Thermal paintball lenses are a type of protective mask lens that helps reduce fogging while playing paintball. They are made of two separate lenses that are separated by a foam gasket around the edge of the inner lens. The outer lens is the same thickness as a regular lens, while the inner lens is thinner and lighter. When there is a big temperature difference, like bringing an object from the cold outside into a warm room, moisture in the air will condense on the cooler object. This is why your paintball lens can fog - the warm, moist air from your head and breathing contacts the cooler lens, causing it to fog up.
Thermal lenses combat this by creating a layer of air between the lenses. This acts as insulation, allowing the inner lens to warm up to the temperature inside the mask without coming into contact with the cooler outer lens. This works much more effectively and for a longer period of time than single-lens masks with an anti-fog coating. However, in very high humidity, a thermal lens may still have some fogging issues. In these cases, using a paintball goggle fan in combination with a thermal lens is the best option for a clear view.