Hi @TomoPi ,
According to AN5983, you must select a material which have to be as trasparent as possibile to the wavelenghts of interest. For example, the lens can be made of silicon, germanium, chalcogenide or polymers, and HDPE (High-density polyethylene).
An option can be use a Frenel lens as the two suggested in the AN5983: TMOS10-12 lens that widen the FOV and TMOS63-10 lens that extend to a longer detectable distance.
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