The document discusses Visual Intelligence's Iris One airborne camera system, which uses a modular Sensor Tool Kit Architecture (iOne STKA) based on multiple camera modules. The key points are: 1) The Iris One uses an Arched Retina Camera Array (ARCA) technology that allows optical axes of individual cameras to intersect at a single perspective center, providing extended ground coverage. 2) Configurations include single, double, and triple ARCA systems, allowing different imaging modes like ortho, multi-spectral, oblique, and stereo imaging. 3) Camera and system calibration is performed to establish metric properties, including geometric calibration of individual cameras and radiometric calibration of the full system.

1. Article
Based on its iOne Sensor Tool Kit Architecture
Visual Intelligence’s Iris One
The Visual Intelligence company has developed a family of modular and scaleable airborne digital
camera systems that are based on its Iris One Sensor Tool Kit Architecture (iOne STKA). This architecture utilizes multiple camera modules that can be equipped with a variety of CCD formats and lenses
to provide extended ground coverage in various alternative configurations. The article reviews the
several different systems that have been developed, including both their hardware and software, and
describes the calibration procedures that are used to establish their metric properties. The company’s
planned future developments are also outlined.
By G
ordon Petrie
I - Introduction & Background
Multiple camera systems comprising various
combinations of vertical and oblique pointing cameras have long been part of the aerial photographic scene. The use of “fans”
of such cameras has been popular for many
years, especially in aerial reconnaissance
operations where they provide extended
cross-track coverage, in some cases, from
horizon-to-horizon. The usual arrangement
consists of a single nadir pointing camera
in combination with either two (or four)
oblique pointing cameras. The shutters of all
three (or five) cameras in the “fan” are triggered to fire simultaneously. Thus all the
resulting photographs in a set will have been
acquired from a single position in the air.
Older examples (from many) of such “fans”
that utilized film cameras are (i) the
Bagley three-lens camera systems manufactured by Bausch & Lomb and used by the
U.S. Army Air Service during the 1920s
[Fig.1]; and (ii) the Tri-Metrogon system
[Fig. 2] employing three Fairchild K-17
standard-format (23 x 23 cm) metric cameras that was used extensively by the U.S.
Army Air Force for aeronautical mapping
and charting purposes during World War II
(WW-II).
Fig. 3 – The Carl Zeiss KRb 8/24 Tri-lens camera has been used in
numerous NATO manned and UAV aircraft. (Source: Carl Zeiss)
Fig. 1 – A Bagley T-1A three-lens camera system with focal lengths
of f = 125 mm for the central (nadir-pointing) lens and f = 178
mm for the two wing (oblique-pointing) lenses. (Source:
Smithsonian National Air & Space Museum)
Turning to more modern times, the Carl
Zeiss KRb 8/24 tri-lens film camera series
[Fig.3] with up to 143° cross-track coverage
have been used extensively by Canadian,
German and French air forces, including a
version that has been used in the Canadair
CL-289 drone (UAV) aircraft. Also the TriMetrogon configuration has continued to be
used, for example in the German Tupolev
Fig. 2 – An example of a Tri-Metrogon camera installation comprising
three separate Fairchild K-17 metric film cameras with f = 152 mm
lenses as mounted in a Boeing B17 bomber aircraft and used during
World War II. (Source: USAF)
30
Fig. 4 – The three Carl Zeiss Jena LMK photogrammetric film cameras mounted in a Tri-Metrogon configuration in the German Tupolev
Tu-154M Open Skies aircraft. (Source: IGI)
Tu-154M Open Skies aircraft, employing
three Carl Zeiss Jena LMK 15/23 mapping cameras [Fig. 4]. Furthermore the later
Carl Zeiss KS-153 reconnaissance cameras have been produced in both three and
five lens forms [Fig. 5] and have been used
widely by NATO air forces, including the
U.S. Navy (on F-14 aircraft) and the
German Luftwaffe (on Tornado aircraft).
Indeed a number of these cameras are still
in operational service.
All of the above examples have utilized film
cameras. However the “fan” configuration
has remained popular in the digital camera era. An example is the DLR-3k camera system [Fig. 6] employing three smallformat cameras, which is in use by the
German Space Agency (DLR). Currently
both IGI (with its Triple-DigiCam system) and
Leica Geosystems within its RCD30 multihead range offer similar three-camera “fan”
configurations, but using medium-format
cameras instead of the small-format cameras
used in the DLR-3k system. There is even an
echo of the WW-II Tri-Metrogon system with
the advent recently of the Russian Tupolev
Tu-214 ON Open Skies aircraft, which is
being equipped with three Intergraph Z/I
DMC-II140 large-format digital cameras
[Fig. 7] in a Tri-Metrogon configuration to
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2. Article
Airborne Camera Systems
[a]
Fig. 5 – (a) A Carl Zeiss KS-153 Tri-lens camera installed in a pod
mounted on a German Tornado aircraft. (b) & (c) Diagrams of the Carl
Zeiss KS-153 reconnaissance camera showing the lens design and ground
coverage in its Tri-lens form [in (b)]; and in its Penta-lens 53 form [in
(c)], providing fans of three and five photographs respectively in the
cross-track direction. (Source: Carl Zeiss)
[a]
[b]
[b]
Fig. 6 – The DLR-3k three-camera system makes use of a “fan” of three
Canon EOS small-format digital cameras with one pointing vertically and
two obliquely to provide wide cross-track coverage. (Source: DLR)
[c]
Fig. 7 – The Tri-Metrogon style arrangement of the three Intergraph
Z/I DMC-II140 large-format cameras that have been installed in the
Russian Tupolev Tu-214ON Open Skies aircraft. (Source: Z/I Imaging)
Fig. 8 – (a) This photo shows the rigid ARCA or “bridge” into which an
array or “fan” of three or five small-format cameras can be inserted.
(b) The geometric arrangement of an ARCA configuration. (Source: Visual
Intelligence)
camera system that is based on the so-called
ARCA (Arched Retina Camera Array) technology on which Visual Intelligence holds a
number of patents. The system features a
rigid ARCA or “bridge” into which an array
or “fan” of three or five digital cameras can
be fitted with a choice of lenses and CCD
formats [Fig. 8 (a)]. This configuration
allows the optical axis of each individual
camera in the array to intersect, passing
through a single perspective centre [Fig. 8
(b)]. This particular arrangement has been
used in the Iris One 50 system (renamed
the Iris One Ortho 19 kps (1) system),
which is the specific model that has been
awarded ‘Digital Aerial Sensor Certification’
by the USGS in 2009.
II – The iOne STKA & ARCA
[Note (1): In this particular context, Dr. Guevara,
provide the required wide-angle coverage
of the ground during its monitoring flights.
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(a) Single ARCA Configuration
The latest additions to this multiple camera
“fan” type of aerial camera system are the
Iris One systems that have been developed
by the Visual Intelligence company based
on its iOne Sensor Tool Kit Architecture (iOne STKA). The company –
which is located in Houston, Texas in the
U.S.A. – was originally called M7 Visual
Intelligence prior to the sale of its sister aircraft component and repair business (M7
Aerospace) in 2010, after which, the company assumed its present title. The Iris One
is a modular and scaleable digital aerial
31
CEO of Visual Intelligence, has coined the term
“kps” (kilo pixel swath), where 1 kps is a swath
that is 1,000 pixels wide. Different ARCA configurations with different lenses, angular coverages
and CCD array formats allow systems to be constructed with swath widths of 11 kps up to 60 kps.]
(b) Double ARCA Configurations
A further and quite distinctive feature of the
scaleable ARCA technology is that an additional ARCA can be added in parallel to the
first with the two ARCAs or “bridges” being
fitted precisely together [Fig. 9 (a)]. This
allows the co-registration of all the images
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3. Article
[a]
[b]
Fig. 9 – (a) Showing two ARCAs or “bridges” fitted together in parallel
in a Lego-like arrangement. (b) Showing how the images produced by
the five colour (RGB) cameras fitted to the first ARCA are co-registered
with the five near infra-red (NIR) images produced by the cameras fitted
to the second ARCA. (Source: Visual Intelligence)
produced by the two fans – as required, for
example, in a multi-spectral version of
the Iris One system. This allows the set of
colour RGB images that have been recorded by the cameras mounted in the first ARCA
or “bridge” to be co-registered with and
superimposed on the corresponding set of
near infra-red (NIR) images that have been
produced by the cameras mounted on the
second ARCA or “bridge” [Fig. 9 (b)]. Visual
[a]
Intelligence calls this its “CoCo” (Co-mounted and Co-registered) system.
Another possibility with the double ARCA
arrangement is that the two sets of images
can be offset with respect to one another,
allowing them to be interlaced in order to
widen the field of view and the cross-track
coverage that can be achieved from a single flight. This particular configuration has
been adopted in the Iris One Hi5 system
[Fig. 10 (a)]. Depending on the focal length
of the lenses that have been selected and fitted to the camera modules and the CCD format size, this system can provide different
swath widths over the ground from a given
flying height. As shown in the two diagrams
[Figs. 10 (b) & (c)], the former provides a
10 km wide swath (using 11 Megapixel
camera modules), while the latter provides
a 12 km wide swath (using 16 Megapixel
camera modules) from a flying height (H) of
12 km (using a jet aircraft!). Fig.11 is a sample colour RGB frame image (described as
a single ARCA Virtual Frame) that has been
acquired by an Iris One system.
(c) Triple ARCA Configurations
three ARCAs or “bridges” of the system –
with their 9 varying-format camera modules
– can be oriented either in the cross-track or
the along-track direction. The pattern of the
resulting ground coverage for each of these
two directions is shown in Figs. 12 (b) & (c).
When each camera module is fitted with a
CCD array having a format size of 6,576 x
4,384 pixels = 29 Megapixels, then the
total coverage of a single set of the 9
images is 19,000 x 12,500 pixels. Using
cameras fitted with f = 85 mm lenses, this
produces an angular coverage of 46° (crosstrack) x 70° (along-track) (12,500 x 19,000
pixels) with the cameras oriented in the
along-track direction [Fig. 12 (b)]. The
60 % longitudinal overlap along the flight
line that is produced when the cameras are
programmed to expose overlapping sets of
colour (RGB) images in a stereo convergent
configuration gives a base:height ratio of
0.6. When the system is rotated by 90
degrees into the cross-track direction
(19,000 x 12,500 pixels) [Fig. 12 (c)], this
yields a base:height ratio of 0.34, similar
to that achieved by the overlapping stereo-
Fig. 11 – A sample wide-swath colour RGB image frame that has been
acquired by an Iris One camera system. (Source: Visual Intelligence)
With the addition of a third ARCA or
“bridge” (interlocked and butted together)
in parallel to the previous two ARCAs, still
more versatile configurations can be envisaged. One of these is the Iris One Stereo
camera system [Fig. 12 (a)]. With this, the
[b]
[b]
Fig. 10 – (a) An Iris One camera system. [N.B. The same enclosure cabinet is used for the Ortho, MS, Oblique, Stereo and Hi-5 models]. (b)
Diagrams showing the alternative ground coverage patterns with interlaced angular coverages that are produced by the Iris One Hi5 system
using cameras having different focal lengths and CCD sizes (11 Mpix or
16 Mpix). (Source: Visual Intelligence)
[a]
[b]
Fig. 12 – (a) An Iris One Stereo camera system showing the 9 cameras mounted on three parallel ARCAs or “bridges”. (b) The pattern of ground coverage that is produced by the 9 camera array when set in the along-track direction. (c) The pattern of the ground coverage that is produced by the 9 camera array in the cross-track direction. (Source: Visual Intelligence)
32
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4. Article
images that are produced by conventional
large-format digital mapping cameras.
lapping stereo-images that are produced by
conventional large-format (non-digital) mapping cameras.
images can then be handled by any standard third-party photogrammetric software
solution.
III – Camera & System
Calibration
The radiometric calibration that is also
carried out in the laboratory (outside under
natural light conditions) for the Iris One system is a relative calibration (not an absolute
calibration) and includes colour and tonal
balancing, shading and aperture correction,
and smearing removal. The resulting data is
incorporated into a radiometric correction
software module for use during later image
processing operations. The module can also
separate the colour RGB images into their
individual component red, green and blue
images and interpolate the missing pixel values produced by the Bayer mosaic filter.
Another possible arrangement of the flexi-
[a]
[b]
One of the main applications of the various
Iris One systems is the topographic mapping
field. Thus the metric side and, in particular,
the calibration of (i) each individual camera in an ARCA or “bridge”; and (ii) that
of the overall camera system as a whole is
obviously a matter of prime concern. The
geometric calibration of each individual camera is carried out in a laboratory
using a calibration cage [Fig. 14] that is fitted out with a set of well distributed and
highly reflective coded targets, whose coordinates have been determined to a precision
of ±1mm in X and Y and ±5 mm in Z.
Photographs of this target field are taken
from a number of different positions and orientations with each individual camera, so
providing a strong convergent geometry.
The images of all the targets that have been
[b]
Fig. 13 – (a) An Iris One Stereo camera system showing the 15 cameras mounted on three parallel ARCAs or “bridges”. (b) The pattern of
the ground coverage that is produced by the 15 camera array oriented in
the along-track direction. (c) The 60 % longitudinal overlap that is produced by two successive sets of exposures from the Iris One Stereo system gives a base: height ratio of 0.6. (Source: Visual Intelligence)
ble three ARCAs or “bridges” of the Iris
One Stereo camera system is to employ
15 small-format cameras [Fig. 13 (a)], as set
out in a paper given by Dr. Hwangbo of
Visual Intelligence at the recent ASPRS 2012
Conference. The pattern of the ground coverage that results if the cameras are oriented in the along-track direction is shown in
Fig. 13 (b). When each camera is fitted with
a CCD array having a 4,008 x 2,672 pixels = 11 Megapixel format, then the total
coverage of a single set of 15 images is
21,460 x 7,438 pixels. Using cameras fitted with f = 135 mm lenses, this produces
an angular coverage of 27° (cross-track) x
70° (along-track). The 60 % longitudinal
overlap along the flight line that is utilized
to produce overlapping sets of colour (RGB)
images in a stereo convergent configuration
gives a base:height ratio of 0.6 [Fig. 13 (c)].
This is similar to that achieved by the over-
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Fig. 14 – The Visual Intelligence calibration cage.
(Source: Visual Intelligence)
recorded on each photograph are then measured. From this information, the value of the
focal length of the lens and the position of
the principal point of the frame image are
then determined for each individual camera,
together with the lens distortion values or
parameters. After each individual camera
has been calibrated, the relative position
and orientation of all the cameras within a
complete ARCA array are then determined
with respect to one other, again using the
known coordinates of the targets in the target field. After this second stage of the calibration process, a single composite “virtual” frame image with a single
perspective centre can be defined using the
measured data from all the component
images that have been generated by the
array of modular cameras mounted on the
ARCA or “bridge”. These “virtual” frame
33
This laboratory calibration is supplemented
by a further in-flight geometric calibration that is carried out over a field of signalized ground control points laid out in a
test area within the Houston metropolitan
region. This test area is overflown by an aircraft at a flying height (H) of 1 km. The Iris
One camera system is programmed to
expose its images with a large (80%) longitudinal and (60%) lateral overlap. Crossstrips are flown as well as the parallel strips
of the main block of aerial photography covering the test area. Automatic image matching of the target and tie points is carried out
on all the overlapping photographs that
have been exposed during the flight. A bundle aerial triangulation and block adjustment employing self-calibration is then
implemented using the BINGO software
from GIP in Aalen, Germany to generate
the final coordinate values and their residuals at the ground control points. Needless
to say, the lever arm corrections that relate
the positions of the GPS antenna and the
IMU to the perspective centre of the “virtual” frame photo will also be determined.
A large number of test flights of the Iris One
system have been undertaken by various
aerial photographic and mapping companies in the U.S.A. Numerous flights have
been undertaken for test purposes over a
range of flying heights by Aerial
Viewpoint, which is based locally in the
Houston area. Further extensive testing has
taken place in cooperation with Techmap,
a mapping company that is based in
Peachtree City, Georgia. Among the larger
American aerial mapping companies,
Sanborn, Fugro Horizons & EarthData and Northrop Grumman 3001
Inc. have all conducted trial flights with Iris
One systems. Further test flights have also
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5. Article
been undertaken by INEGI, the
Mexican national mapping
agency. Purchasers of Iris One
systems
include
Aviation
Supplies, a leading supplier of
aircraft solutions in China. In the
U.S.A., McKim & Creed, a
surveying and mapping company based in Raleigh, North
Carolina – has acquired several
iOne Infrastructure Mapping
Systems (iOne IMS).
IV – System Software
Fig. 15 – An Iris One camera system (at left) with its compact MaxCube data collection, camera control and data processing server (at right) – with its laptop controller located on top of
the main cabinet. (Source: Visual Intelligence)
The Iris One system is of course
driven and controlled by software. The main components that
[a]
carry out the in-flight data acquisition operations are provided through the so-called
Visual Navigator software. This features
three modules – (i) for image data acquisition; (ii) for flight line management and
camera control; and (iii) for data management respectively. A second software system, called Isis, carries out the subsequent
image data processing, including the radiometric correction software mentioned above.
It features two modules. (i) The Isis Sky
module carries out the processing of the
acquired imagery in conjunction with the
DGPS and IMU data that has been collected simultaneously in-flight. If an existing
DEM is available, e.g. from USGS in the
United States, this module can also carry out
the generation of an orthophoto in-flight. (ii)
The Isis Earth module is used to carry out
a more accurate ortho-rectification using
ground control points and more refined
GPS/IMU data in a post-flight processing
operation that is carried out later on the
ground.
Within this context, Visual Intelligence has
teamed up with the MaxVision company,
based in Madison, Alabama, to create an
image processing system that will satisfy the
rather demanding requirements of the Visual
Navigator and Isis Sky software modules,
especially in respect of implementing its inflight image processing capability. The computer that is being used for the purpose is
the compact MaxCube II mobile “super
server” [Fig. 15]. This ruggedized computer
can be supplied with two Intel Xeon processor units providing up to twelve powerful 64bit CPUs and up to eight removable hard
drives, each with a 3 terabyte storage
capacity. All of this processing power and
storage capacity is contained in a cabinet
that is close to one cubic foot in size and
has a low power consumption.
[b]
V - Current & Future
Developments
A recent development has been the introduction of the Iris One Infrastructure
Metric-Mapping System – having the titular acronym iOne IMS. This comprises a
single rigid ARCA or “bridge” into which is
inserted a pair of nadir-pointing camera
Fig. 16 – (a) Showing the ARCA or “bridge” on which the twin nadirpointing RGB and NIR cameras and the two oblique-pointing cameras are
mounted – as used in the Iris One Infrastructure Metric-Mapping System
[iOne IMS]. (b) Showing an iOne IMS system with the protective cover
in place over the cameras. (c) Showing the arrangement of the four
lenses – two nadir-pointing and two oblique-pointing – of an iOne IMS
camera system from the underside. {Source Visual Intelligence)
34
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6. Article
[a]
can utilize the standard camera hole on a
fixed-wing aircraft.
Another closely related development has
been the co-mounting of an Iris One camera
system with a RIEGL VQ-580 airborne
topographic laser scanner [Fig. 17]. The two
devices are mounted closely together on a
common base plate fitted with anti-vibration
dampeners. The spatial relationship of the
two devices, including their relationship to
the accompanying GPS/IMU sub-system, is
determined very precisely through the measurement of the lever arm offset during the
system calibration.
Visual Intelligence has also developed the
concept of a five- or nine-camera system
[Fig. 18] that it calls its “360° orthostereo-oblique” system. This generates
ground coverage in the form of a “Maltese
Cross” that is similar to the systems developed by Pictometry and Track’Air, but is
based on the ARCA technology. Further
developments of this architecture are currently focussed on the development, miniaturization and production of very light
weight and compact camera systems [Fig.
19] that are fully metric and can be utilized
in UAVs; in ground vehicles; and in mobile
devices.
[b]
VI – Conclusion
Fig. 17 – (a) The Iris One and RIEGL VQ-580 combination mounted side-by-side on a common base plate which is placed on a set of anti-vibration
dampers – as viewed from the side at left and from above at right. (b) An iOne IMS camera system mounted alongside a RIEGL VQ-580 laser scanner.
(Source: Visual Intelligence)
modules of varying-format sitting side-by
side; the one a colour RGB camera; the
other an NIR camera. These are flanked by
a pair of oblique-pointing cameras, that are
pointing in opposite directions [Fig. 16].
Essentially this arrangement is similar to the
Tri-Metrogon configuration discussed above.
Since the system is designed specifically for
the corridor mapping of infrastructure, the
ARCA or “bridge” on which the cameras
are mounted is oriented in the along-track
(flight) direction. The system can be
installed in a specially constructed pod that
is fitted to the underside of a helicopter or it
Fig. 18 – The “360° ortho-stereo-oblique” system
(Source: Visual Intelligence)
Visual Intelligence has already developed a
most interesting series of airborne digital
frame camera systems based on its
scaleable and modular iOne Sensor Tool Kit
Architecture. For the future, it will be very
interesting to follow the concepts that are
currently in the research and development
stage and to see them come to fruition.
Fig. 19 – A prototype of a three-camera ARCA or “bridge” made of carbon-fibre. (Source: Visual Intelligence)
36
Gordon Petrie is Emeritus Professor of Topographic Science
in the School of Geographical & Earth Sciences of the University
of Glasgow, Scotland, U.K. E-mail – Gordon.Petrie@glasgow.ac.uk;
Web Site – http://web2.ges.gla.ac.uk/~gpetrie
September 2012