- Injecting a gas (usually nitrogen) with a very low moisture content into the body of an optical instrument to create a positive internal pressure. This prevents the entrance of moisture, which could cause fogging of the optics. While the nitrogen has the same index of refraction as the air it replaces, the fact that it is under some level of pressure may cause an index change that must be compensated for.
- Denoting a spherically shaped surface; curved outward.
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