With the COVID-19 pandemic raging, interest in germicidal technologies has surged as well. Ultraviolet radiation, specifically in the UV-C (100‒280 nm) band, is one technology that has gone from a very niche subject to the front pages of major newspapers because UV-C energy can deactivate the virus.  So let’s consider the facts about germicidal UV-C and the sources that can produce such energy, including LEDs.

This column was prompted in part by a document we posted on the Center for Lighting Enabled Systems & Applications (LESA) website at Rensselaer Polytechnic Institute.  We posted the top 10 questions that we have been asked relative to germicidal UV-C and a coronavirus such as SARS-CoV-2. Being that this column is in LEDs Magazine, we will revisit part of our top 10 list and add information that is specific to the use of UV-C LEDs in germicidal applications.

The first question we are generally asked is whether UV-C radiation can kill SARS-CoV-2.  The fact is that a virus is not technically alive, so killing it is not the goal.  Rather, we seek a technology that can deactivate or suppress the virus.  And yes, UV-C radiation applied correctly can deactivate the coronavirus.

“Applied correctly” is the key phrase.  The application depends on whether you seek to disinfect a surface, water, or air.  Moreover, the germicidal efficacy depends on UV-C dosage.  Dosage is determined by the radiometric power or watts delivered by a UV-C source, the distance from the target, and the duration of UV-C exposure.  Distance is easily understood if, for instance, you want to disinfect a surface.  In an air disinfection application, such as within an HVAC air duct, the distance would not be easy to estimate, as air disinfection can happen along the length of the duct through which UV-C radiation, along with the air, passes.  Distance becomes an even more complex issue were you to try and use a UV-C source to disinfect all of the air in a room.  Digging into the details, UV-C irradiance is measured in watts per unit area (typically W/cm2 in the US) and dose is calculated by multiplying the irradiance by the exposure time in seconds to get energy per unit area [typically joules (J)/cm2].

Moving to UV-C sources, LEDs are progressing as an alternative to mercury-discharge lamps in all types of UV-C applications.  But just as early visible-light LEDs struggled to usurp legacy sources, UV-C LEDs face similar challenges.  Generally speaking, UV-C LEDs have relatively higher cost, lower output power, and shorter lifetime. We’d expect the LED manufacturers to overcome those limitations over time.

However, LEDs are viable in some UV-C applications.  The technology is already used embedded inside various water-disinfection systems ranging from a home kitchen faucet to pumps for water processing to water supply for soda dispensers.  Indeed, mercury lamps would be tough to deploy in such applications due to size and potential contaminant concerns.

Most of the UV-C systems that have been in the news of late, however, are more focused on surface or air disinfection. And most of those systems use legacy lamps. For example, LEDs Magazine covered a United Airlines application for disinfecting surfaces in the jet cockpit and a Jet Blue application that uses robots to disinfect the passenger cabin of a jet.

The output power restrictions of UV-C LEDs would limit the effective use of the components in some such applications, or the number of LEDs required could be prohibitively expensive.  Other applications such as deployment in HVAC ducting might be a different story.

When you think about LEDs relative to a legacy source such as a mercury lamp, you have to think about how a system based on the source would operate.  Source lifetime is very important for a mercury lamp because lamp warmup time and lifetime reduction with repeated on/off switching will require continuous operation even if the utilization rate (e.g., airflow) isn’t continuous.

LEDs can be powered on and off and achieve virtually full power output instantaneously. Cycling power does not damage the components. In an HVAC system, the UV-C LEDs could be turned off when air is not circulating.  So the consideration of product lifetime changes.  There is also evidence that LEDs used in pulse mode can deliver greater germicidal efficacy in some applications.  Some research has shown such efficacy improvement in water disinfection and E.coli.  The reason is not fully understood at this point and more research is needed.

We are also asked about LEDs at the so-called far UV-C wavelengths in the 222-nm range.  For now, there are no LEDs available at such short wavelengths and the challenges of delivering such LED components appear stout.  Our message, however, remains that only a thorough analysis of the application and the model of system usage will determine whether LED or legacy sources are the best choice in a project.

Author: Robert Karlicek, PhD, Director of the Center for Lighting Enabled Systems & Applications (LESA) for LEDs Magazine