Unlike filters in optical region, there are harder requirements for astronomical IR filters.
HgCdTe (MCT) imaging sensor requires IR filters to have very wide blocking range, often from 0.4 μm to 3.0 μm, sometimes ~ 5.0 μm.
Vacuum and cryogenic environment also requires IR filter to be made with monolithic substrate to avoid the abrasion of cement bythermal shock.
Eventually incredible number of thick interference film layers are necessary to fulfill these requirements.
These days thermal blocking filter is asked as well as Mauna Kea Observatories Y, J, H, and Ks filters.
We provided the thermal blocking filters to Dunlap Institute for Astronomy and Astrophysics in Canada in 2014. It was coated by the state-of-the-art magnetron sputtering technology on the monolithic fused silica substrate, and showed "beautiful" shape of passband and excellent blocking performance in IR (thermal) wavelengths.
Transmission curve was also measured and evaluated under the 77 K cryogenic condition with the witness sample at Asahi plant.