Search Interpretations


Mr. Kiminori Hyodo

Deputy General Manager, Regulation & Certification

Koito Manufacturing Co., Ltd.

4-8-3, Takanawa

Minato-Ku Tokyo


Dear Mr. Hyodo:

This responds to your letter, in which you seek clarification as to the definition of the optical axis for a lower beam headlamp using light-emitting diodes (LEDs) under Federal Motor Vehicle Safety Standard (FMVSS) No. 108, Lamps, Reflective Devices, and Associated Equipment. Specifically, you asked how one would determine the optical axis for a LED lower beam headlamp, where respective LEDs provide different light intensities or beam configurations. As discussed below, it is our opinion that the optical axis for a visual/optical aim headlamp is the reference axis necessary to assure proper horizontal and vertical alignment of the optical aiming equipment. It is up to the manufacturer to decide how to determine that axis and to select the location of the required marking. This interpretation would apply to any visually/optically aimed headlamp regardless of light source type.

By way of background, the National Highway Traffic Safety Administration (NHTSA) is authorized to issue FMVSSs that set performance requirements for new motor vehicles and items of motor vehicle equipment (see 49 U.S.C. Chapter 301). NHTSA does not provide approvals of motor vehicles or motor vehicle equipment.  Instead, manufacturers are required to self-certify that their products conform to all applicable safety standards that are in effect on the date of manufacture. FMVSS No. 108 specifies requirements for original and replacement lamps, reflective devices, and associated equipment.  NHTSA selects a sampling of new vehicles and equipment each year to determine their compliance with applicable FMVSSs.  If our testing or examination reveals an apparent noncompliance, we may require the manufacturer to remedy the noncompliance, and may initiate an enforcement proceeding if necessary to ensure that the manufacturer takes appropriate action.

Your letter asked what constitutes the location of the optical axis in a situation in which LEDs of varying intensity are used in a headlamp. You described a situation in which one LED from an array of LEDs serves to provide the dominant intensities toward the center of the beam pattern and determines a major characteristic of a cut-off. You asked if it would be appropriate to use that element to determine the optical axis. You also ask this question assuming a condition where respective LEDs are directed differently to constitute respective parts of the low-beam illumination. These questions caused the agency to closely examine the meaning of optical axis in order to assure proper headlamp aiming.

In your letter, you cited a prior agency interpretation (December 21, 2005 letter to Mr. Takayuki Amma) regarding lower beam headlamps using several LEDs of equal light intensities, and our conclusion that the optical axis shall always correlate to the actual photometric output of the lamp. In view of your latest inquiry, we reexamined our 2005 interpretation relative to the determination of the optical axis. In our 2005 interpretation, we expressed agreement that the optical center would serve as an optical axis of a lower beam headlamp. We also agreed with your recommended approach and said that for LED lower beam headlamps, the optical center should be determined as the geometric center of the portion of the lens that is illuminated by the LED light sources. While we continue to believe this could be a valid approach, manufacturers may choose other methods as well. For example, with LED light sources of varying intensity, a manufacturer could conclude that the geometric center of the illuminated lens might not be accurate for marking the lamp for aiming purposes.

The agency notes that the term optical axis as used in FMVSS No. 108 may be inconsistent with the encyclopedic definition of the phrase. For visually/optically aimed headlamps, the term optical axis, as it is used in Standard No. 108, refers to the reference axis (a.k.a. mechanical axis) of the headlamp. Given this, we have reexamined a second point from the 2005 letter, in which we stated that the center of the emitted light is always taken to be the center of the optical axis. In saying this, we were quoting a January 14, 1976 letter of interpretation to the Department of California Highway Patrol. Upon closer examination, the 1974 letter addressed the optical axis (i.e., the mechanical or reference axis) in turn signals, not headlamps. As turn signals are symmetrical, the center of light emitted should always constitute the reference axis. However, as headlamps are asymmetrical, the quoted portion of the 1974 letter does not apply to headlamps.

Paragraph S7.8.5.3(f) of FMVSS No. 108 requires that a visually/optically aimed headlamp include a mark or markings identifying the optical axis of the headlamp. The location of this mark or markings is to be determined by the headlamp manufacturer. Once chosen, the mark establishes the reference axis that will be used to assure proper horizontal and vertical alignment of the aiming screen or optical aiming equipment with the headlamp being aimed. NHTSA will use this mark to identify the reference axis, and will conduct its compliance testing accordingly.

Finally, we note that this interpretation broadens flexibility for manufacturers under the rule and, as such, does not amount to a change implicating compliance issues for manufacturers currently relying on the opinions in our previous letters.

If you have any further questions, please contact Ari Scott of my staff at (202) 366-2992.


Anthony M. Cooke

Chief Counsel