Interpretation ID: aiam2516
Director of Engineering
Trailer Manufacturers Association
401 North Michigan Avenue
Chicago
IL 60611;
Dear Mr. Reed: This is in reply to your letter of December 20, 1976 asking for clarification of the statement in my letter to you dated December 3, 1976, that 'the light emitted by one bulb must not be perceived as performing the function of the other in addition to its design function.'; It is evident from your letter and others that our previou interpretations of the term 'optical combination' have been found to be ambiguous and lacking in the objective criteria that a Federal motor vehicle safety standard must provide. We have reviewed the matter, and now wish to modify our previous interpretation. In our view a lamp is 'optically combined' when the same light source (i.e. bulb) and the same lens area fulfill two or more functions (*e.g.* taillamp and stop lamp, clearance lamp and turn signal lamp). A dual filament bulb would be regarded as the 'same light source'. In determining conformance, the photometric requirements for clearance and taillamp functions, where two bulbs are located in a single compartment, must be met with only the bulb energized that is designed to perform the specific function. But the 15 candlepower maximum under Standard No. 108, however, would be determined with both the taillamp and clearance lamp bulb energized. Further, the lamp must be located to meet requirements for both clearance and taillamps. Our re- interpretation means that the issue of light spill-over from one area of the lamp to another is irrelevant to conformance.; Sincerely, Frank Berndt, Acting Chief Counsel