Lens distortion correction with a calibration harp

Plumb line lens distortion correction methods permit to avoid numerical compensation between the camera internal and external parameters in global calibration method. Once the distortion has been corrected by a plumb line method, the cam era is ensured to transform, up to the distortion precision, 3D straight lines into 2D straight lines, and therefore becomes a pinhole camera. This paper introduces a plumb line method for correcting and evaluating camera lens distortion with high precision. The evaluation criterion is defined as the average standard deviation from straightness of a set of approximately equally spaced straight strings photographed uniformly in all directions by the camera, so that their image crosses the whole camera field. The method uses an easily built "calibration harp," namely a frame on which good quality strings have been tightly stretched to ensure a very high physical straight ness. Real experiments confirm that our method produces high precision corrections (less than 0.05 pixel), approximating the distortion with a large number of degrees of freedom given by a polynomial model of order eleven.