The first gas masks mostly used circular lenses made either of glass, mica or cellulose acetate. With the exception of the latter, these materials were quite brittle and needed frequent replacement. Later on, the Triplex lense style (two layers of glass and one layer of cellulose acetate in between) became more popular, and alongside the simpler cellulose acetate they became the standard into the 1930s. Panoramic lenses were not popular until the 1930s, but there are some examples of those being used even during the war (Austro-Hungarian 15M). Later, polycarbonate started being used for its strength.