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, Projecting Long Term Lumen Maintenance of LED Light Sources). There are some limitations with this approach:

  • LED package manufacturers do not collect lumen maintenance data for every package permutation.
  • Other degradation mechanisms (e.g., lens degradation, operation in an improper environment, dirt accumulation) impact overall luminaire lumen maintenance.
  • Output lumens are measured in the controlled environment of a test lab vs. the actual operating environment of the final product.
  • And perhaps most importantly, customers, in the end, are not concerned with lumen output per se. Customers want their employees to be able to see so they are safe and can do their work. As Jordon Musser, Senior Director of LED Products and Technology at Flex opined, "customers don't care about lumens, they care about the light levels in their space. Customers want to know their foot-candles are going to be at a certain level for a certain amount of time."

It's important to recognize that there are parameters outside the manufacturer's 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.

Driver expected life
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:

(Image courtesy of 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.

Luminaire efficacy
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.

Efficacies of commercial LED packages measured at 25°C and 35A/cm2 input current density (DOE Solid-State Lighting, 2017 Suggested Research Topics Supplement: Technology and Market Context September 2017)

TM-21 L70 projection
As mentioned above, 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).

(Image courtesy of 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, shared this perspective: "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.

Yoelit Hiebert has 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.

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