Our “lives have been flipped, turned upside down and I’d like to take a minute, just sit right there, let me tell you how.” We’ve become so confused with air…and water and surfaces…
While the pandemic certainly is not entertaining like the Fresh Prince of Bel-Air, it has rapidly enabled advances in ultraviolet C-band (UV-C) LED technology for disinfection. Germicidal UV brings new opportunities but also new challenges and characteristics to bear. All levels of the supply chain must understand, transparently share, and not play games with these points or risk jeopardizing the adoption of germicidal UV systems — or worse, public safety.
As semiconductor devices, LEDs will enable smaller, more environmentally-friendly systems that can cycle immediately and, in some cases, last much longer than traditional technologies. Soon, UV LEDs will provide energy savings as well. Transparency of the UV LED characteristics laid out here is critical for adoption.
Radiant flux is the radiated energy of an LED at a specified input current and voltage. Comparing apples to apples, one must understand the total power at which the flux is stated. Too many manufacturers state just the radiant flux or the radiant flux at a given current (i.e., 60 mW at 350 mA). Unfortunately, all UV LEDs are not close to equal as forward voltage (VF) can vary dramatically between LED manufacturers (i.e., as low as 5V to upwards of 8V at 350 mA).
Wall-plug efficiency (WPE) is the ratio of radiant flux to input power. As UV-C LEDs are still in their infancy, WPE is low in comparison to mercury-vapor lamps. The WPE variance between LED manufacturers is also significant, upwards of 2x at similar power levels. However, the future remains bright as LED manufacturers that survive will continue to improve. It is a matter of time before WPE of LEDs surpasses that of mercury vapor.
With regard to wavelength, 254 nm is no longer the only choice. LED manufacturers can adjust recipes to achieve various wavelengths. As such, there are now UV-C LEDs ranging from 220–280 nm. I cannot say “commercially available,” as many are lab samples, are not available in production/volume, or simply do not last more than a few hours. The key takeaway is the shorter the wavelength, the lower the flux, efficiency, and lifetime.
Virucidal power is defined as the virucidal efficiency times the radiant flux. Virucidal efficiency versus wavelength is relevant to germicidal inactivation. However, it alone does not tell the complete story of an LED. 265 nm may be the most efficient germicidal inactivation wavelength, assuming the same radiant flux per wavelength. Unfortunately, that is an incomplete assumption as radiant flux varies by wavelength. Given the ability of LEDs to achieve many different wavelengths, virucidal power is most applicable for LEDs.
Junction temperature occurs at the junction of the LED die, where the electrons collide, generating photons and heat. LEDs do not like heat, and this junction is the most critical point of the LED when discussing output and lifetime.
System dose is calculated by multiplying the irradiance (mW/cm2) by time (seconds). The lack of dosage sharing has been a very irresponsible tactic over the past year. Too many claims have been made with respect to SARS-CoV-2 inactivation without referencing dosage. “Inactivate 99.9% SARS-CoV-2 in 5 seconds!” This is like saying, “I kicked a 50-yard field goal” but conveniently not stating that it took 100 attempts. A GUV system should reference the dose and efficiency when claiming inactivation rates.
With regard to lifetime, LEDs typically do not have catastrophic failures but degrade over time. Manufacturers will reference lifetime hours as “Lxx” where xx is a percentage of radiant flux remaining. Heat is typically the greatest cause for depreciation. Unfortunately, many manufacturers have conveniently forgotten this in lifetime statements. Claiming L70 10,000 hours is useless. For a lifetime claim to be valuable, one must also state the junction temperature. This allows end users to understand the lifetime of the LED in their thermally-stabilized system.
All LED characteristics are relevant and incomplete promotions fail to communicate the full story. A radiant flux without input power is hiding something. A wavelength by itself is not an ideal solution. A dosage without efficiency is incomplete. A lifetime claim without temperature is useless. To every LED manufacturer out there, please share the whole story. To every system designer out there, please ask these details and share your story. We can all do better!
Get to know our expert
ERIK SWENSON is a seasoned professional with 14 years in the solid-state lighting industry. As general manager for Nichia America Corporation, Swenson has helped lead the adoption of Nichia LEDs throughout various markets including general illumination, UV, video display, and more. He has been an active voice of Nichia within the SSL market, having spoken at several industry and manufacturer’s conferences and trade shows. After receiving a degree in electrical engineering from Purdue University, Swenson practiced various engineering facets within the automotive industry for 6 years prior to joining Nichia in a commercial role.
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