The use of sapphire material for optical sensing, emitting or detecting has been increasing considerably in the last 20 years, hence the term "window". Sapphire offers a high-quality optical surface finish, the hardness to prevent scratches, and high temperature processing capabilities. Although the index of light transmission is relatively low in comparison to other materials, this can be compensated by the materials used on the sensor, emitter, and anti-reflection coatings on one or two surfaces of the window.
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