Nitrogen filled night vision devices
— filling the housing with a special inert gas with a minimum of water vapor has two main objectives. Firstly, such an environment is as chemically inert as possible, which prevents corrosion of sensitive parts. Secondly, zero humidity means that the lenses will not fog up from the inside during temperature changes. In addition, this feature is directly related to dust and moisture resistance: gas filling implies a sealed enclosure, which by definition provides some degree of protection against dust and moisture.
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