Summary Description

Al₂O₃ is a relatively low refractive index material (1.760 @ 0.6 microns) with excellent durability and temperature range. It is non-hygroscopic with excellent abrasion and chemical resistance. Because of its hardness it is difficult to polish flat to interferometric specifications but is easily polished to window quality. It is on the GRAS list and is an excellent choice as a window for near-IR measurements on process streams. Al₂O₃ is bi-refringent and all applications should be carefully evaluated for the effects of bi-refringence on the intended measurement.

Advantages

1. Excellent durability
2. Excellent chemical resistance

Disadvantages

1. Bi-refringence
2. Cost for large elements (>3 cm)

Physical Data

Melting Point: 2040 °C
Density: 4.0 g/cm³
Solubility in H₂O: Insoluble
Hardness: 1370 kg/mm²
Appearance: Clear

Refractive Index¹

Specific index listed; Generic: 1.760 @ 0.6 microns

Spectral Range

Short Wavelength Limit: 40,000 cm^-1 (1 mm)
Long Wavelength Limit: 1608 cm^-1 (1 mm), 1834 cm^-1 (4 mm)

Coatings

Al₂O₃ detector windows are often coated with a BBAR (broad band anti-reflective) coating covering the 2–6 micron spectral region. When used as sampling windows they are usually not coated since the coating is much less durable than the uncoated window.

Typical Uses

• IR detector windows (especially InSb detectors)
  Liquid cell windows (particularly near-IR measurements on process streams)
  Infrared optical applications represent a small portion of the market for sapphire

Notes

Short and Long Wavelength Ranges for which transmissivity is greater than 50% of stated crystal thickness

1. D.A. Burch; Journal of the Optical Society of America. Vol. 55, p 625, 1965