Accuracy Enhancement of Vision Metrology through Automatic Target Plane Determination

In digital close-range photogrammetry, commonly referred to as vision metrology, circular targets are often used for high precision applications. The most common target type used consists of retro-reflective material, which provides a high contrast image with flash photography. The measurement of image coordinates of signalised targets continues to be a factor limiting the achievable accuracy of high-precision vision metrology systems. Mathematical algorithms are used to determine the centres of imaged targets in 2D space. These 2D centroids are then used in a triangulation process to calculate the target position in 3D space. This computational process assumes that the targets represent perfect ‘points’ in space. However, in practice target-thickness and target-diameter can adversely effect this assumption, leading to the introduction of systematic errors and to incorrect calculation of 3D position. The paper presents the development of a target plane determination process, which will serve to automatically correct for these errors. This will also lead to high accuracies within the bundle adjustment via an improved mathematical model.