California Code, Vehicle Code - VEH § 26101

(a) A person shall not sell or offer for sale for use upon or as part of the equipment of a vehicle any device that is intended to modify the original design or performance of any lighting equipment, safety glazing material, or other device, unless the modifying device meets the provisions of Section 26104.

(b) A person shall not use upon a vehicle, and a person shall not drive a vehicle upon a highway that has installed a device that is intended to modify the original design or performance of a lighting, safety glazing material, or other device, unless the modifying device complies with Section 26104.

(c) This section does not apply to a taillamp or stop lamp in use on or prior to December 1, 1935, or to lamps installed on authorized emergency vehicles.