LEDs Come Up Short vs. Lifetime Claims
September 10, 2018
A little more than a year ago, the Daily Mail ran an article claiming that 25% of the expensive long-life bulbs came up short on the lifetime claims—with some even falling below the European Union’s legal minimum life of 6,000 hours. These were bulbs that claimed between 15,000 and 25,000 hours of use. The article discussed tests on five samples of 46 types of bulbs by European partners and consumer watchdogs. The test involved turning bulbs on for 165 minutes and off for 15 minutes until failure. The results of the test:
· Five types stopped working before 6,000 hours
· Five with claims of at least 25,000 hours stopped before 10,000 hours
· 66 out of 230 samples failed before 10,000 hours even though claims were for at least 15,000 hours of use
The U.S. Department of Energy’s websitediscusses tests for a variety of bulb types. For instance, it states that, “CFLs are tested according to LM-65, published by the Illuminating Engineering Society of North America (IESNA). A statistically valid sample of lamps is tested at an ambient temperature of 25° C using an operating cycle of 3 hours on and 20 minutes off. The point at which half the lamps in the sample have failed is the rated average life for that lamp. For 10,000 hour lamps, this process takes about 15 months.”A bulb claiming 50,000 hours would need to be tested for 5.7 years so there seem to be no real sure-fire ways to test the long-term performance/viability of the LED.
Table 1: Light Source vs. Lifetime (L70)
The table above is from a DOE report. And, while the estimated useful life of a high-power LED looks awesome, the source of the data is the lamp manufacturer and some are claiming more than 100,000 hours! According to the DOE, “electrical and thermal design of the LED system or fixture determine how long LEDs will last and how much light they will provide. Driving the LED at higher than rated current will increase relative light output but decrease useful life. Operating the LED at higher than design temperature will also decrease useful life significantly.” But under what real circumstances can an accurate life/performance/cost figure be concluded. There have been rumblings of late regarding the utility of the L70 metric in characterizing the expected life of LED luminaires. The question is whether it might be time for lighting manufacturers to recalibrate performance metrics to align more closely with customer expectations. For those unfamiliar with L70 metric, it is the point in time, expressed in hours, at which a lighting source is expected to provide only 70% of initial lumens. Rather than abruptly going dark, as is the case for incandescent lamps for example, LED lumen output degrades over time, hence the need for the metric.
In ideal operating conditions, LEDs can continue to emit light for many tens of thousands of hours. Because of this the L70 metric is calculated, arrived at via application of a limited set of lumen maintenance test data for the LED packages used in the luminaire (IES LM-80-15, IES Approved Method: Measuring Luminous Flux and Color Maintenance of LED Packages, Arrays and Modules) to an extrapolation protocol (IES TM-21-11
It's important to recognize the parameters outside the manufacturers control that directly impact light levels. The physical environment of a space (e.g., paint and furniture color) can impact lighting levels at the work place as can operation in environments not suitable for the product. Given the above, what can customers do to help ensure that the LED lighting products they are thinking about buying will perform "at a certain level for a certain amount of time"? A white paper recently published by Flex provides some recommendations. Drivers are often the most life-limited component of the lighting system and heat is the biggest contributor to premature failure. So it's important to confirm expected life at the temperature of the intended environment, which can be determined via a graph like this one:
Figure 1: LED Driver IC Lifetime vs. Temperature
Source:Flex Lighting Solutions
Note the dramatic drop-off in expected life at driver case temperatures above 75°C in this typical example. Since case temperature increases with increasing ambient temperature, the customer can expect that the higher the ambient operating temperature, the more often a luminaire driver will need to be replaced. With continual improvements in LED package efficacy as shown in the graph below, luminaire efficacies of 150 lm/W are becoming increasingly common. While not an absolute indicator, lower efficacy products may be indicative of the use of lower quality LED packages or other cost-cutting measures that may impact long-term lumen maintenance.
Figure 2: LED Efficacy Improvements vs. Time
Efficacies of commercial LED packages measured at 25°C and 35A/cm2input current density (DOE Solid-State Lighting, 2017 Suggested Research Topics Supplement: Technology and Market ContextSeptember 2017)
TM-21 is the IES approved method for projecting long-term lumen maintenance of LED light sources. The key metric is the "reported hours" at a specified lumen maintenance level (e.g., L70). The TM-21 methodology takes into account both the drive current applied to the LED packages and the case operating temperature of those packages. Reported hours are limited to six times the total test duration of the LM-80 LED package lumen maintenance data upon which the projection is based (e.g., if the LM-80 test duration is 6000 hours, the reported hours may not exceed 36,000 hours – see table below).
Table 2: Interpolation Report – Lifetime vs. Temperature
Source: Flex Lighting Solutions
In addition to the L70 metric limitations mentioned earlier, the TM-21 methodology does not account for "hard" failures in luminaire circuitry or LEDs that stop emitting light completely, failures of LED drivers as discussed above, or chromaticity shift. An understanding of what information a TM-21 report both can and cannot provide is important as part of a purchase evaluation.
Wade Johnson, Sr. Business Development Manager, North America for Flex Lighting Solutions, said: "The primary function of artificial lighting is to provide humans with the ability to see where they would otherwise be unable. Since LED technology has begun dominating the lighting industry we have lost track of this primary function. We have become enamored with LED’s ability to utilize sensor and IoT technology. The lighting industry’s inability to accurately communicate to the consumer exactly how long they will be able to see in a specific environment lit by LED lighting is a fundamental failure." The LM-80/TM-21 methodology was developed at a time when LED lighting was still relatively new and there was a pressing need for some standardization of how manufacturers communicated expected life to their customers. But as LED technology evolves, improves, and becomes ever more prevalent, perhaps the time has come for the industry to take another look at the way long term performance is characterized so that it perhaps might more closely align with what, in the end, customers really care about.
Source of the Article is Yoelit Hiebertwho worked in the field of LED lighting for the past 10 years and has experience in both the manufacturing and end-user sides of the industry.