Selecting the UV LED Wavelength for Purification Applications

Recently there have been articles about how UV LED technology will revolutionize water and surface purification. However, like with every new technology (or application of existing technology in a new market), there are elements of confusion and misinformation. The key challenge for the adoption of emerging technologies is to determine the right balance between technology readiness, pricing and market acceptance. As an example, it will take several years before UV LED performance and pricing reach an appropriate level for targeting municipal water treatment. Applications for which UV LEDs are ready today include water dispensers, ice makers, tumblers, humidifiers, vacuum cleaners, medical instruments and area/hospital lighting, amongst others.

The main value proposition of UV technology is to purify without potentially harmful chemicals.  UV LED technology is ideal for these applications since they are compact, instant-on, low voltage devices with superior reliability over traditional UV lamps. For water and surface purification applications, low-pressure mercury lamps are traditionally used, which have a peak wavelength of 254 nm. This has caused a significant proportion of the market to believe that LEDs must also emit light at 254 nm to be effective. The reality is that 254 nm is not the peak absorption wavelength of bacteria and viruses but is simply a convenient wavelength for mercury lamps.  In fact, the peak absorption wavelength of bacteria and viruses is around 265 nm. Most UV-C LEDs have a peak wavelength between 275 nm and 280 nm and are just as effective as 254 nm for purification purposes.

For water purification, there is no debate that UV-C technology is required to eliminate the threat of bacteria and viruses. However, for surface purification, the market includes UV-C (i.e. 275 nm) and UV-A (i.e. 405 nm) solutions. The market perception is that 405 nm LEDs are safer and lower cost but the reality is quite different.  If the required amount of light to achieve the same impact on bacteria is compared, then the perception is proven to be inaccurate. In fact, 405 nm light requires about 1200 times more light than at 275 nm, which means that due to the required light density at 405 nm, the 275 nm UV-C LED will be significantly lower cost and safer at the system level.

It's important to understand the main differences between 275 nm versus 405 nm LEDs for surface purification. The first is how organisms are affected by the different wavelengths. With 275 nm LEDs, the light penetrates through the cell walls of all organisms such as bacteria, viruses, and mold where the light disrupts the structure of their DNA molecules, prohibiting reproduction, rendering organisms inert. 405 nm light targets specific chemical compounds (porphyrins) found in cells of certain bacteria to cause an oxidation reduction reaction where this does not kill the bacteria but rather inactivates them.

This means that 275 nm light is lower cost, safer and targets a wider range of organisms for equivalent performance. The main clear advantage of UV-A LEDs for luminaire designs is that UV-A light is compatible with standard diffusors, PMMA and PC optics where UV-C light requires Quartz glass or silicone optics. The bottom line is that an impractical number of UV-A LEDs are required to reach a > 99.9% bacterial inactivation rate where this is easily achieved with 275 nm LEDs. Although there is a market for luminaires with UV-A LEDs providing some level of purification, with the rise antibiotic resistant bacteria infecting millions of people per year, it's clear that UV-C LEDs should be the technology of choice for surface and water purification in the years to come.