Sapphire (Al2O3)


Sapphire (Al2O3)

Sapphire (Al2O3) is a single crystal aluminum oxide (Al2O3) with Mohs hardness of 9, it is one of the hardest materials. This extreme hardness of sapphire makes it difficult to polish using standard techniques. High optical quality finishes on sapphire are not always possible. Since Sapphire is very durable and has good mechanical strength, it is always used as window material where scratch resistance is required. Its high melting point, good thermal conductivity and low thermal expansion provide excellent performance in high temperature environments. Sapphire is chemically inert and insoluble to water, common acids, and alkalis for temperatures up to 1,000 °C. It is commonly used in IR laser systems, spectroscopy, and rugged environmental equipment.

Material Properties

Refractive Index

1.755 @ 1.064 µm

Abbe Number (Vd)

Ordinary: 72.31, Extraordinary: 72.99

Thermal Expansion Coefficient (CTE)

8.4 x 10-6 /K

Thermal Conductivity


Mohs Hardness




Lattice Constant

a=4.75 A; c=12.97A

Melting Point


Transmission Regions & Applications

Optimum Transmission Range Ideal Applications
0.18 - 4.5 μm Commonly used in IR laser systems, spectroscopy and rugged environmental equipment


The right graph is transmission curve of 10 mm thick, uncoated sapphire substrate

Tips: Sapphire is slightly birefringent, general purpose IR windows are usually cut in a random way from crystal, however an orientation is selected for specific applications where the birefringence is an issue. Usually this is with the optic axis at 90 degrees to the surface plane and is known as "zero degree" material. Synthetic optical sapphire has no colouration.


For more in-depth specification data, please view our catalog optics to see our complete selection of optics made from sapphire.