The physical, chemical and optical properties of
sapphire make it one of the most versatile materials from which various types
of windows can be fabricated. Some
applications, such as precision optical components, however, demand the highest
level of quality while others such as mechanical applications permit use of
less perfect material.
Why
are there different grades?
Natural sapphire is a gemstone variety of the mineral corundum, or
aluminum oxide (Al2O3), commonly referred to as alumina
(α-alumina) or aloxide, one of nature's most abundant compounds.
How
sapphire is graded?
Sapphire quality is
graded based on its optical and physical properties. There is no single
globally accepted grading system that is used by all manufacturers of synthetic
sapphire.
In
another commonly used grading system for sapphire, shown below, grades 1
through 4 are considered to be of optical quality while grades 5 and 6 are
considered technical quality.
- Grade 1:Free
of insertions, block boundaries, twins, micro-bubbles and scattering
centers.
- Grade 2:
Free of insertions, block boundaries and twins. Individual scattering
centers (micro-bubbles < 10 µm located no closer to each other than 10
mm) are allowed.
- Grade 3:
Free of insertions, block boundaries and twins. Individual bubbles < 20 µm located no
closer to each other than 10 mm are allowed.
- Grade 4:Free
of insertions, block boundaries and twins. Bubbles < 20 µm located no
closer to each other than 2 mm, well as bubble clusters (which may include
individual bubbles to 50 µm) < 200 µm scattered no closer to each other
than 10 mm within an effective volume of 20x20x20 mm are allowed.
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