MgAl<sub>2</sub>O<sub>4</sub> is a candidate for sintered windows, domes and lenses for UV, visible, and IR applications. However, exact
Mie calculation shows that for imaging uses with a window thickness of e.g. 5 mm even IR transmission will not tolerate
smallest amounts of 0.01% of 50-100 nm small pores, and the impact of such pores is even worse at shorter wave
lengths. Principles of solid state sintering suggest that smallest pores should be eliminated more easily than larger ones.
It is, however, observed that a significant population of 50-100 nm small nanopores exists in undoped transparent spinel
ceramics after hot-isostatic pressing with the higher concentration the finer the particles of the raw spinel powder are. On
the other hand, it is demonstrated that sintering densification is governed not only by the size of the ceramic powder particles
and the homogeneity of their mutual coordination but also by the state of the crystal lattice. Taking advantage of
this latter effect, sintered spinel ceramics were derived by reactive sintering of undoped MgO/Al<sub>2</sub>O<sub>3</sub> mixtures resulting
in an in-line transmittance which fits comparable spinel single crystals from 200nm wave length up to the IR range.
New grades of sintered (polycrystalline) corundum ceramics have been shown to exhibit a ballistic shielding power close to SiC/B4C composites when manufactured with a grain size of about 500 nm. It is demonstrated here that these Al<sub>2</sub>O<sub>3</sub> ceramics become transparent when their residual porosity is decreased to less than 0.05 %. Specifically, in the IR range between about 2 and 6 μm their transmissivity equals that of sapphire approaching the upper theoretical limit for wavelengths of 2.5-4.5 μm. This opens the way to new possible applications such as IR domes. These optically and mechanically homogeneous ceramics can be manufactured with a wall thickness up to 15 mm by a wet casting approach. The technology enables the manufacture of complex hollow spheres which after sintering are transparent in visible light without polishing.