Nowadays, stereoscopic devices are widely used for precise three-dimensional (3-D) measurements of hard-to-reach elements in industrial and biomedical applications. The most common approach for their implementation is the utilization of prism-based optical systems, which allows simultaneous acquisition of two images from different viewpoints on a single sensor. Normally, they are equipped with a wideband (WB) white-light source, but contrast visualization of the inspected object and, therefore, accurate quantitative characterization of its parameters, often requires narrow-band (NB) spectral imaging. We show that the standard geometrical calibration may lead to significant measurement errors when obtained using WB illumination and applied to NB images. As the criterion to evaluate the calibration error, we have chosen the difference between reference and measured lengths of the 1 mm segment along transverse x, y , and longitudinal z axes. If WB calibration data are applied to NB images, the measurement bias increases from the middle of the working spectral range to its edges and can reach significant values: up to 0.1 mm along x axis and 0.15 mm along z axis in 10 to 25 mm distance range. To overcome this, we propose the calibration and image processing procedures based on a proper choice of a few spectral bands for calibration and interpolation of the calculated calibration parameters. Results of multiple experiments using stereo video endoscope confirmed that the proposed technique allows a decrease in the measurement bias by three times in comparison to conventional WB calibration for all wavelengths of the visible range, which essentially improves the measurement accuracy. The impact of WB calibration on random errors of measurements and the quality of image rectification was also analyzed and shown to be insignificant.
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