Sapphire
Sapphire is a superior window material in many ways. Because of its extreme surface hardness, sapphire can be scratched by only a few substances (such as diamond or boron nitride) other than itself. Chemically inert and insoluble in almost everything except at highly elevated temperatures, sapphire can be cleaned with impunity. For example, even hydrogen fluoride fails to attack sapphire at temperatures below 300°C. Sapphire exhibits high internal transmittance all the way from 150 nm (vacuum ultraviolet) to 6000 nm (middle infrared). The external transmittance of sapphire is shown in the figure below. |
||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||
|
External transmittance for 1-mm-thick uncoated sapphire |
||||||||||||||||||||||||||||||
|
Because of its great strength, windows made from sapphire can be much thinner
than windows of other glass types, and therefore are useful even at wavelengths
that are very close to their transmission limits. Because of the exceptionally
high thermal conductivity of sapphire, thin windows can be very effectively
cooled by forced air or other methods. Conversely, sapphire windows can easily
be heated to prevent condensation. Sapphire is single-crystal aluminum oxide (Al2O3). Because of its hexagonal crystalline structure, sapphire exhibits anisotropy in many optical and physical properties. The exact characteristics of an optical component made from sapphire depend on the orientation of the optic axis or c-axis relative to the element surface. Sapphire exhibits birefringence, a difference in index of refraction in orthogonal directions. The difference in index is 0.008 between light traveling along the optic axis and light traveling perpendicular to it. The dispersion relationship for the ordinary ray in sapphire is given by the following equation: |
||||||||||||||||||||||||||||||
 |
 |
|||||||||||||||||||||||||||||
|
where |
||||||||||||||||||||||||||||||
|
 |
|||||||||||||||||||||||||||||
|
The transmission of sapphire is limited primarily by losses caused by surface reflections. The high index of sapphire
makes magnesium fluoride almost an ideal single-layer antireflection coating. When a single layer of magnesium
fluoride is deposited on sapphire and optimized for 550 nm, total transmission of a sapphire component can be
kept above 98% throughout the entire visible spectrum. |
||||||||||||||||||||||||||||||
|
Sapphire Constants |
||||||||||||||||||||||||||||||
|
Density: 3.98 g-cm-3 at 25°C Young's Modulus*: 3.7 x1010 dynes/mm2 Poisson's Ratio*: - 0.02 Moh Hardness: 9 (by definition) Specific Heat at 25ºC: 0.18 cal/g-°C Coefficient of Linear Expansion (0°C to 500°C): 7.7 x 10-6/°C Softening Point: 1800°C * Sapphire is anisotropic in many of its properties which require tensor description. These values are averages over many directions.
|
||||||||||||||||||||||||||||||
| Back to Top | Previous Next |
| Optics Guide Copyright 2002 Melles Griot Inc. |