Unlike the filter for optical region, there are hard requirements for IR filter which is mounted in vacuum and cryogenic dewar. Both requirements, monolithic substrate and long blocking range for astronomical IR filter make the fabrication difficult.
These days, the required blocking range becomes longer even from optical wavelength to e.g. ~ 2.6um due to the spread of excellent HgCdTe imaging sensor.
In addition, recently larger size is required like wide field CCD mosaic camera. We are currently developing IR filter which can meet monolithic substrate, long blocking range and larger size at the same time.
WFCAM Z, Y / The Mauna Kea near-infrared J, H, K filter set
The Mauna Kea near-infrared J, H and K filters were defined by Dr. Alan Tokunaga, IfA, University of Hawaii and become the standard photometric system in NIR region like SDSS in the optical. WFCAM Z and Y filters are also standard for IR astronomers. Each passband is achieved by dielectric interference film coated by ion assisted deposition, so spectral response is stable under 77 K cryogenic and vacuum condition.