Aerospace & Defense Technology - April 2021 - 27

Tech Briefs
A Comparative Evaluation of the Detection and Tracking
Capability Between Novel Event-Based and Conventional
Frame-Based Sensors
This research establishes a fundamental understanding of the characteristics of event-based sensors
and applies it to the development of a detection and tracking algorithm.
Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio

he detection and tracking of moving
objects in free space is an area of
computer vision that has benefited
greatly from years of research and development. To date, there are many different algorithms available with new and
improved revisions being developed on
a regular basis. With such maturity, it is
not surprising that such algorithms provide very effective solutions in a wide
range of applications. There are, however, select scenarios in which traditional detection and tracking algorithms break down. In many cases it is
not attributable to the algorithm, but
rather the fundamental operation of a
frame-based sensor.
Despite extensive research, traditional frame-based algorithms remain
tied to predefined frame rates that lead
to image artifacts such as motion blur
and sensor characteristics such as low
dynamic range, speed limitations and
the requirement to process large data
files often filled with copious amounts
of redundant data.
Event-based sensors, also known as
silicon retinas or neuromorphic sensors, are revolutionary optical sensors
that operate fundamentally differently
to traditional frame-based sensors and
offer the potential of a novel solution
to these challenges. Inspired by the
functionality of a biological retina,
these sensors are driven by changes in
low-light intensity and not by artificial frame rates and control signals.
Within these sensors each pixel behaves both asynchronously and independently, enabling events to be generated with microsecond resolution in
response to localized optical changes
as they occur.
As noted, event-based sensors specifically aim to mimic the biological
retina and subsequent vision processing of the brain. While the retina is the

ciliary body

eye muscle

Bipolar cell

Horizontal cell

vitreous humor
lens
iris

retina
choroid
scelera

aqueous humor
light

Fovea

Rod

Ganglion cell

Light

cornea

Cone

pupil
suspensory
ligaments

optic disk
optic nerve

Inner & Outer plexiform layer
Amacrine cell

eye muscle

(a)

(b)

(a) Cross section of an eyeball; (b) Schematic of the human retina

photosensitive tissue of the eye, for it
to work properly the entire eyeball is
needed. Figure (a) shows a schematic
crosssection of a typical eye. Light entering the eye passes through the
cornea and into the first of two humors. The aqueous humor is a clear
mass that connects the cornea with the
lens, helping to maintain the shape of
the cornea. Between the aqueous
humor and lens lies the iris, a colored
ring of muscle fibers. The iris forms an
adjustable aperture, called the pupil,
which is actively adjusted to ensure a
relatively constant amount of light enters the eye at all times.
While the cornea has a fixed curvature, the shape of the lens can be actively moulded to adjust the eye's focal
length as needed. On the back side of
the lens is the vitreous humor that
again helps to maintain the shape of
the eye. Light entering the pupil then
forms an inverted image on the retina.
The retina contains the photosensitive
rods and cones and is a relatively
smooth, curved layer with two distinct
points; the fovea and optic disc.
Densely populated with cone cells, the
fovea is positioned directly opposite
the lens and is largely responsible for
color vision. The optic disc, a blind

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spot in the eye, is where the axons of
the ganglion cells leave the eye to form
the optic nerve.
From the photoreceptors, neural responses pass through a series of linking cells, called bipolar, horizontal
and amacrin cells (see Figure (b)).
These cells combine and compare the
responses from individual photoreceptors before transmitting the signals
to the retinal ganglia cells. The linkage between neighboring cells provides a mechanism for spatial and
temporal filtering, facilitating relative, rather than absolute, judgment
of intensity, emphasizing edges and
temporal changes in the visual field of
view. It is the network of photoreceptors, horizontal cells, bipolar cells,
amacrin cells and ganglion cells that
can discriminate between useful information to be passed to the brain, and
redundant information that is best
discarded immediately.
This work was done by James P. Boettiger of the RAAF for the Air Force Institute of Technology. For more information, download the Technical Support
Package (free white paper) at www.
aerodefensetech.com/tsp under the
DAQ, Testing & Sensors category.
AFRL-0299
27

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Aerospace & Defense Technology - April 2021

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