Applications with precalibrated camera

Robot calibration. At KUKA Robotertechnik of Augsburg industrial robots have been reliably measured, adjusted and calibrated on the assembly line at two specially installed workplaces during the past two years [30]. To measure the required positions and orientations, a photogrammetric metrology system consisting of one or two RolleiMet-ric Réseau Scanning Cameras (RSCs) are mounted on a rugged tripod (Fig.6.16). Using a shiftable standard CCD sensor, these cameras reach a resolution of 4200×4200 picture elements at an image format of 50

×50mm² with an accuracy of better than 1µm in image space. The orientation of the single images in relation to the entire image is done

Figure 6.16: Robot adjustment.

in an optical-numerical way by a réseau measurement. Besides, this principle, which is described in Riechmann [8], allows the focusing of the camera without changing the interior orientation.

The cameras are controlled by a commercial PC with a standard frame grabber, running under Windows NT. The PC serves for operator prompting, for processing and outputting results and for connection to the robot control. The measurement system is basically independent of the robot control.

The interior orientation of the cameras is determined once in a spe-cial calibration measurement. With this known interior orientation, it is possible to determine the orientation of the cameras. Various target plates 450mm× 450mm in size are used, with reflective targets as control points, which are also identified as tools for the robot. A sec-ond target plate of 600 mm×600 mm with an adapter serves for prior determination of the robot base and external orientation of the cam-era. To transfer the different coordinate systems, highly precise bores in the target plates are used with special adapters. A mechanical pre-cision measuring machine serves as a higher-order metrology system for measuring the bores.

After orientation the online measurement of the robots is possible.

The quality of the system orientation is verified by special measure-ments. A recalibration of the system normally is necessary only in time periods of some months.

6.8 Summary 173 Other applications. Other photogrammetric applications for the 3-D capture of objects can be found, for example, in accident photography and in architecture. In these fields, it is primarily scale drawings or rectified scale photos (orthophotos) that are obtained from the pho-tograms. The cameras employed are generally calibrated for different focus settings using the methods described in the foregoing. An ex-ample is the RolleiMetric ChipPack with a resolution of 2000 ×2000 sensor elements. Special metric lenses, which guarantee reproducible focus setting by mechanical click stops of the focusing ring, keep in-terior orientation constant for prolonged periods. The data of inte-rior orientation are entered in the software and thus used for plotting and all computations. This guarantees high-precision 3-D plotting with minimum expense in the phase of image acquisition.

6.8 Summary

The use of digital cameras for measurement purposes requires the knowledge about different parameters, describing the interior camera model and the exterior camera positions and orientations. The deter-mination of the interior and exterior camera parameters is defined as calibration and orientation of the measuring system. It has been shown, that—depending on the application—different strategies for the cali-bration and orientation exist. Different mathematical models for the description of optical measuring systems are usable. A focal point has been the description of an integrated model, which defines the trans-formation from image-space to object-space by six parameters of the exterior orientation and different parameters for the camera geometry.

Effects from electronical, mechanical or optical influences (e.g., lens dis-tortion) are corrected by the model. The described models have been used for many applications and are sufficient for a wide range of cam-eras. Current developments of digital cameras for measuring purposes are using large image-sensors with higher resolution. On the other hand, accuracy requirements are increasing for many applications. For this reason future improvements and extensions of the mathematical camera model can be necessary and helpful, taking into account spe-cial problems of large sensors, such as sensor flatness or patching of smaller sensor parts to complete sensors.

6.9 References

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[2] Brown, D. C., (1966). Decentering distortion of lenses. Photogrammetric Engineering,32:444–462.

[3] Schafmeister, R., (1997). Erste Erfahrungen mit der neuen Rollei Q16 Met-ricCamera. InPublikationen der Deutschen Gesellschaft für Photogramme-trie und Fernerkundung (DGPF), Vol. 1, pp. 367–378. Berlin: DGPF.

[4] Lenz, R. and Lenz, U., (1990). Calibration of a color CCD camera with 3000×2300 picture elements. ISPRS Symposium, Com. V. Close-Range Photogrammetry meets Machine Vision, Zürich.Proc. SPIE,1395:104–111.

[5] Richter, U., (1993). Hardware-Komponenten für die Bildaufnahme mit höchster örtlicher Auflösung. InPublikationen der Deutschen Gesellschaft für Photogrammetrie und Fernerkundung., Vol. 1, pp. 367–378. Berlin:

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[7] Holdorf, M., (1993). Höchstauflösende digitale Aufnahmesysteme mit Réseau Scanning und Line-Scan-Kameras. InSymposium Bildverarbeitung

’93,, pp. 45–51. Esslingen: Technische Akademie Esslingen.

[8] Riechmann, W., (1993). Hochgenaue photogrammetrische Online-Objekterfassung. PhD thesis, Technical University of Brunswick.

[9] Bösemann, W., Godding, R., and Riechmann, W., (1990). Photogrammetric investigation of CCD cameras. ISPRS symposium, close-range photogram-metry meets machine vision, Zürich.Com. V. Close-Proc. SPIE,1395:119–

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[10] Lenz, R., (1988). Zur Genauigkeit der Videometrie mit CCD-Sensoren. In Proc. 10. DAGM-Symp. Mustererkennung 1988, Informatik Fachberichte 180, H. Bunke, O. Kübler, and P. Stucki, eds., pp. 179–189, DAGM. Berlin:

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[11] Beyer, H., (1992). Advances in characterization and calibration of digital imaging systems. International archives of photogrammetry and remote sensing. 17th ISPRS Congress, Washington.Com. V,29:545–555.

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[13] Wester-Ebbinghaus, W., (1989). Mehrbild-Photogrammetrie — räumliche Triangulation mit Richtungsbündeln. InSymposium Bildverarbeitung ’89, pp. 25.1–25.13. Technische Akademie Esslingen.

[14] Rüger, Pietschner, and Regensburger, (1978). Photogrammetrie—

Verfahren und Geräte. Berlin: VEB Verlag für Bauwesen.

[15] Wester-Ebbinghaus, W., (1980). Photographisch-numerische Bestimmung der geometrischen Abbildungseigenschaften eines optischen Systems.

Optik,3:253–259.

[16] Lenz, R., (1987). Linsenfehlerkorrigierte Eichung von Halbleiterkameras mit Standardobjektiven für hochgenaue 3D-Messungen in Echtzeit. In Proc. 9. DAGM-Symp. Mustererkennung 1987, Informatik Fachberichte

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[17] Conrady, A., (1919). Decentered lens systems.Royal Astronomical Society, Monthly Notices,79:384–390.

[18] Brown, D., (1976). The bundle adjustment—progress and perspectives.

Helsinki 1976. InInternational Archives of Photogrammetry, Vol. 21(3), p. 303041.

[19] Fryer, J., (1989). Camera calibration in non-topographic photogrammetry.

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69. American Society of Photogrammetry and Remote Sensing.

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851–855.

[21] Gerdes, R., Otterbach, R., and Kammüller, R., (1993). Kalibrierung eines digitalen Bildverarbeitungssystems mit CCD-Kamera. Technisches Messen,60(6):256–261.

[22] Wester-Ebbinghaus, W., (1985). Bündeltriangulation mit gemeinsamer Ausgleichung photogrammetrischer und geodätischer Beobachtungen.

Zeitschrift für Vermessungswesen,3:101–111.

[23] Fellbaum, M., (1996). PROMP —A new bundle adjustment program using combined parameter estimation.International Archives of Photogramme-try and Remote Sensing,31(B3):192–196.

[24] Wester-Ebbinghaus, W., (1985). Verfahren zur Feldkalibrierung von pho-togrammetrischen Aufnahmekammern im Nahbereich.DGK-Reihe B,275:

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7 Three-Dimensional Imaging