Interpretation ID: nht92-3.22

DATE: 10/08/92

FROM: T. KOUCHI, -- DIRECTOR & GENERAL MANAGER, AUTOMOTIVE EQUIPMENT DEVELOPMENT & ADMINISTRATION DEPARTMENT, STANLEY ELECTRIC CO., LTD.

TO: PAUL JACKSON RICE -- CHIEF COUNSEL, NHTSA

TITLE: RE.: PHOTOMETRIC TEST METHOD OF HMSL

ATTACHMT: ATTACHED TO LETTER DATED 12-1-92 FROM PAUL J. RICE TO T. KOUCHI (A40; STD. 108)

TEXT: We Would like to ask you some questions about photometric test method of High-Mounted Stop Lamp (HMSL) using Light Emitting Diodes (LEDs) as light sources.

BACKGROUND: HMSL is required to conform to SAE J186a which is referenced in Table III of FMVSS 108. We know, however, that SAE J1889 has been published as a technical guide to standard practice for LED lighting devices. So, in making photometric measurement of HMSL incorporating LEDs, we usually follow testing method described in Sec. 3.1.5 (Photometry Test) of that SAE document.

PROBLEMS: When we conduct photometric measurements of HMSL incorporating LEDs according to the test method of SAE J1889 as mentioned above, the lamp must be energized, for the measurement of photometric minimums, "until either internal heat buildup saturation has occurred or 30 minutes has elapsed, whichever occurs first" as recommended in Sec. 3.1.5.3. However, since it is rather difficult to determine the exact time of internal heat buildup saturation every time when measuring lamps, we make it a rule to conduct measurement 30 minutes after the lamp energization. We think it is reasonable that the lamp should be energized for stabilization beforehand, because the light output of a lamp changes with time at the beginning of operation due to the unique character of LEDs. But, after energization for such a long time as 30 minutes, the light output decreases by approximately 30% of the initial value depending on the type of lamp.

So we must always allow a margin of the same percentage when designing initial light output of the lamp, which necessitates increase in the number of LEDs used, lamp size, product cost, and, therefore, user's expense.

SOLUTION: To solve the above mentioned problems, and for reasons as will be stated in the following, we consider that the warm-up time of HMSL incorporating LEDs should be 5 minutes for measurement of photometric minimums.

Reasons: 1) The time duration is usually within a minute or 3 minutes at the longest that a driver keeps to step on the brake pedal. We do not have the statistical data about this time duration, but some members of our staff who drive a car everyday gave us similar values.

2) The light output of the lamp does not always stabilize in 5 minutes, but it is not necessary to take a warm-up time above the time duration the lamp is continuously operated in real driving.

3) SAE J575 "Warpage Test on Devices with Plastic Components" prescribes, in Table 1, operating cycles of individual signal lamps during the warpage test, and it specifies 5 on-5 off operation for a stop lamp. This specification of 5-minute energization deserves attention because it seems to be established taking into consideration actual way of usage of stop lamps.

Based on the above explanation, we would like you to revise FMVSS 108 by adding a new provision, in certain place thereof, specifying that HMSL incorporating LEDs shall be energized for 5 minutes before measurement of photometric minimums. Your view on this matter is highly appreciated.

Furthermore, regarding actual steps of photometric measurement, we ask you to permit the following. We would like to have your comment on this matter also.

STEPS: 1. Measure luminous intensity at H-V axis after the lamp was energized for 5 minutes.

2. After the light output stabilized, measure luminous intensities at all test points including H-V axis.

3. From the results of Steps 1 and 2, calculate the decrease rate of the luminous intensity at H-V axis from the time the lamp was energized for 5 minutes to the time the light output stabilized.

4. Multiply the luminous intensities at test points excluding H-V axis obtained at Step 2 by the reciprocal of the decrease rate calculated at Step 3, to estimate the luminous intensities to be measured at respective test points after the lamp was energized for 5 minutes. Then the individual estimated values are deemed the measured photometric values and they are recorded.