Adaptive camera calibration for a focus adjustable liquid lens in fiber optic endoscopy
Benefitting from recent innovations in the smartphone sector, liquid optics in very compact designs have been cost-effectively introduced to the market. Without mechanical actuation, a focus variation can be adjusted within fractions of a second by curving a boundary layer between two liquids by applying a pulse width or amplitude modulated potential. Especially in the field of endoscopy, these innovative optical components open up many application possibilities. Conventional, mechanical zoom lenses are not very common in endoscopy and can only be miniaturized at considerable effort due to the necessary actuation and the complex design. In addition, the mechanical response is slow, which is a particular disadvantage in hand-held operation. A calibrated camera provides a two-dimensional camera pixel translated into a three-dimensional beam and, together with distortion correction enables the extraction of metric information. This approach is widely used in endoscopy, for example, to measure objects in the scene or to estimate the camera position and derive a trajectory accordingly. This is particularly important for triangulation-based 3D reconstruction such as photogrammetry. The use of liquid lenses requires a new data set with an adapted camera calibration for each focus adjustment. In practice, this is not feasible and would result in an extensive calibration effort. This paper therefore examines, on the basis of an experimental setup for automated endoscopic camera calibration, the extent to which certain calibration parameters can be modelled and approximated for each possible focal adjustment, and also investigates the influence of a liquid lens on the quality of the actual calibration.
Institute of Measurement and Automatic Control (Germany)
Lennart Hinz works as a research associate at the Institute for Measurement and Automatic Control at the Leibniz University Hannover. He is part of the DFG Collaborative Research Center Transregio 73 and he is researching fiber optic modular measurement systems for industrial inline applications. The aim of his subproject is to develop a measuring system based on the principle of fringe projection in combination with flexible optical fibres. The usage of optical fibres allows a spatial separation of the measuring head and the camera-projector unit. Therefore, a measuring head of small dimensions can perform measurements in running forming plants between each forming cycle. The main task of his subproject is the design of the optical setup, the construction of appropriate measuring heads within in the context of the measuring task and the development of algorithms processing the fringe pattern by including the transmission properties of the used optics.
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