Biomedical Engineering Reference
In-Depth Information
on a row of the FPA, one can map across the row
the spatial distance associated with one temporal
sample. The readout then shifts down to the next
row, and that distance is the center-to-center dis-
tance between detectors on a column of the FPA.
Whether this mapping is useful depends on the
application.
Though many modern camera and imaging
systems use a digital interface to pass data to
computer, recording, and/or display devices,
there are still many older (and some newer
special-purpose) systems that make use of an
analog interface at some point in the signal chain.
This requires special consideration. A common
example is the relatively inexpensive analog
video camera (e.g., RS-170, RS-330, NTSC, CCIR,
PAL, or SECAM) that processes (i.e., modulates)
the output from the FPA into a specific analog
video format to be carried by a cable to a com-
puter, whereupon a specialized analog-to-digital
(A/D) converter (called a
frame grabber
)
11
turns the
analog image data into discrete digital pixel data.
Two issues are predominant: bandwidth and res-
ampling. The analog bandwidth
B
allocated to the
video signal
12
puts a limit on the horizontal reso-
lution the camera can provide (regardless of the
number of photodetectors on the FPA), such that
the varying analog voltage level in the video sig-
nal cannot have more than 2
B
independent values
per second
[27]
. The resampling that occurs (the
first sampling was at the FPA, the second at the
frame grabber) almost always means that a pixel
of digital image data has no direct correspond-
ence to a particular photodetector location on the
FPA. For some applications, this can have serious
ramifications to the design.
How the interface between optical analysis
and electronic analysis is handled is up to the
designer and the particular application, but the
process warrants significant thought to avoid
erroneous conclusions.
1.2.3 Recommended Approach
This section has presented a very brief over-
view of optics and photodetectors in a practi-
cal way for the purpose of either designing or
analyzing a vision sensor. Repetitive calcula-
tions can be made easily using a numerical
analysis program such as MATLAB®, which
can also provide insightful plots. More exten-
sive optical analysis, simulation, and design can
be achieved with a program designed specifi-
cally for optics, such as Zemax®. Tools such as
MATLAB and Zemax are extremely valuable
for this purpose, can save a great deal of time,
and help avoid dead ends for potential design
approaches. A particularly handy figure of
merit for an imaging system that is easy to cal-
culate and takes into account both optics and the
detector array is
F
λ/
d
, where
F
is the f-number,
λ
is the wavelength under consideration, and
d
is the detector size of one element of the FPA in
the given direction, as discussed earlier
[12, 13]
.
With this figure of merit,
F
λ/
d
<
1
results in a
detector-limited system, and
F
λ/
d
>
1
results in
an optics-limited system. When
F
λ/
d
2
, there
is no aliasing possible, but this condition may
result in too much image blur. See Holst
[12, 13]
for more detail.
1.3 BIOMIMETIC APPROACHES TO
VISION SENSORS
Essentially all vision sensors developed by
humans in the past and present are in some way
biomimetic vision sensors. That is, humans stud-
ied how various animals
see
the world around
them, then applied known principles of optics
and light detection to mimic certain aspects of
how animal vision systems evolved and thus
created artificial vision sensors.
13
11
There are optional frame grabbers for digital cameras, without the A/D circuitry.
12
The video signal specifics, such as scan rate, blanking interval, and bandwidth, must be known.
13
Two optical techniques, zoom lenses and Fresnel lenses, have not yet been found in nature.
