes arranged spherically. More
faithful to biological designs than previous work, these eyes contain separate ommatidia, each with
its own lens, waveguide, and photoreceptor. The difficulties lie primarily with shaping the polymers
appropriately. Hemispherical substrates are produced by natural surface tension forces on droplets
of liquid polymer which are then allowed to cure onto a flexible membrane. A micromolded lenslet