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Quality control in large volume roll-to-roll manufacturing for printed electronics, photovoltaic cells, battery and display production demand high-resolution inline imaging. Frequently, defects, such as surface inhomogeneities, are hard to identify due to low image contrast using conventional 2D imaging. Photometric stereo, surface reconstruction based on reflectance behavior, can complement 2D observations by acquiring multiple illumination directions at line rates ≥100k lines/s provided by contemporary line-scan cameras. We extended the concept of time delay integration (TDI) in multi-line-scan cameras with time-multiplexed multi-directional illuminations - short multi-TDI. This improves signal-to-noise ratio (SNR) and dynamic range, which has the potential to visualize various production defects, otherwise difficult to identify in 2D. We implemented multi-TDI using diode line-lasers, which represent a power-efficient, bright illumination with a small mechanical form factor suitable for fast intensity modulation. Their beam can be fused to several lines acquired by multi-line-scan cameras. However, coherent illumination comes along with laser speckles, especially at high spatial resolutions. Our approach aggregates light over time during object-motion, thus inherently reducing speckle while simultaneously acquiring multiple illumination directions. This work examines parameters (laser intensity, number of light sources and number of aggregated lines) useful to outperform LED-based illumination in terms of acquisition speed, cost and integration size. Further, we demonstrate the achievable reduction in speckle contrast to quantify achievable speckle reduction. In summary, our proposed multi-TDI speckle reduction approach for laser illumination enables increased image quality in high-resolution inline photometric stereo, facilitating defect identification in roll-to-roll manufacturing processes for optimized production quality and reduced production resources.
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