Interpretation ID: aiam2455
Executive Vice President
Wesbar Corporation
Box 577
West Bend
WI
53095;
Dear Mr. Weber: This is in reply to your letter of October 28, 1976, asking severa questions with respect to that portion of paragraph S4.4.1 of Motor Vehicle Safety Standard No. 108 which prohibits the optical combination of clearance and tail lamps. You have also expressed your concern, in a recent telephone conversation with Mr. Vinson of my staff, about the ACUTEK interpretation of October 22, 1976, to be discussed below.; The distinction between the hypothetical lamp in Question 2 of you letter of September 7, 1976, and the Acutek lamp is that there is no opaque barrier wall in the former, separating the tail lamp bulb and the clearance lamp bulb, while in the latter the barrier rises to the base of the bulb. Since Standard No. 108 does not require separate compartments (i.e., an opaque barrier) for tail lamps and clearance lamps, it is obvious that the prohibition against optical combination means that (a) a single bulb may not perform both functions and (b) a single bulb must not be perceived as performing both functions. This was the rationale behind Mr. Driver's advice to Acutek that the available data indicated 'that when the tail lamp bulb [on the Acutek lamp] is activated independently from the clearance lamp bulb, and vice versa, there is no appreciable amount of incidental light emitted from the lens of the clearance lamp,' and that 'the amount of light 'spill' appears to be so small that it would not be interpreted (by a driver following the vehicle on which it is installed) as illuminating the lens of the tail lamp when operated in the clearance lamp mode, and vice versa.'; If you apply this general principle to the questions you asked then think you will have the answers. The principle is necessarily dependent upon the candlepower output of any lamp to which it is applied, a value not given in your questions. Thus, the principle cannot be quantified and the determination of the extent of light spill is necessarily subjective, and certification is dependent upon a manufacturer's good faith in attempting to achieve compliance.; After reviewing this matter I must admit that I am curious as to th safety rationale behind the prohibition. Paragraph S4.4.1 had its genesis in a similar provision in Bureau of Motor Carrier Safety regulations (49 CFR 393.22(b)(3), formerly 393.22(c)) and was adopted in conformance with it. The Society of Automotive Engineers, however, does not prohibit combining these lamps. If clearance lamps are mounted below 72 inches--the maximum allowable mounting height for tail lamps--it may be that they could be combined with tail lamps, without any detriment to safety, and at a saving to the consumer. Perhaps you would like to comment on this.; Sincerely, Frank A. Berndt, Acting Chief Counsel