Tsai's radial alignment constraint (RAC) method has been shown as a viable candidate for camera calibration. The solution procedure, however, will fail if the camera plane is parallel to the plane defined by a calibration board which is used to provide calibration data. Tsai also devised an algorithm that handled the exactly parallel case. In this paper, we show that by a slight modification of the RAC-based camera calibration method, the unknown parameters of the camera can still be calibrated even when the sensor plane is near-parallel to the camera calibration board. An application example of the proposed approach is the calibration of a selectively compliant assembly robot arm (SCARA) with a hand-mounted camera, where the extrinsic parameters need to be repeatedly calibrated to reconstruct robot end-effector poses at various robot measurement configurations.
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