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296 Biomimetics: Biologically Inspired Technologies

Incident light

Liquid crystal SLM

Unfocused areas
Focused area
Image plane
(a) (b)
Figure 11.6 (a) A foveated imaging system where incident light rays are directed to a single imaging plane. The
SLM changes the index of refraction to focus light from a specific direction. (b) Sample image from such a system.
(From Martinez, T., Wick, D., and Restaino, S. Optics Express 2001: 8(10), 555–560. With permission.)

center of their field of view than towards the periphery. This is known as foveated imaging and
allows for a relatively wide angle of view with the option of detailed resolution by scanning.
Optical engineers are often faced with a similar challenge of increasing the field of view while
maintaining resolution. Traditional methods include decreasing the entrance pupil size (which
increases the f/# at the expense of resolving power and illumination) and adding optical elements
(increasing the complexity, size, and weight of the system). Martinez et al. (2001) have devised an
artificial foveated viewing system as a unique solution.
Their design uses liquid crystal spatial light modulators (SLM), which are used to manipulate
optical wavefronts. Voltages applied to liquid crystals alter the index of refraction such that
aberrations are corrected. However, only aberrations from a limited range of angles can be
corrected at one time (Figure 11.6). Rays of light from different field angles are not corrected,
and the result is a region of high resolution surrounded by areas of low resolution.
By appropriately varying the SLM, the optical system effectively scans with a narrow field of
view of high resolution while maintaining peripheral vision much like the human eye. An additional
benefit of this optical system is a decreased bandwidth requirement for transmitting digital images,
as only a portion of the entire field of view has high resolution. The low resolution areas may serve a
purpose as well; they may be used as an initial assessment of whether an area warrants high
resolution probing.

11.3 COMPOUND EYES

The appeal of insect compound eyes may be due, in part, to their being so different than our own.
On the surface they also appear to be more diverse and complex with anywhere from a single
ommatidium (individual eye unit) in the ant species Pomera punctatissima to over 10,000 per eye in
some species of dragonflies. Again, the various manifestations of compound eyes are customized to
the needs of their users. In general, compound eyes are broadly divided into two categories,
superpositional and appositional.
The individual facets of appositional compound eyes are optically isolated, and each of them
provides part of a scene. The result is a series of images slightly offset from one another (see Figure
11.7). The advantage of this arrangement is that the images are processed in parallel, leading to