Cal. Code Regs. tit. 13, § 741
Fog taillamps shall not be optically combined with any lighting function other than a tail lamp or reflex reflector. The projected luminous lens area in the H-V direction shall not exceed 140 cm2 (21.7 in.2).
Note: Authority cited: Section 26103, Vehicle Code. Reference: Sections 24012 and 26103, Vehicle Code.