Interpretation ID: nht89-3.7

TYPE: INTERPRETATION-NHTSA

DATE: 09/29/89

FROM: STEPHEN P. WOOD -- ACTING CHIEF COUNSEL NHTSA

TO: SHUICHI WATANABE -- MANAGER, AUTOMOTIVE LIGHTING HOMOLOGATION SECT. STANLEY ELECTRIC CO., LTD. JAPAN

TITLE: NONE

ATTACHMT: LETTER DATED 3/27/89 FROM SHUICHI WATANABE OF STANLEY ELECTRIC CO OF JAPAN TO ERIKA JONES OF NHTSA RE: MEASUREMENT OF INCIDENT LIGHT ANGLE

TEXT: Dear Mr. Watanabe:

This is in reply to your letter with respect to measurement of the incident light angle prescribed by Motor Vehicle Safety Standard No. 108 for license plate lamps. You have submitted six Figures for our consideration and reply. I regret the delay in r esponding.

Paragraph 6.5 (not 5.3.3) of SAE Standard J587 OCT81 License Plate Lamps (Rear Registration Plate Lamps) states:

"When a single lamp is used to illuminate the plate, the lamp and license plate holder shall bear such relation to each other that at no point on the plate will the incident light make an angle of less than 8 deg. to the plane of the plate, this angle be ing measured from the edge of the light emitting surface of the device farthest from the surface of the plate (see Fig. 3)."

With respect to your Figure A, you comment that, in order to determine the farthest point on the light emitting surface, "only the distance '1' between license plate and a plane runs parallel to it should be considered and not by distance 'm' nor 'n'".

In examining Figure 3 of SAE J587, you will note that the incident angle is measured by a line that extends from the edge of the light emitting surface of the lamp that is the farthest from the most distant point on the plate that the light can be expect ed to reach, to that point. With respect to your Figure A, the line "n" that extends from the edge of the lamp "p" to the bottom of the plate replicates the line shown in Figure 3, and thus is the correct one, not the perpendicular line "1".

Similarly, with respect to Figure B, a round license plate lamp, a line must be drawn from the bottom of the plate (such as line "n" in Figure A) to the farthest light emitting surface surface of the lamp (as shown in Figure 3, not your Figure B) in orde r to illustrate the farthest point.

With respect to Figures C and D, you have asked: "But if the farthest point can exist so may on a line or a plane . . . how could it be determined? Should it be for instance, left end, right end or center of them?" Figure C appears to depict an elongate d lamp in which "p"s are depicted at the left end, right end, and center. In this instance, "P2" is the edge of the light farthest from the surface of the plate, as represented by corner "Q". Though "Po" represents an edge of the lamp, at no place on t he plate is it the point "farthest from the surface of the plate". Thus, the measurement from "Q" to the tangent of the light emitting surface near "P2" is the line to be used to measure the 8 degree minimum angle.

Figure D appears also to represent an elongated lamp but one in which the lens area is directed more towards the plate. In this Figure, "P" is the edge of the lamp that is farthest from the plate. But because of the configuration of the lamp, light fro m this point is interrupted by a part other than a lens and cannot shine directly upon the plate. Your points "P1", "P0", and "P2" appear to indicate the points on the lamp surface where such interruption ceases and light shines directly upon the plate. Therefore, it is our opinion that in this instance "P2" is the edge of the lamp farthest from the plate, as represented by corner "Q".

You have also asked the same question with respect to Figures E and F. These Figures represent lamp designs with two light sources. However, in Figure E, the two light sources appear incorporated in a single lamp, and the requirements of paragraph 6.5 still apply. Thus, a line from Q, tangent to the light emitting surface of the lens near the point "P3" (and not your line "Q-"P3"), is the correct reference for angle measurement. Figure F, however, depicts the light sources as compartmentalized, and thus may be regarded as a two-lamp device. Paragraph 6.6 of SAE J587 establishes the requirement for two or more license plate lamps. It states:

"When two or more lamps are used to illuminate the plate, the minimum 8 deg incident light angle shall apply only to that portion of the plate which the particular lamp is designed to illuminate. The incident light angle shall be measured in the same wa y as provided in paragraph 6.5."

Under these circumstances the light emitting edge of the lamp farthest from the surface of the plate is the furthest edge of the lamp illuminating that portion of the plate, here represented by "P2". Thus, the angle to be measured would be determined wi th reference to a tangent line to the light emitting surface near "P2", from corner "Q".

I hope that this has answered your questions.

Sincerely,