New UV standard targets component testing for performance optimization

LEDs Magazine also spoke with the National Institute of Standards and Technology (NIST) optical radiation group leader Cameron Miller, who leads the UV Working Group within the IES Testing Procedures Committee, on the importance of LM-92. Miller highlighted the need for the standard to offer a usable framework of definitions and methods that can be understood by test equipment suppliers, test and measurement technicians, and LED manufacturers alike.

NIST’s role, Miller said, is to “help U.S. industries flourish, move forward, and take away barriers. Part of that’s providing documentary standards that give everybody a level playing field.”

Miller, who leads a team of scientists that specializes in photometry and radiometry and collaborates with industry representatives to develop standardized methods for measuring optical radiation, explained that consensus standards — like LM-85, LM-80, and LM-92 — provide common methodologies and best practices for testing, evaluation, and performance that can be repeated by technicians, product developers, and third-party test services providers. “There are certain examples where [a practice] has been around so long and everybody does it,” Miller noted. “That’s accepted, but it’s not quite the common process.”

Although the IUVA is not a recognized SDO, Miller acknowledged, IES is, and the IES Testing Procedures Committee provided the right connections between IUVA members well-versed in UV-C technology, the accredited standards development process, and the parties who would be responsible for measuring and documenting UV-LED optical and electrical properties.

Missing link for UV LED measurement

“[IUVA] and [IES] had a shortlist of standards we wanted to work on to support the community,” Miller said. “This was one of [those] that needed to get out sooner than later.” He described the UV-C LED market as being in an emerging phase much like visible white-light LEDs were in the late 1990s. “There’s a lot of defect states. … There are still a lot of anomalies that the visible LED folks have eliminated by better manufacturing capabilities. With the UV LEDs, with the wider bandgap materials, it’s still a learning process in many ways. It’s rapidly advancing, but it’s still [germinating].”

Arkesso president and semiconductor industry consultant Mike Krames has expressed similar sentiments in presentations and articles featured by LEDs, although he approached the prospects by comparing visible-light and UV device architectures and applying lessons learned from blue LED advances and fabrication techniques. In 2018, Krames wrote that recent epitaxial structure developments should remove some of the limitations of photon extraction efficacy with UV LEDs, therefore increasing power-conversion efficiency and eventually making mass production less expensive.

Still, Miller explained, established measurement methods for visible LEDs, such as those documented for thermal characterization in LM-85, didn’t present entirely accurate data when applied to UV LEDs under test. That has potentially held back market expansion and commercialization of UV LEDs for disinfection applications.

“When you would apply your first pulse of electricity to some UV LEDs — actually, it’s the larger fraction — you would see the forward voltage go significantly high and then relax,” with up to a 20% shift on the measurement results, Miller explained. With LM-85, “the basis was that the forward voltage really told you what was going on with the junction temperature, and that was not the case with these UV LEDs,” he said. The junction temperature is a key indicator of how much optical radiation an LED is capable of emitting, which ultimately could make or break the real-world disinfection claims of UV-LED components and finished systems.

LEDs will not detail the test methodology here as we expect to soon publish a longer feature article on LM-92 and what LED device technology advances and challenges led to its development.

Stepping up UV standards – What’s next?

Next steps involve educating the LED product development and test/measurement sectors, Miller said, adding that “the tools [for moving ahead with this LM-92 methodology] exist, and there are companies that make perfectly acceptable products. It’s a matter of how they are used.” Further documentation on equipment calibration and characterization of detectors is forthcoming, he said.

Additionally, IES and IUVA are currently developing standards for other UV light sources, such as low-pressure mercury tubes, excimer lamps, and pulsed-xenon lamps; as well as more guidance on finished UV disinfection systems. An International Standards Organization standard, ISO 15727, for linear low-pressure mercury lamps currently exists, Miller said, but he is not aware of any other accredited standards for UV light source measurement.

ANSI/IES/IUVA LM-92-22 is available from the IES Standards library.

More on UV component/systems design and test

Disinfection demands a complete UV-C LED picture

How does UV light measurement change lighting design perspective?

UV LED sources demand calibration to precise measurements for safe operation

CARRIE MEADOWS is managing editor of LEDs Magazine, with 20+ years’ experience in business-to-business publishing across technology markets including solid-state technology manufacturing, fiberoptic communications, machine vision, lasers and photonics, and LEDs and lighting.

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