I was recently talking to an academic about creating ultra-cheap Fresnel lenses (ie << $100 per 1m sq). There is inevitably at least one patent in this field, but I believe the following isn’t infringing on that.
In a similar way to the form of old-style LP records, a sheet of plastic (transparent) would have one of its surfaces cut with a needle-like tool moving in a spiral path.
The angle of the cutter relative to the surface would change with time in order to generate the required refractive profiles in the groove.
This could make use of low-grade plastic (maybe even recycled material, initially repolished). No need for any injection moulding tooling; at most a pc controlling a relatively simple spiral cutter.
