A general perception in the industry is that the normal expected lifetime of LEDs is 100,000 hours of operation. This perception, while not always true, comes from years of testing of LEDs under controlled laboratory conditions, then utilizing the data collected to make a lifetime projection. In real world applications the primary factors that affect the operating life of the LEDS are temperature and drive current.  

 

There are two temperature factors to consider one being the ambient temperature that the product is expected to experience, the other is the LED operating temperature. The operating temperature is derived from the ambient temperature and LED self-heating based on the current at which the LED is being driven. Manufacturers in most instances have little control over the operating ambient; however they must be aware of the expectations.  Manufacturers do have full control, while designing a product, over the current level that the LEDS will be driven, the heat generation as a result, and the thermal management design of the product.

 

LED manufacturers now have several industry standard procedures available for testing LEDs that provides information that allows lumen maintenance or useful lifetime projections of the LEDs in an application. These procedures, developed under the guidance of the Illuminating Engineering Society of North America (IESNA), include IESNA LM-80, LM-79, and TM-21 reports.

 

The LM-80 report is generated by the discrete LED manufacturer where large samples of LEDs are operated under controlled ambient temperatures and controlled LED temperatures and currents. Light output data is collected at defined intervals to determine any light output depreciation. The LM-80 standard requires that sample sets be operated at several different temperatures and several different drive currents. The information from these data sets is important to make useful fixture life calculation.

 

The next report, LM-79, is generated by the actual operation of an LED fixture under controlled conditions. . LM-79 calls out appropriate procedures for “Electrical and Photometric Measurements of Solid-State Lighting Products”. It prescribes methodologies for the measurement of total flux, light distribution, electrical power, and color. When LM-79 testing is combined with UL /ANSI 1598:2008 In-Situ (ISTMT) temperature measurements, the LED drive current and package temperature are measured.  Dialight, under lab code 201033-0, is able to generate NVLAP- accredited LM-79 photometric reports that help validate the performance of its innovative LED lighting solutions. This represents the highest standard of accreditation for LED luminaire testing. Dialight is also able to generate accredited in-situ (ISTMT) temperature measurements necessary for validating its industry-leading performance warranties.

 

Lastly is the TM-21 report. The TM 21 calculation takes the light output verses temperature and drive current data from the LM-80 report and combines this information with the LED drive current and In-Situ temperature information from the LM-79 report to project lumen depreciation for the LED assembly. An end result of the TM-21 report is the projected L70 point of the LED assembly. L70 is defined as the point at which the LED output has depreciated to 70% of the initial output and may be considered the LED end of useful life.

 

Dialight has designed the new long life LED Traffic Signal devices to far outperform any current LED traffic signal assembly offered in the market today. This new product is offered with a 15 year warranty and a 20 year expected life. To achieve this, the selection of LEDs, the operating current of the LEDs and the LED heat-sinking played a vital role in the overall design. Manufacturers of LEDs are now producing high power LEDs that are capable of 1000 mAmps (mA) operation for extended periods of time. With the optimal LED configuration design, Dialight’s ITE compliant LED traffic signal modules are designed with LEDs operating at less than half of those currents. Combine this lower operating current with enhanced heat-sinking of the LED printed circuit board and the result is a very low temperature rise by self-heating.

 

The standard ITE operating temperature for testing purposes is in a split chamber where the temperature in front of the signal lens in 49o C and the temperature on the rear of the module is 74 o C. For this evaluation Dialight tested the yellow long life LED traffic Signal Head at the prescribed ITE temperatures. The yellow traffic ball was selected for test since it is operated at the highest LED drive current, resulting in the “worst case condition” for lumen depreciation. After thermal stabilization the measured LED source temperature (Ts) was 101 o C.This was the In-Situ temperature used for the TM-21 calculation. Since the LM-80 reports from the LED manufactures cannot test for all potential drive currents, TM-21 requires that the next higher drive current, which in this case is 500 mA, be used from the LM-80 reports for the calculation. The chart below graphically shows the output from the TM-21 calculator for the projected life of the Integrated LED traffic Head.