Interpretation ID: nht90-4.87

TYPE: Interpretation-NHTSA

DATE: December 17, 1990

FROM: Paul Jackson Rice -- Chief Counsel, NHTSA

TO: M. Iwase -- General Manager, Technical Administration Department, Koito Manufacturing Company

TITLE: None

ATTACHMT: Attached to letter dated 11-13-90 to Paul Jackson Rice from M. Iwase

TEXT:

This is in reply to your letter of November 13, 1990, with respect to photometric measurement procedures for a L.E.D. center high-mounted stop lamp (CHMSL).

According to your letter, "photometric output of L.E.D. lighting device decreases as the time passes after activation, as shown in Figure 1." Your Figure 1 demonstrates that L.E.D.s at five minutes have a relative photometric output of slightly more than 70 percent of the output when the lamp is activated. After 20 minutes, it would appear to be only slightly more than 60 percent of initial output. You have asked at what time after activation of the CHMSL its photometrics are to be measured; in your opinion, five minutes is a reasonable time. You support your opinion with three arguments, which I will not summarize here since they are not relevant to my response.

Standard No. 108 does not specify when the CHMSL photometric test is to occur. Neither SAE Recommended Practice J186a, Supplemental High Mounted Stop and Rear Turn Signal Lamps, January 1977, the applicable standard that is incorporated by reference, nor any other section of Standard No. 108, requires the photometric test to be conducted at any particular time. The standard does require that CHMSLs and other lamps be designed to conform to its requirements; therefore, we expect the CHMSL to meet the minimum photometric specifications at whatever point in time it is tested after its activation. If Koito wishes to test at five minutes after activation, it may do so.

The purpose of the CHMSL is to reduce the frequency and severity of rear end collisions. Thus, its initial activation is the one that is most critical to highway safety. Although the short survey of continuous brake application times that Koito conducted, and which is referenced in your letter, showed one continuous brake application that exceeded four minutes, at such a period in time the warning message of the light would have been long delivered to the driver following. Thus, the fact that the L.E.D. diminishes over a continuous period of time would not appear to affect its purpose, as long as the CHMSL conforms to the minimum photometric requirements upon each application of the brake pedal, no matter how long the previous brake application and no matter how short the interval between brake applications.

I hope that this answers your question.