Extended lens model calibration of digital still cameras

Digital still cameras have been widely adopted for close range photogrammetry and machine vision applications. Due to the advantages of on board storage of digital images, portability and rapid data processing, digital still cameras have virtually replaced medium format film cameras for measurement tasks such as structural monitoring and industrial metrology. As for any photogrammetric application, the accuracy of the derived object data is dependent on the accuracy of the camera calibration, amongst many other factors. For photogrammetric applications in which accuracy is not of paramount importance or the object is larger than a few metres in size, use of a simple model of lens distortion in conjunction with the collinearity equations is sufficient. However, the combination of very close ranges and the large distortions typically associated with the lenses used with digital still cameras requires an extended lens model to account for variation of distortion within the object space. The fidelity of the calibration model becomes particularly important where stringent tolerances are set, for example in aerospace inspection tasks. This paper reviews previous research into distortion variation and outlines an investigation of the modelling of this systematic error. The calibration procedure, utilising a straight line calibration range, is described. Experimental results for Kodak DCS420 and DCS460 digital still cameras, including an assessment of the effect and repeatability of the distortion variation, are presented. 1