Ade from Optics Warehouse explains Nitrogen Purging and why it is used in the manufacturing process of Optics.
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Nitrogen is used in Rifle Scopes along with many other Optics; Binoculars, Spotting Scopes etc.
Quite simply, this is done to create a positive pressure to remove any ambient air laden moisture/oxygen that is evident in the manufacturing process, which would then become trapped, and be far more susceptible to cause issues such as internal fogging.
Over time, mold would live and build on the internal lenses, along with possible corrosion, when encountering differing temperatures, pressures and conditions whilst in use.
Bushnell state they repeat this process around 24 times on each and every Optic to ensure their products are as 'pure Nitrogen filled' as possible.
You may still experience external fogging on both lenses - this is not a faulty Optic - but simply differences between the temperature of the Optic and the ambient outside temperature, which soon clears.
However, prolonged use in cold climates (where the head is in close proximity to the eye ocular) can cause this to affect the appearance of any Optic e.g. a Hunter who has been physically active walking/etc in high altitude, chilly, conditions and quickly mounting his Optic e.g Rifle Scope for a shot.
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To see our range of Optics [CLICK HERE].
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(Image courtesy of Bushnell.)