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A thermal lense is actually two lenses are squished together, but purged with nitrogen in the middle. The nitrogen doesn't have a low specific heat, so it will not transfer temperature as well between the two lenses. The payoff? When you exhale and your warm breath goes up into the mask, it doesn't hit the colder, outside temperature air on the outside of the lense. This means that the colder outside temperature that would exceede the dew point of your warm breath and produce the vapor, doesn't interact with the outside air. Simply put, a thermal lense reduces your chance of "fog".
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