2.1. Vegetation illumination
2.1.1. Beam generation
Each laser module uses three 4 mm collimated laser sources, two red (635 nm and 685 nm) lasers and one near-infrared (785 nm), with output power levels of 30 mW, 50 mW and
50 mW, respectively. Within a module, each laser is independently mounted onto an alignment stage using alignment screws, so that all laser beams are aligned along the same optical axis. Two fixed,thin-film beam combiners are used to combine the laser beams, as reported by Askraba et al. (2013). Once all laser beams emerging from the laser module are collinear, overlapped and their polarisation directions are aligned, all laser stages are secured with locking screws to prevent the lasers from moving during dynamic tests.
The collimated beams emerging from the laser module enter a multi-spot beam generator, which consists of an optical cavity coated with a reflective top (back) surface and non-uniform transmissive bottom (front) surface (Askraba et al., 2011, 2013). The beam spot field of view coverage was improved, in comparison to previously reported beam spot generators (Paap, 2014) by using a longer cavity inclined at a greater angle of 23. This configuration increased the linear beam spacing from 12 mm in earlier prototypes to 15 mm in the final prototype and also reduced the gap between the two beams that are the closest to both sides of the line scan sensor. The non-uniform transmissive front surface was a fundamental improvement that enhanced overall system performance, leading to 4 mm wide collimated beams of almost similar intensities.
2.1. Vegetation illumination2.1.1. Beam generationEach laser module uses three 4 mm collimated laser sources, two red (635 nm and 685 nm) lasers and one near-infrared (785 nm), with output power levels of 30 mW, 50 mW and50 mW, respectively. Within a module, each laser is independently mounted onto an alignment stage using alignment screws, so that all laser beams are aligned along the same optical axis. Two fixed,thin-film beam combiners are used to combine the laser beams, as reported by Askraba et al. (2013). Once all laser beams emerging from the laser module are collinear, overlapped and their polarisation directions are aligned, all laser stages are secured with locking screws to prevent the lasers from moving during dynamic tests. The collimated beams emerging from the laser module enter a multi-spot beam generator, which consists of an optical cavity coated with a reflective top (back) surface and non-uniform transmissive bottom (front) surface (Askraba et al., 2011, 2013). The beam spot field of view coverage was improved, in comparison to previously reported beam spot generators (Paap, 2014) by using a longer cavity inclined at a greater angle of 23. This configuration increased the linear beam spacing from 12 mm in earlier prototypes to 15 mm in the final prototype and also reduced the gap between the two beams that are the closest to both sides of the line scan sensor. The non-uniform transmissive front surface was a fundamental improvement that enhanced overall system performance, leading to 4 mm wide collimated beams of almost similar intensities.
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