Aug 3, 2020

The evolving metrics of light from LEDs

Category: Illumination

Over the last twenty years, we’ve been party to an evolving discussion about light from LEDs. Most conversations center around light output and quality of light. In both cases, performance needs and requirements have spurred a continuum of innovation. While some advances are simply treated as ongoing improvements, such as improving lumen output by a small percentage, others impact the entire industry—for example, Lumileds introduction of hot-testing (85°C) of LEDs. As LED innovation continues, so does the process by which LEDs are evaluated—case in point, usable light.

Usable Light
The lumen and efficiency numbers reported on LED datasheets today are so far beyond what they were just a decade ago that one might be forgiven for thinking that the datasheet lumen values tell the whole story. They do not. While it’s well understood that output depends primarily on current and thermal conditions, the optical design of the LED itself is playing an increasingly important role in determining luminaire performance. Let’s consider LEDs designed for operation in a streetlight. It’s critical that all the light from the LED move in a forward direction so that it can be managed to illuminate the surface below. This would be Usable Light. Any light emitted from the LED that is broader than 90 degrees from the center will be moving in the wrong direction and not be usable in the application. These are lost lumens, paid for but not delivering any value.

Figure: Percentage of total flux versus half-width angle for LUXEON HL2X, reference cosine and a similar high power part from a competitor.

For example: If the objective is 10,000 lumens and just 5% of the LEDs initial light output is lost because it can’t be managed, then 10,000 datasheet lumens become 9500 available field lumens. Additional LEDs, board space, etc. would be required to achieve the target. It quickly becomes apparent that we must know more than merely the published datasheet lumen numbers when evaluating and comparing LEDs to make an informed decision.

As Lumileds engineers designed the new LUXEON HL2X, the concept of Usable Light was top of mind. The package’s optical design causes all of the light output to be usable and available to the application. Other LEDs may emit more raw lumens, but if you can’t use the lumens, why pay for them?