The Visual Intelligence company has developed a family of modular and scaleable airborne digitalcamera systems that are based on its Iris One Sensor Tool Kit Architecture (iOne STKA). This architec-ture utilizes multiple camera modules that can be equipped with a variety of CCD formats and lensesto provide extended ground coverage in various alternative configurations. The article reviews theseveral different systems that have been developed, including both their hardware and software, anddescribes the calibration procedures that are used to establish their metric properties. The company’splanned future developments are also outlined.

I - Introduction & Background

Multiple camera systems comprising variouscombinations of vertical and oblique point-ing cameras have long been part of the aeri-al photographic scene. The use of “fans”of such cameras has been popular for manyyears, especially in aerial reconnaissanceoperations where they provide extendedcross-track coverage, in some cases, fromhorizon-to-horizon. The usual arrangementconsists of a single nadir pointing camerain combination with either two (or four)oblique pointing cameras. The shutters of allthree (or five) cameras in the “fan” are trig-gered to fire simultaneously. Thus all the

resulting photographs in a set will have beenacquired from a single position in the air.Older examples (from many) of such “fans”that utilized film cameras are (i) theBagley three-lens camera systems manu-factured by Bausch & Lomb and used by theU.S. Army Air Service during the 1920s[Fig.1]; and (ii) the Tri-Metrogon system[Fig. 2] employing three Fairchild K-17standard-format (23 x 23 cm) metric cam-eras that was used extensively by the U.S.Army Air Force for aeronautical mappingand charting purposes during World War II(WW-II).

Turning to more modern times, the CarlZeiss KRb 8/24 tri-lens film camera series[Fig.3] with up to 143° cross-track coveragehave been used extensively by Canadian,German and French air forces, including aversion that has been used in the CanadairCL-289 drone (UAV) aircraft. Also the Tri-Metrogon configuration has continued to beused, for example in the German Tupolev

Tu-154M Open Skies aircraft, employingthree Carl Zeiss Jena LMK 15/23 map-ping cameras [Fig. 4]. Furthermore the laterCarl Zeiss KS-153 reconnaissance cam-eras have been produced in both three andfive lens forms [Fig. 5] and have been usedwidely by NATO air forces, including theU.S. Navy (on F-14 aircraft) and theGerman Luftwaffe (on Tornado aircraft).Indeed a number of these cameras are stillin operational service.

All of the above examples have utilized filmcameras. However the “fan” configurationhas remained popular in the digital cam-era era. An example is the DLR-3k cam-era system [Fig. 6] employing three small-format cameras, which is in use by theGerman Space Agency (DLR). Currentlyboth IGI (with its Triple-DigiCam system) andLeica Geosystems within its RCD30 multi-head range offer similar three-camera “fan”configurations, but using medium-formatcameras instead of the small-format camerasused in the DLR-3k system. There is even anecho of the WW-II Tri-Metrogon system withthe advent recently of the Russian TupolevTu-214 ON Open Skies aircraft, which isbeing equipped with three Intergraph Z/IDMC-II140 large-format digital cameras[Fig. 7] in a Tri-Metrogon configuration to

30September 2012

A r t i c l e

By Gordon Petrie

Fig. 1 – A Bagley T-1A three-lens camera system with focal lengthsof 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)

Fig. 2 – An example of a Tri-Metrogon camera installation comprisingthree separate Fairchild K-17 metric film cameras with f = 152 mm lenses as mounted in a Boeing B17 bomber aircraft and used duringWorld War II. (Source: USAF)

Fig. 3 – The Carl Zeiss KRb 8/24 Tri-lens camera has been used innumerous NATO manned and UAV aircraft. (Source: Carl Zeiss)

Fig. 4 – The three Carl Zeiss Jena LMK photogrammetric film cam-eras mounted in a Tri-Metrogon configuration in the German Tupolev

Tu-154M Open Skies aircraft. (Source: IGI)

Based on its iOne Sensor Tool Kit Architecture

Visual Intelligence’s Iris One

provide the required wide-angle coverageof the ground during its monitoring flights.

II – The iOne STKA & ARCA

(a) Single ARCA ConfigurationThe latest additions to this multiple camera“fan” type of aerial camera system are theIris One systems that have been developedby the Visual Intelligence company basedon its iOne Sensor Tool Kit Archi -tecture (iOne STKA). The company –which is located in Houston, Texas in theU.S.A. – was originally called M7 VisualIntelligence prior to the sale of its sister air-craft component and repair business (M7Aerospace) in 2010, after which, the com-pany assumed its present title. The Iris Oneis a modular and scaleable digital aerial

camera system that is based on the so-calledARCA (Arched Retina Camera Array) tech-nology on which Visual Intelligence holds anumber of patents. The system features arigid ARCA or “bridge” into which an arrayor “fan” of three or five digital cameras canbe fitted with a choice of lenses and CCDformats [Fig. 8 (a)]. This configurationallows the optical axis of each individualcamera in the array to intersect, passingthrough a single perspective centre [Fig. 8(b)]. This particular arrangement has beenused in the Iris One 50 system (renamedthe Iris One Ortho 19 kps(1) system),which is the specific model that has beenawarded ‘Digital Aerial Sensor Certification’by the USGS in 2009.

[Note (1): In this particular context, Dr. Guevara,

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 configu-

rations with different lenses, angular coverages

and CCD array formats allow systems to be con-

structed with swath widths of 11 kps up to 60 kps.]

(b) Double ARCA ConfigurationsA further and quite distinctive feature of thescaleable ARCA technology is that an addi-tional ARCA can be added in parallel to thefirst with the two ARCAs or “bridges” beingfitted precisely together [Fig. 9 (a)]. Thisallows the co-registration of all the images

A r t i c l e

31Latest News? Visit www.geoinformatics.com September 2012

Fig. 5 – (a) A Carl Zeiss KS-153 Tri-lens camera installed in a podmounted on a German Tornado aircraft. (b) & (c) Diagrams of the CarlZeiss KS-153 reconnaissance camera showing the lens design and groundcoverage 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 thecross-track direction. (Source: Carl Zeiss)

Fig. 6 – The DLR-3k three-camera system makes use of a “fan” of threeCanon EOS small-format digital cameras with one pointing vertically and

two obliquely to provide wide cross-track coverage. (Source: DLR)

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 anarray or “fan” of three or five small-format cameras can be inserted.

(b) The geometric arrangement of an ARCA configuration. (Source: VisualIntelligence)

[a]

[b]

[c]

[a]

[b]

Airborne Camera Systems

produced by the two fans – as required, forexample, in a multi-spectral version ofthe Iris One system. This allows the set ofcolour RGB images that have been record-ed by the cameras mounted in the first ARCAor “bridge” to be co-registered with andsuperimposed on the corresponding set ofnear infra-red (NIR) images that have beenproduced by the cameras mounted on thesecond ARCA or “bridge” [Fig. 9 (b)]. Visual

Intelligence calls this its “CoCo” (Co-mount-ed and Co-registered) system.

Another possibility with the double ARCAarrangement is that the two sets of imagescan be offset with respect to one another,allowing them to be interlaced in order towiden the field of view and the cross-trackcoverage that can be achieved from a sin-gle flight. This particular configuration hasbeen adopted in the Iris One Hi5 system[Fig. 10 (a)]. Depending on the focal lengthof the lenses that have been selected and fit-ted to the camera modules and the CCD for-mat size, this system can provide differentswath widths over the ground from a givenflying height. As shown in the two diagrams[Figs. 10 (b) & (c)], the former provides a10 km wide swath (using 11 Megapixelcamera modules), while the latter providesa 12 km wide swath (using 16 Megapixelcamera modules) from a flying height (H) of12 km (using a jet aircraft!). Fig.11 is a sam-ple colour RGB frame image (described asa single ARCA Virtual Frame) that has beenacquired by an Iris One system.(c) Triple ARCA Configurations

With the addition of a third ARCA or“bridge” (interlocked and butted together)in parallel to the previous two ARCAs, stillmore versatile configurations can be envis-aged. One of these is the Iris One Stereocamera system [Fig. 12 (a)]. With this, the

three ARCAs or “bridges” of the system –with their 9 varying-format camera modules– can be oriented either in the cross-track orthe along-track direction. The pattern of theresulting ground coverage for each of thesetwo directions is shown in Figs. 12 (b) & (c).When each camera module is fitted with aCCD array having a format size of 6,576 x4,384 pixels = 29 Megapixels, then thetotal coverage of a single set of the 9images is 19,000 x 12,500 pixels. Usingcameras fitted with f = 85 mm lenses, thisproduces an angular coverage of 46° (cross-track) x 70° (along-track) (12,500 x 19,000pixels) with the cameras oriented in thealong-track direction [Fig. 12 (b)]. The60 % longitudinal overlap along the flightline that is produced when the cameras areprogrammed to expose overlapping sets ofcolour (RGB) images in a stereo convergentconfiguration gives a base:height ratio of0.6. When the system is rotated by 90degrees into the cross-track direction(19,000 x 12,500 pixels) [Fig. 12 (c)], thisyields a base:height ratio of 0.34, similarto that achieved by the overlapping stereo-

32September 2012

Fig. 9 – (a) Showing two ARCAs or “bridges” fitted together in parallelin a Lego-like arrangement. (b) Showing how the images produced bythe five colour (RGB) cameras fitted to the first ARCA are co-registered

with the five near infra-red (NIR) images produced by the cameras fittedto the second ARCA. (Source: Visual Intelligence)

A r t i c l e

Fig. 10 – (a) An Iris One camera system. [N.B. The same enclosure cab-inet is used for the Ortho, MS, Oblique, Stereo and Hi-5 models]. (b)Diagrams showing the alternative ground coverage patterns with inter-laced angular coverages that are produced by the Iris One Hi5 system

using cameras having different focal lengths and CCD sizes (11 Mpix or16 Mpix). (Source: Visual Intelligence)

Fig. 11 – A sample wide-swath colour RGB image frame that has beenacquired by an Iris One camera system. (Source: Visual Intelligence)

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 cover-age 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 cam-

era array in the cross-track direction. (Source: Visual Intelligence)

[a]

[b]

[a]

[b]

[a]

[b]

[b]

images that are produced by conventionallarge-format digital mapping cameras.

Another possible arrangement of the flexi-

ble three ARCAs or “bridges” of the IrisOne Stereo camera system is to employ15 small-format cameras [Fig. 13 (a)], as setout in a paper given by Dr. Hwangbo ofVisual Intelligence at the recent ASPRS 2012Conference. The pattern of the ground cov-erage that results if the cameras are orient-ed in the along-track direction is shown inFig. 13 (b). When each camera is fitted witha CCD array having a 4,008 x 2,672 pix-els = 11 Megapixel format, then the totalcoverage of a single set of 15 images is21,460 x 7,438 pixels. Using cameras fit-ted with f = 135 mm lenses, this producesan angular coverage of 27° (cross-track) x70° (along-track). The 60 % longitudinaloverlap along the flight line that is utilizedto produce overlapping sets of colour (RGB)images in a stereo convergent configurationgives a base:height ratio of 0.6 [Fig. 13 (c)].This is similar to that achieved by the over-

lapping stereo-images that are produced byconventional large-format (non-digital) map-ping cameras.

III – Camera & SystemCalibration

One of the main applications of the variousIris One systems is the topographic mappingfield. Thus the metric side and, in particular,the calibration of (i) each individual cam-era in an ARCA or “bridge”; and (ii) thatof the overall camera system as a whole isobviously a matter of prime concern. Thegeometric calibration of each individu-al camera is carried out in a laboratoryusing a calibration cage [Fig. 14] that is fit-ted out with a set of well distributed andhighly reflective coded targets, whose coor-dinates have been determined to a precisionof ±1mm in X and Y and ±5 mm in Z.Photographs of this target field are takenfrom a number of different positions and ori-entations with each individual camera, soproviding a strong convergent geometry.The images of all the targets that have been

recorded on each photograph are then mea-sured. From this information, the value of thefocal length of the lens and the position ofthe principal point of the frame image arethen determined for each individual camera,together with the lens distortion values orparameters. After each individual camerahas been calibrated, the relative positionand orientation of all the cameras within acomplete ARCA array are then determinedwith respect to one other, again using theknown coordinates of the targets in the tar-get field. After this second stage of the cali-bration process, a single composite “vir-tual” frame image with a singleperspective centre can be defined using themeasured data from all the componentimages that have been generated by thearray of modular cameras mounted on theARCA or “bridge”. These “virtual” frame

images can then be handled by any stan-dard third-party photogrammetric softwaresolution.

The radiometric calibration that is alsocarried out in the laboratory (outside undernatural light conditions) for the Iris One sys-tem is a relative calibration (not an absolutecalibration) and includes colour and tonalbalancing, shading and aperture correction,and smearing removal. The resulting data isincorporated into a radiometric correctionsoftware module for use during later imageprocessing operations. The module can alsoseparate the colour RGB images into theirindividual component red, green and blueimages and interpolate the missing pixel val-ues produced by the Bayer mosaic filter.

This laboratory calibration is supplementedby a further in-flight geometric calibra-tion that is carried out over a field of sig-nalized ground control points laid out in atest area within the Houston metropolitanregion. This test area is overflown by an air-craft at a flying height (H) of 1 km. The IrisOne camera system is programmed toexpose its images with a large (80%) longi-tudinal and (60%) lateral overlap. Cross-strips are flown as well as the parallel stripsof the main block of aerial photography cov-ering the test area. Automatic image match-ing of the target and tie points is carried outon all the overlapping photographs thathave been exposed during the flight. A bun-dle aerial triangulation and block adjust-ment employing self-calibration is thenimplemented using the BINGO softwarefrom GIP in Aalen, Germany to generatethe final coordinate values and their residu-als at the ground control points. Needlessto say, the lever arm corrections that relatethe positions of the GPS antenna and theIMU to the perspective centre of the “virtu-al” frame photo will also be determined.

A large number of test flights of the Iris Onesystem have been undertaken by variousaerial photographic and mapping compa-nies in the U.S.A. Numerous flights havebeen undertaken for test purposes over arange of flying heights by AerialViewpoint, which is based locally in theHouston area. Further extensive testing hastaken place in cooperation with Techmap,a mapping company that is based inPeachtree City, Georgia. Among the largerAmerican aerial mapping companies,Sanborn, Fugro Horizons & Earth -Data and Northrop Grumman 3001Inc. have all conducted trial flights with IrisOne systems. Further test flights have also

A r t i c l e

33Latest News? Visit www.geoinformatics.com September 2012

Fig. 13 – (a) An Iris One Stereo camera system showing the 15 cam-eras mounted on three parallel ARCAs or “bridges”. (b) The pattern of

the ground coverage that is produced by the 15 camera array oriented inthe along-track direction. (c) The 60 % longitudinal overlap that is pro-duced by two successive sets of exposures from the Iris One Stereo sys-

tem gives a base: height ratio of 0.6. (Source: Visual Intelligence) Fig. 14 – The Visual Intelligence calibration cage. (Source: Visual Intelligence)

[a]

[b]

[b]

been undertaken by INEGI, theMexican national mappingagency. Purchasers of Iris Onesystems include AviationSupplies, a leading supplier ofaircraft solutions in China. In theU.S.A., McKim & Creed, asurveying and mapping compa-ny based in Raleigh, NorthCarolina – has acquired severaliOne Infrastructure MappingSystems (iOne IMS).

IV – System Software

The Iris One system is of coursedriven and controlled by soft-ware. The main components that

carry out the in-flight data acquisition oper-ations are provided through the so-calledVisual Navigator software. This featuresthree modules – (i) for image data acquisi-tion; (ii) for flight line management andcamera control; and (iii) for data manage-ment respectively. A second software sys-tem, called Isis, carries out the subsequentimage data processing, including the radio-metric correction software mentioned above.It features two modules. (i) The Isis Skymodule carries out the processing of theacquired imagery in conjunction with theDGPS and IMU data that has been collect-ed simultaneously in-flight. If an existingDEM is available, e.g. from USGS in theUnited States, this module can also carry outthe generation of an orthophoto in-flight. (ii)The Isis Earth module is used to carry outa more accurate ortho-rectification usingground control points and more refinedGPS/IMU data in a post-flight processingoperation that is carried out later on theground.

Within this context, Visual Intelligence hasteamed up with the MaxVision company,based in Madison, Alabama, to create animage processing system that will satisfy therather demanding requirements of the VisualNavigator and Isis Sky software modules,especially in respect of implementing its in-flight image processing capability. The com-puter that is being used for the purpose isthe compact MaxCube II mobile “superserver” [Fig. 15]. This ruggedized computercan be supplied with two Intel Xeon proces-sor units providing up to twelve powerful 64-bit CPUs and up to eight removable harddrives, each with a 3 terabyte storagecapacity. All of this processing power andstorage capacity is contained in a cabinetthat is close to one cubic foot in size andhas a low power consumption.

V - Current & FutureDevelopments

A recent development has been the intro-duction of the Iris One InfrastructureMetric-Mapping System – having the tit-ular acronym iOne IMS. This comprises asingle rigid ARCA or “bridge” into which isinserted a pair of nadir-pointing camera

A r t i c l e

34September 2012

Fig. 15 – An Iris One camera system (at left) with its compact MaxCube data collection, cam-era control and data processing server (at right) – with its laptop controller located on top of

the main cabinet. (Source: Visual Intelligence)

Fig. 16 – (a) Showing the ARCA or “bridge” on which the twin nadir-pointing RGB and NIR cameras and the two oblique-pointing cameras aremounted – 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 fourlenses – two nadir-pointing and two oblique-pointing – of an iOne IMS

camera system from the underside. {Source Visual Intelligence)

[a]

[b]

modules of varying-format sitting side-byside; the one a colour RGB camera; theother an NIR camera. These are flanked bya pair of oblique-pointing cameras, that arepointing in opposite directions [Fig. 16].Essentially this arrangement is similar to theTri-Metrogon configuration discussed above.

Since the system is designed specifically forthe corridor mapping of infrastructure, theARCA or “bridge” on which the camerasare mounted is oriented in the along-track(flight) direction. The system can beinstalled in a specially constructed pod thatis fitted to the underside of a helicopter or it

can utilize the standard camera hole on afixed-wing aircraft.

Another closely related development hasbeen the co-mounting of an Iris One camerasystem with a RIEGL VQ-580 airbornetopographic laser scanner [Fig. 17]. The twodevices are mounted closely together on acommon base plate fitted with anti-vibrationdampeners. The spatial relationship of thetwo devices, including their relationship tothe accompanying GPS/IMU sub-system, isdetermined very precisely through the mea-surement of the lever arm offset during thesystem calibration.

Visual Intelligence has also developed theconcept of a five- or nine-camera system[Fig. 18] that it calls its “360° ortho-stereo-oblique” system. This generatesground coverage in the form of a “MalteseCross” that is similar to the systems devel-oped by Pictometry and Track’Air, but isbased on the ARCA technology. Furtherdevelopments of this architecture are cur-rently focussed on the development, minia-turization and production of very lightweight and compact camera systems [Fig.19] that are fully metric and can be utilizedin UAVs; in ground vehicles; and in mobiledevices.

VI – Conclusion

Visual Intelligence has already developed amost interesting series of airborne digitalframe camera systems based on itsscaleable and modular iOne Sensor Tool KitArchitecture. For the future, it will be veryinteresting to follow the concepts that arecurrently in the research and developmentstage and to see them come to fruition.

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

A r t i c l e

36September 2012

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-vibrationdampers – 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)

Fig. 18 – The “360° ortho-stereo-oblique” system (Source: Visual Intelligence)

Fig. 19 – A prototype of a three-camera ARCA or “bridge” made of car-bon-fibre. (Source: Visual Intelligence)

[a]

[b]