Interpretation ID: nht76-2.46

DATE: 01/14/76

FROM: AUTHOR UNAVAILABLE; R. B. Dyson; NHTSA

TO: Department of California Highway Patrol

TITLE: FMVSS INTERPRETATION

TEXT: This is in response to your letter of December 8, 1975, asking four questions, the answers to which would provide an interpretation of Standard No. 108 with respect to separation distance of a turn signal lamp from the nearest edge of a Type 2 headlamp.

SAE Standard J588d, Turn Signal Lamps, June 1966, incorporated by reference in Standard No. 108, requires in pertinent part that "The optical axis (filament center) of the front turn signal lamp shall be at least 4 inches from the inside diameter of the retaining ring of the headlamp unit providing the lower beam . . . ." We agree with your opinion that the reference to filament center may have been added because of the difficulty of determining the location of the optical axis in certain instances. In the vast majority of cases, however the filament center is on the optical axis, and the addition of the provision assists in determining compliance with the requirement.

You have asked:

"1. Is the filament center always to be taken as the center of the optical axis?"

The answer to this question is no. In some instances the filament center will not be on the optical axis. When this is the case the standard is ambiguous as to whether distance is measured from the optical axis or the filament center. While we prefer the optical axis, under the present wording either must be viewed as legally supportable.

"2. Is the center of the emitted light always to be taken as the center of the optical axis?"

The answer is yes.

"3. If the answers to the above two questions are no, does the vehicle manufacturer have the choice as to which method is most favorable to him?"

Yes, because of the ambiguity the manufacturer may choose either the optical axis or filament center as the point of measurement.

"4. What is the optical axis of a two- or three- compartment lamp?"

The optical axis of a multi-compartment lamp is the center of the light emitted by the array, treated as a single complex light source. The "half-value" method you described in your letter is a valid method of finding the optical axis of a complex light source as well as that of a simple one.

Finally you have asked whether, if we agree with the need for clarification, the letter can be considered a petition for rulemaking or whether a formal petition should be submitted.

We agree that clarification is needed and accordingly plan to issue a notice of proposed rulemaking in the near future.

Yours truly,

ATTACH.

December 8, 1975

Frank Berndt -- Chief Counsel, National Highway Traffic Safety Administration

Dear Mr. Berndt:

When inspecting 1976 passenger cars, we discovered a problem in measuring the distance of a front turn signal lamp from the nearest edge of the Type 2 headlamp. Federal Standard No. 108 requires that turn signal lamps meet the 4-inch minimum spacing in SAE J588d. Standard No. 108 also permits lamps to be mounted closer than the 4-inch interval if they emit 2-1/2 times the minimum candlepower otherwise specified.

SAE J588d clearly states that the distance shall be measured from the optical axis of the turn signal lamp to the inside diameter of the retaining ring from the headlamp providing the low beam. It then, unfortunately, makes the requirement ambiguous by a parenthetical reference to the filament center.

The SAE wording was satisfactory when it was adopted a number of years ago, because lamp designs then had the optical axis coincident with the filament. More recent designs have kept the filament 4 inches from the headlamp but have used the ambiguity as a loophole to allow the optical axis to be unreasonably close to the headlamp.

The 4-inch separation was adopted by SAE after a number of complaints about the lack of effectiveness of some turn signals that were snuggled up against the headlamps. The brightness of the adjacent low beam headlamps washed out the turn signals so they would not attract an oncoming driver's attention unless he was looking almost directly at them. The SAE Lighting Committee made nighttime demonstrations of turn signals at various distances from the headlamps in view of a proposal that the edges of the lamps be separated by a minimum distance such as 2 or 2 inches. A jury-type judgment indicated that the present requirement was barely acceptable usually and would allow vehicle manufacturers sufficient design freedom in placing the lamps on vehicles.

The attached drawing illustrates the absurdity of the "filament center" interpretation for modern-day turn signals (and incidentally the skill and ingenuity of lamp designers). Figure I shows a current lamp with a filament center meeting the 4-inch requirement but with an optical center much closer to the headlamp. Figure II illustrates a left-hand version of the same lamp with a filament center that does not meet the 4-inch requirement but with an optical center farther removed from the headlamp. The second lamp provides a more effective signal from an opposing driver's viewpoint, but it would be illegal if measured from the filament center.

The filament center reference apparently was added to the SAE standard because of an assumed difficulty in determining the location of the optical axis. An axis of any object usually passes through a point of symmetry. In the case of a symmetrical light beam meeting turn signal photometric requirements, the optical axis falls in a plane on either side of which is one-half of the total light output. The optical axis is easily located by measuring the intensity of the lamp at HV and then sliding an opaque card with a straight edge across the face of the lens until the photometer reading is one-half the HV value.

In view of the foregoing discussion, we would appreciate your interpretation of Standard No. 108 with respect to the following questions:

1. Is the filament center always to be taken as the center of the optical axis?

2. Is the center of the emitted light always to be taken as the center of the optical axis?

3. If the answers to the above two questions are no, does the vehicle manufacturer have the choice of which method is most favorable to him?

4. What is the optical axis of a two- or three-compartment lamp?

If you agree with the need for clarification, can this letter be considered a basis for your initiating a proposed change in Standard No. 108 or must this Department submit a formal petition for a rulemaking?

Very truly yours,

WARREN M. HEATH -- Commander, Enforcement Services Division

Enclosure

cc: Lou Owen, NHTSA; Francis Armstrong, NHTSA

(Graphics omitted)