Amorphous hydrogenated hard carbon (a-C:H) is a promising new optical coating material for passive infrared materials. It offers the rare combination of extreme hardness, chemical inertness, and optical transparency over a wide spectral range. We give an overview of the optical properties of rf-plasma deposited a-C:H coatings and compare them with vacuum-evaporated infrared coatings. For many applications, a-C:H solves the problem of a mechanically and chemically resistant 8 to 12 Am coating despite its moderate absorption in the 6 to 20 Am range. The tunability of the refractive index between 1.8 and 2.2 allows single layer coatings on Si and Ge with zero reflection. State-of-the-art applications, possible future developments, as well as remaining technological problems of a-C:H are discussed.