Significant improvement in the output of white LEDs opens the opportunity for these devices to be used for forward lighting applications. The Lighting Coordinating Committee formed the LED RID Task Force in early 2002 to address the application of white LEDs in forward lighting applications. Membership included lamp makers, bulb makers, vehicle manufacturers, LED manufacturers and other interested parties. This document, in many ways, is the companion to SAE J1889 that was developed for signal lamp applications.
The scope of the LED RID standard includes all forward lighting devices on a vehicle including headlamps, fog lamps, cornering lamps, and auxiliary high beam headlamps. These devices can use either replaceable or non-replaceable LED modules.
Similar to SAE J1889, each of the forward lighting devices are required to meet the mechanical, photometric and environmental specifications specific for that device along with additional requirements necessary to insure that LEDs and related electronics perform as expected. SAE J575 tests and requirements are used extensively in this standard with some variations to deal with the unique characteristics of LED devices.
As with LED signal and marking lamps, the LED forward illumination device’s temperature may influence the output of the lamp. A specific test was devised to evaluate the performance of the lamp after one minute of operation and again after the lamp output stabilized. Many alternative methods were considered before finalizing the method now detailed in the standard. Each device will be evaluated at a single test point to determine the change in output as well as the time that stabilization of the beam occurred. This was done to keep the test time short and preserve the accuracy required to determine the final output of the device. If all of the test points had to be tested at one minute, it could require several minutes to make the photometric scan versus the few seconds to test the one point.
Life testing was a concern due to the unique nature of LEDs and LED modules. The Task Group needed to establish the criteria for end-of-life. Would the outage of one LED constitute the end-of-life or the point at which the device no longer meets photometric requirements? It was decided that the outage of either an LED or an LED module would constitute end-of-life. LED sources are expected to be very long life sources and any outage of a single LED will result in a significant reduction in lamp output and would certainly be the case if a module became non-functional.
A tremendous amount of life data is available for halogen headlamp bulbs over the years and the outage modes are well understood. Although LEDs have been used in automotive applications for several years, they have not been used for road illumination. It is expected that vehicle use data will be required to understand the outage modes for high power white LEDs. An additional complication of LED systems over bulb systems is in the number of LEDs required to create the beam pattern, the electronic control circuitry necessary to provide the correct power to the LEDs, and the interconnects required to power each LED. Additional complexity of LED headlamps may generate a higher potential for outage or other functional issues.
As more testing is conducted and designs reach production, this area will require modification to include practical experience. It was also agreed that the requirements for life should fall into the “guidelines” section of the document similar to that in SAE J1383.
LED color and color-shift over time were key areas of discussion. The white LEDs expected to be used for forward lighting applications behave differently over time than halogen and HID sources. They also possess different spectral power distributions compared with halogen and HID. Color changes with temperature and time are experienced with LEDs although testing to date has shown that the changes are within the CIE white boundaries. To deal with the spectral characteristics of LEDs, it was decided that LEDs should have the same “red content” requirements as HID bulbs. As the lighting industry develops appropriate spectral content requirements for other colors, these will be added to the standard.
Thermal shock requirements of LED sources were other major topics of discussion. This was based on the mechanical interconnects associated with the LEDs and related electronic control module. It was decided to remove specific requirements for thermal shock, as this test was in some ways redundant to the thermal cycle test and warpage test already required by most device standards. Application specific tests may be required.
The discomfort glare concerns for LED lamps are derived from the very high color temperature possible with these sources. Research has shown that these effects are relatively small. LEDs may be manufactured with color temperatures in excess of 7000 Kelvin. Since there is already great concern with the color temperature of HID light sources, a much higher value may cause even greater concerns. Although the concern was high, it was not possible to determine what maximum color temperature was acceptable. Additional research is necessary to establish the need for a requirement and if there are other factors that relate to this issue such as lamp size and optical system.
The test section on Lumen Maintenance was brought directly from the latest balloted version of SAE J2009. The test for discharge sources requires that the luminous flux maintenance shall be measured at 75% of design life. The requirement section for these sources is that the lumen output shall be a minimum of 60% of the original measurement. LED white light sources will typically exceed this value and it was decided that the lumen output requirement should be higher with a value of 80% of initial measurement.