LED automotive lighting applications require unique SAE standards (MAGAZINE)
Jianzhong Jiao summarizes the broad landscape of standards for LED lighting specific to automotive applications, which includes unique characteristics in terms of lumen maintenance, reliability, lifetime, and color spectrum.
JIANHZHONG JIAO summarizes the broad landscape of standards for LED lighting specific to automotive applications, which includes unique characteristics in terms of lumen maintenance, reliability, lifetime, and color spectrum.
The development of AlInGaP (aluminum gallium indium phosphide) materials technology in the early 1990s led to higher efficiency and better reliability in yellow-to-red light-emitting diodes (LEDs). Around the same time, LED technology for various illumination applications was on the rise. Automotive exterior lighting was one of the first such applications starting with center high-mounted stop lamps (CHMSLs). In the late 1980s, the SAE (Society of Automotive Engineers) International Lighting Committee started to develop LED automotive lighting standards and their work continues today. Most automotive exterior lighting lamps are considered safety devices, and as such, these lamps are regulated at a federal or state level. Due to the nature of safety requirements, the introduction of LEDs brought together several different perspectives that became the central focus of LED lighting standard development with the SAE Lighting Committee.
SAE LED lighting standards have two categories: component or packaged LED-level (LED package) standards and system-level standards including the entire lamp, i.e., a signaling lamp or headlamp. (Note that in the auto industry the system-level lighting product is referred to as a lamp as opposed to a luminaire). The system-level standards only address lamp testing and performance when LED sources are used. Other source requirements are not included.
Auto signaling applications
The first established LED automotive lighting standard was SAE J1889, a standard for LED signal devices including all signaling and marking lamps such as stop lamps, tail lamps, turn signals, and daytime running lamps (DRLs). This standard has been updated several times over the years.
A key item in SAE J1889 addresses how procedures should differ when testing a signaling lamp using LEDs versus a signaling lamp using incandescent sources, as LED lamps include electronic components while incandescent lamps do not. Since LEDs are a temperature-dependent light source and the light output reduces as the junction temperature increases, the testing requirements must capture this characteristic to ensure the lamp installed on a vehicle fulfills the photometric performance requirements. A photometry test is conducted when the lamp is thermally stabilized.
Furthermore, most automotive lamps have both minimum and maximum photometric level requirements (light intensity values at angular test points). There is an intensity ratio requirement for combined function lamps as well. For example, the intensity ratio between a stop function versus a tail function must have levels distinguishable by the human eye. These functional intensity ratio requirements are captured in the various SAE lamp documents. Also, SAE J1889 addresses the areas of initial light intensity output, stabilized light intensity output, device performance of minimum and maximum values, as well as light intensity ratio requirements.
Forward lighting standards
In the early 2000s, after breakthroughs in white LED technology made LED headlamps possible, the SAE Lighting Committee developed and established an additional LED lighting standard, SAE J2650, which addresses the need for LED forward illumination devices such as headlamps and fog lamps.
The tests to determine maximum and minimum photometry level ratios and thermal stability are similar to those of SAE J1889; however, two additional performance requirements have been added for LED forward lighting devices. The first addition is the red color content test because it is important to effectively render the color red in the roadway environment including obstacles, pedestrians, and signage - most notably the red stop sign.
The light spectrum emitting from headlamps must contain sufficient red color content to render these signs effectively for drivers to properly recognize the signs. Therefore, SAE J2650 includes a requirement for red color content as a percentage of the full visible-light spectral power distribution.
The second addition is the requirement of lumen maintenance over lifetime. Typically, SAE lighting standards do not address product lifetime requirements, but in this case the consideration was based on the fact that LEDs are longer-life light sources. As an LED light output declines over lifetime, the guideline was designed to ensure that LED headlamps maintain a recommended minimum photometric output of 80% of initial light output over the rated lifetime.
In 2007, the SAE Lighting Committee started to develop standards for testing LED luminous flux maintenance. After years of work, SAE J2938 was published in 2012. In most cases for automotive lamps designed with LED sources, the LEDs are often operated at or near the maximum junction temperature allowance.
An LED's operating current is typically designed near its maximum junction temperature. Due to the variability of vehicle operational and environmental conditions, this standard was designed to ensure LED lamps meet minimum photometric requirements even during the worst operational conditions. Thus, for automotive lamps, the LED case temperature is determined for the condition where the junction temperature reaches the maximum or within 10° of the maximum. Even though in reality LEDs may be operated in lower junction temperature regimes (resulting in extended luminous flux maintenance life), the SAE J2938 standard recommends testing that captures the worst case scenario.
Automotive OEMs and lamp manufacturers have set the expectation that LEDs should last the lifetime of the vehicle. Although some automakers may have measured lamp usage over the lifetime of a vehicle for certain vehicle models, the industry did not have a reliable and comprehensive database to set such a lifetime standard. In 2008, the University of Michigan Transportation Research Institute (UMTRI) published two reports that provided an average-based summary on the real-world frequency of use of lighting equipment in passenger vehicles in the United States.
Auto lamp life
The UMTRI reports set the basis and rationale for the testing duration recommended in SAE J2938. For passenger vehicles in the US over the life of the vehicle, the longest operational duration was a tail lamp at slightly less than 2,000 hours. For DRLs, the duration is approximately three times as long as a tail lamp. Therefore, SAE J2938 recommends a minimum of 2,000 hours for the duration of LED lumen maintenance testing. Longer durations can be used for DRL or heavy-duty vehicle lighting applications. If longer-term lumen maintenance projections are needed for these cases, IES TM-21 is recommended.
Also in 2007, the application for LED replaceable light sources became a topic of standard development. For current automotive lamps, most use replaceable light sources (bulbs). At the time that LEDs became bright enough to be used for exterior lighting, one approach was to integrate individual LEDs into a replaceable socket design such that the LED bulb (replaceable light source) could be used in multiple vehicle platforms or as service parts. This offers the possibility of a range of light sources, which then can be used by all device manufacturers - openly traded and manufactured by a variety of suppliers.
SAE traditionally specifies miniature replaceable light sources (bulbs) in SAE J573. Different from the conventional bulbs, LED integrated bulbs may provide additional advantages for light pipe lamps, reflector optic lamps, thin lamp designs, and other advanced optical designs. Based on these practices, the SAE Lighting Committee developed specifications that address optical, mechanical, and other characteristics to standardize these types of bulbs.
The standardized LED replaceable light sources for signal lighting applications can now also be found in SAE J573. As the automotive lighting market is increasingly a global market, the LED replaceable light source technical data and specifications, which are included in SAE J573, are harmonized with the datasheet in UN Regulation 128.
The latest document developed by the SAE Lighting Committee for LED lighting is SAE J3014, which addresses highly accelerated failure testing (HAFT) for LED-containing lamps. HAFT, sometimes referred to as accelerated life testing (ALT) or highly accelerated life testing (HALT), has been an industry practice for a long time in both automotive and general lighting applications. The purpose of HAFT is to identify failure types that provide insight into design faults or the desired rated life of the lighting product.
In LED lighting applications, lumen maintenance alone may not be the definitive measure of reliability. Other types of failures within the LED lamp assembly also need to be identified as quality or reliability concerns. With the range of potential impacts from the environment and usage in automotive applications, the committee identified three major stressors that can be used to induce early failures in LED lamps: temperature, vibration, and input voltage. This test is used to realize the operating and destructive limits of a product when subjected to extreme thermal, electrical, and vibration conditions.
The temperature changes in HAFT testing will produce rapid, uncontrolled humidity swings that will act as a stressor on the lamps being tested. However, humidity is not considered as an independent stressor because it is a function of temperature and the formation of condensation naturally occurring as temperature is lowered. Controlling humidity requires air exchange and conditioning. Controlling humidity during temperature changes is slow and test equipment challenging, and during rapid temperature transitions it is not possible to control humidity. SAE J3014 recommends the analysis of the test results may also help recognize variations and uncover factors that may affect the intended performance. However, the test results do not and should not be used to predict operational life of the product.
A reliable method for projecting LED and LED lighting product long-term reliability is highly desired in all applications and several industry expert groups continue to work toward this objective today. Standards for testing and performance have long been part of the design considerations for LED usage in various applications. Existing standards on discrete silicon-based components were used to qualify LED components, like AEC Q101. The latest activities on dedicated LED component qualification tests at IEC are focused to consider the technology and implemented in IEC 60810. As a pioneer application, LED automotive lighting standards can provide valuable references for developing LED general lighting standards.
In the next issue we will focus on the global view of automotive standards in a follow-up article entitled "New global automotive qualification standards emerge for LED applications."
|DR. JIANZHONG JIAO, director of regulations and emerging technologies at OSRAM Opto Semiconductors, Inc., is an internationally recognized lighting expert. He has been actively involved in LED and SSL standard development activities. He serves as the past chairman of the SAE Lighting Committee, past chairman of NGLIA, past chairman of the NEMA SSL Technical Committee, active member of IESNA Testing Procedure Committee, Roadway Lighting Committee, and Computer Committee, ANSI SSL Working Groups, Standard Technical Panel of UL8750, standard committees in IEEE, CIE USA, SEMI, JEDEC and other organizations. He can be reached at firstname.lastname@example.org.|