30 CFR § 18.46
Headlights.
(a) Headlights shall be constructed as explosion-proof
enclosures.
(b) Headlights shall be mounted to provide illumination where it will
be most effective. They shall be protected from damage by guarding or
location.
(c) Lenses for headlights shall be glass or other suitable material
with physical characteristics equivalent to 1/2-inch thick tempered glass,
such as "Pyrex." Lenses shall meet the requirements of the tests prescribed
in §18.66.
(d) Lenses permanently fixed in a ring with lead, epoxy, or
equivalent will be acceptable provided only lens assemblies meeting the
original manufacturer's specifications are used as replacements.
(e) If a single lead gasket is used, the contact surface of the
opposite side of the lens shall be plane within a maximum deviation of 0.002
inch.