Biomimetic Vision Sensors - Engineered Biomimicry - page 2

instead uses many small-aperture lenses, each
coupled to a small group of photodetectors. This
is the type of eye found in insects in nature and
has only recently been mimicked for use as alter-
native vision sensors [1, 3, 5] . However, knowle-
dge of the optics and sensing in a camera eye is
very helpful in understanding many aspects of
the compound eye.
Using just two categories—camera eyes and
compound eyes—can be somewhat oversimpli-
fied. Land and Nilsson describe at least 10 dif-
ferent ways in which animal eyes form a spatial
image [1] . Different animals ended up with dif-
ferent eyes due to variations in the evolutionary
pressures they faced, and it is believed that eyes
independently evolved more than once [1] .
Despite this history, the animal eyes we observe
today have many similar characteristics. For
example, a single facet of an apposition com-
pound eye in an insect is quite similar to a very
small version of the overall optical layout of the
camera eye in a mammal.
Mammals evolved to have eyes that permit a
high degree of spatial acuity in a compact organ,
along with sufficient brain power to process all
that spatial information. While mammals with
foveated vision have a relatively narrow field of
view for the highest degree of spatial acuity,
they evolved ocular muscles to allow them to
scan their surroundings, thereby expanding
their effective field of view; however, this
required additional complexity and brain func-
tion [1] . Insects evolved to have simple, modular
eyes that could remain very small yet have a
wide field of view and be able to detect even the
tiniest movement in that field of view [1] . The
insect brain is modest and cannot process large
amounts of spatial information, but much pre-
processing to extract features such as motion is
achieved in the early neural layers before the
visual signals reach the brain [1] .
In general, the static spatial acuity of com-
pound eyes found in nature is less than most
camera eyes. Kirschfeld famously showed that
a typical insect compound eye with spatial

acuity equal to that of a human camera eye
would need to be approximately 1 m in diame-
ter, far too large for any insect [6] . Each type of
eye has specific advantages and disadvantages.
As previously mentioned, the camera eye and
the compound eye are the two most common
types of eye that designers have turned to when
drawing upon nature to create useful vision
sensors.
Before getting into the specifics of these two
types of vision systems, we first need to discuss
image formation and imaging parameters in gene-
ral, using standard mathematical techniques to
quantify how optics and photodetectors interact,
and then show how that translates into a biomi-
metic design approach. Separate discussions of
biomimetic adaptations of mammalian vision
systems and insect vision systems are provided,
along with strengths and weaknesses of each.
The design, fabrication, and performance of a
biomimetic vision system based on the common
housefly, M. domestica , are presented.

1.2 IMAGING, VISION SENSORS,
AND EYES

We have found that one of the most common
problems encountered in designing a biomi-
metic vision sensor is a misunderstanding of
fundamental optics and image-sampling con-
cepts. We therefore provide a brief overview
here. This chapter is by no means an exhaustive
reference for image formation, optical engineer-
ing, or animal eyes. In just a few pages, we cover
information that spans many topics. We include
only enough detail here that we feel is impor-
tant to most vision sensor designers and to pro-
vide context for the specific biomimetic vision
sensor discussion that follows. For more detail,
see [1-3, 5, 7-17] . We assume incoherent light in
this discussion; coherent sources such as lasers
require a slightly different treatment. Nontra-
ditional imaging modalities such as light-field
cameras are not discussed here.