Interpretation ID: 7252
Chairman, TSEI Engineering Committee (Lights)
Transportation Safety Equipment Institute
P.O. Box 1638
Englewood Cliffs, NJ 07632-0638
Dear Mr. Murphy:
This responds to your letter of April 30. 1992, requesting an interpretation of Motor Vehicle Safety Standard No. 108. You have asked "whether the lens leg of various lamp assemblies may be included in the calculation" of the minimum effective projected luminous lens area required of certain lamps by Standard No. 108.
Specifically, you have enclosed "Figure 1" which "shows that the last optic against the lens leg projects light outward beyond the lens leg and yet the light may be beneficial to meeting the twenty degree outward test points for stop, tail, turn lamps." Accordingly you have concluded "that this light, though low in intensity due to its distance from the filament, may be significant as far as meeting the photometric requirements of the lamp."
NHTSA adopted a definition of "effective projected luminous lens area" on May 15, 1990 (55 FR 20158), to mean "that area of the projection on a plane perpendicular to the lamp axis of that portion of the light-emitting surface that directs light to the photometric test pattern. . . ." No exceptions were made to the definition. In rejecting a petition for reconsideration to include lens parts, such as the rim (or leg), in the calculation of lens area in those instances where the rim transmits unobstructed light, NHTSA explained on December 5, 1990 (55 FR 50182), that areas that do not contribute "significantly" to light output should not be included in the lens area calculation. It commented that "the optical parts of the reflector and lens are designed to achieve that purpose", and that "lens rims or legs do not contribute to the optical design" but instead "take up surface area that can reduce the area of the optically designed part of the lens if they are allowed to be included in the computation of minimum lens area."
In the comments that both you and we have quoted above, NHTSA has tried to differentiate between optical parts that are specifically designed to contribute to the optical design of a lamp and those whose contribution is only incidental and secondary. Those comments express clearly the agency's opinion that a lens leg, such as shown in your Figure 1, is an optical part that contributes only incidentally to the optical design of a lamp. However, the agency's opinions, as expressed in the preambles on this subject, are not the most definitive answer to your question. Instead, with reference to Figure 1, whether the additional lens area may be included in the computation of the minimum effective projected luminous lens area is determined by the definition of that term set forth in S4 of Standard No. 108. If the lens leg in Figure 1 meets that definition, it may be included in the computation. If not, it may not be included in the computation.
Sincerely,
Paul Jackson Rice Chief Counsel ref.108 d:5/27/92