30 CFR §18.46

1. (a)Headlights shall be constructed as explosion-proof enclosures.
2. (b)Headlights shall be mounted to provide illumination where it will be most effective. They shall be protected from damage by guarding or location.
3. (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.
4. (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.
5. (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.