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For high-precision measurements through a inspection window, a 3D scanner based on fringe projection profilometry is being researched. The 3D scanner combines a micromirror array projector and two telecentric cameras. The affine camera model is commonly used to calibrate telecentric imaging systems, in which a single magnification factor is introduced and optimized for each lens. However, 3D reconstructions based on this model indicated that reconstruction uncertainties in the peripheral areas of the measuring volume are significanty affected by a possible inspection window. These uncertainties may occur due to the model-based determination of the magnification factor and the reduction in parallelism of the visible rays within the telecentric lens that can occur at larger working distances. To address this issue, a new method for calculating and identifying the influence of the magnification factor on the 3D point scaling for telecentric measuring systems is proposed. First, the measuring system is calibrated using an affine camera model. Then, the reconstructed 3D target points are used to estimate the magnification factor locally and assessing the influence of an inspection window in the optical path. In order to further investigate the influence of the inspection window on the imaging performance of the cameras the focus is estimated locally within the measuring volume. Initial measurements using these methods reveal that scale variations and the reduction of focus can be quantified locally and a model based correction as well as the removal of poorly reconstructed points is feasible.
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