With transparent ceramic, optical properties beyond the possibilities of glass can be achieved.
The figure shows the spectra of different transparent ceramics with a thickness of 1 mm. Transparent ceramics can be transparent from the UV to the IR range. The maximum transmission depends on the refractive index and can be increased with an anti-reflection coating. The refractive index is highest for cubic-ZrO2 (green line) and lowest for Spinel (MgAl2O4, red line).
In the case of non-cubic crystal systems (Al2O3, tetragonal ZrO2), the birefringence causes the light to split up in each crystallite. The birefringence scatters the light and reduces the transmission. This light scattering can be avoided by using very small crystallites, smaller than the wavelength of the light.
Cubic crystal systems (YAG, Y2O3, 10Y-ZrO2, MgAl2O4) show no birefringence.
In contrast to glass ceramics, transparent ceramics do not contain glass and are therefore harder, stronger and more thermally resistant than glass ceramics.