(a) MSHA may waive testing of materials for windows or lenses except
headlight lenses. When tested, material for windows or lenses shall meet the
test requirements prescribed in §18.66 and shall be
sealed in place or provided with flange joints in accordance with
(b) Windows or lenses shall be protected from mechanical damage by
structural design, location, or guarding. Windows or lenses, other than
headlight lenses, having an exposed area greater than 8 square inches, shall
be provided with guarding or equivalent.