Proto Paintball makes a great quality thermal lens for your mask. The Proto Thermal lens is very easy to install and as of 2009 are compatible with all previous models of the Proto Goggle Systems which include the EL, FS, Axis and Axis Pro mask. The Proto Lens currently is available in five different colors as some of the previous colors have been discontinued due to their lack of popularity.
It is always a great idea to have a spare Proto Thermal Lens in your gear bag. You never know when you will have a problem with your stock lens and need to throw in a quick replacement.