Optical products are often filled with a dry gas to prevent the condensation on the inside of the housing when exposing them to temperature extremes. If there is even a slight sign of air inside, there is a certain % of moisture present.
Usually, they’re filled with either argon or nitrogen gas, which have the same effect – to prevent the moisture and internal fogging without affecting the optical properties.
In addition, these gases also prevent the formation of fungus which would destroy the optics. Internal dewing was the biggest problem in older binoculars when exposed to lower temperatures because they weren’t watertight and contained air. Newer binoculars are therefore all airtight and filled with dry nitrogen or argon.
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