Abstract
TV cameras involve some intrinsic parameters such as the image coordinates of principal point, the principal distance, and the distortion of lens. The calibration of these camera parameters is one of the most important techniques for 3D measurements based on visual information. The information derived from 2D image is visual angle, and we should evaluate the accuracy of calibration by using the visual angle. When we evaluate the accuracy of calibration based on the visual angle, which direction should we take as the reference of the visual angle? Nominal visual angles have been usually used for evaluations, that is, true values of the coordinates of the principal point have been used as the reference of the visual angle. But, in this way, the optimal setup condition of a fiducial chart varies from a camera to another, that is, the condition varies with the length of principal distance : the fiducial chart is a plane, on which fiducial marks are drawn, and is moved along the depth direction. In practical use, it makes calibration procedures troublesome, and, so, it is desired the optimal setup condition not to vary t between cameras with various principal distances. This paper proposes a novel concept of the evaluation method with the visual angle : the calibrated coordinates of principal points are taken as the references of the visual angle. We refer this angle to the effective visual angle. It yields a useful result that the optimal setup condition does not vary with the lengths of principal distance.
