  Thermal imaging and sensing technologies offer critical information about our
thermally radiant world, and in recent years, have seen dramatic increases in
usage for a range of applications. However, the cost and technical finesse of
manufacturing infrared optical components remain a major barrier towards the
democratization of these technologies. In this report, we present a solution
processed plasmonic reflective filter or PRF as a scalable and inexpensive
thermal infrared optic. The PRF selectively absorbs sunlight and specularly
reflects thermal infrared TIR wavelengths with performance comparable to
state-of-the-art TIR optics made of materials like Germanium. Unlike
traditional infrared optical components, however, the PRF can be conveniently
fabricated using inexpensive materials and a dip and dry chemical synthesis
technique, and crucially, has manufacturing costs that are orders of magnitude
lower. We experimentally demonstrate the core optical functionality of the PRF,
as well as its integration into infrared imaging and sensing systems without
compromising their thermographic or radiometric capabilities. From a practical
standpoint, the inexpensive and convenient fabricability of the PRF represent a
significant advance towards making the benefits of thermal imaging and sensing
systems more affordable and accessible. Scientifically, our work demonstrates a
previously unexplored optical functionality and a new direction for versatile
chemical synthesis in designing optical components.
