Hybrid optics -- diffractive structures on a refractive substrate -- have some specific applications in the lower of the two thermal infrared wavebands. These applications include optical systems where simplicity and/or low mass are major requisites. In many cases, however, the use of diffractive surfaces is limited to those substrate materials that can be diamond-machined successfully. The paper examines the optical merits of several substrate materials and gives design examples where hybrid optics have major advantages relative to conventional optics in terms of mass, thermal defocus, and complexity.
Philip J. Rogers, Philip J. Rogers,
"Use of hybrid optics in the 3 to 5-u infrared band", Proc. SPIE 2540, Current Developments in Optical Design and Engineering V, (15 September 1995); doi: 10.1117/12.219514; https://doi.org/10.1117/12.219514