California Code of Regulations
Title 13 - Motor Vehicles
Division 2 - Department of the California Highway Patrol
Chapter 2 - Lighting Equipment
Article 3 - Construction Requirements
Section 641 - Optic Indexing

Lenses, interior reflectors, removable sockets, double-filament bulbs, and sealed and semisealed optical units shall be indexed into adjacent components to prevent rotation and misinstallation. Housings for sealed and semisealed optical units designed for mounting in either of two positions rotated 180 deg apart (such as fog lamps above or below a bumper) shall be indexed so the optical units are in their design position when the housing is mounted in either of the two positions. Indexing is not required for lenses that are symmetrical about the H-V axis.

Note: Authority cited: Section 26103, Vehicle Code. Reference: Sections 24012 and 26103, Vehicle Code.