The researchers describe the HSI element as a folded metasurface platform comprising three reflective and one transmissive dielectric metasurfaces all integrated on a gold-mirrored glass substrate about 1mm thick, in one lithographic step. Looking closer, the metasurfaces consist of α-Si nanoposts with rectangular cross sections, resting on the fused silica substrate and capped by a 2μm thick layer of SU-8 photoresist. The metasurfaces are designed to collectively disperse and focus light of different wavelengths and incident angles on a focal plane parallel to the glass substrate. Light to be analysed enters the HSI through an input aperture in the front gold mirror and is deflected into the substrate and vertically dispersed by the first metasurface. The other two reflective metasurfaces together with the transmissive one focus light with different wavelengths and horizontal incident angles to diffraction-limited spots on a detector array plane parallel to the substrate. The last metasurface is transmissive and simultaneously acts as the output aperture to collect the spectrum images.