“The COVID-19 pandemic has inspired engineers to consider ultraviolet (UV) light for disinfecting and sterilizing products to “inactivate” SARS-CoV-2, the virus that causes COVID-19. Traditional disinfection and sterilization products use low-pressure mercury vapor lamps that irradiate with the desired form of the UV-A spectrum to eliminate pathogens. But LEDs offer many advantages, including higher efficiency, greater light output, longer lifetime, and lower lifetime cost.
Author: Steven Keeping
The COVID-19 pandemic has inspired engineers to consider ultraviolet (UV) light for disinfecting and sterilizing products to “inactivate” SARS-CoV-2, the virus that causes COVID-19. Traditional disinfection and sterilization products use low-pressure mercury vapor lamps that irradiate with the desired form of the UV-A spectrum to eliminate pathogens. But LEDs offer many advantages, including higher efficiency, greater light output, longer lifetime, and lower lifetime cost.
UV-A LEDs are relatively easy to manufacture, by retrofitting blue LEDs into the near-visible spectral range, which has been used in industrial curing applications for over a decade. But SARS-CoV-2 inactivation requires higher-energy UV-C.
Commercial UV-C LEDs have become available in the past few years. However, we cannot consider these devices to be direct replacements for conventional mercury vapor lamps, as this presents many new design challenges. For example, in order to ensure normal operation, disinfection and sterilization products have high requirements on radiation flux and strict control. In addition, although UV-C LED kills bacteria and viruses, it is also harmful to the human body, so adequate protection is an important part of the entire design.
This article will briefly discuss the types of UV radiation and their role in disinfection and pathogen control. Then, the advantages of LEDs as radiation sources and the associated design challenges are presented. The article will then introduce solutions to these challenges using UV LED examples from OSRAM Opto Semiconductors, Inc., Everlight Electronics, and SETi/Seoul Viosys.
Why use UV light to kill pathogens?
Ultraviolet radiation fits into the electromagnetic spectrum between visible light and X-rays, including high-energy short-wavelength (400 – 100 nm) photons of the same energy. Radiation wavelength is inversely proportional to frequency: the shorter the wavelength, the higher the frequency (Figure 1).
Figure 1: In the electromagnetic spectrum, ultraviolet radiation is only lower than visible light, with wavelengths between 100 and 400 nm, and is divided into three types A, B, and C. (Image credit: Government of Canada).
According to the interaction of UV radiation with biological materials, UV light is divided into three types: UV-A (400 to 315 nm), UV-B (314 to 280 nm), UV-C (279 – 100 nm). The sun produces all three kinds of UV rays, but the UV rays that humans are exposed to are mainly UV-A, because very little UV-B penetrates the earth’s ozone layer, and UV-C does not penetrate the ozone layer. However, there are several artificial methods for generating these three types of UV light, such as mercury vapor lamps and, more recently, UV LEDs.
UV-C radiation was an established pathogen eradication technology long before the current pandemic. Conventional products use mercury vapor lamps as the UV light source. A recent study on the efficacy of UV-C disinfection against SARS-CoV-2 showed that UV light with a wavelength of approximately 250 – 280 nm is preferentially absorbed by the RNA of the virus, with a total dose of 17 joules per square meter (J/m2), with a pathogen inactivation rate reached 99.9%. It should be noted that although this level of radiation cannot directly kill the virus, it can fully destroy the RNA of the virus so that it cannot replicate, so it can limit the ultraviolet radiation that people receive and will not cause any harm to people.
UV light source
The traditional UV light source is a mercury vapor lamp. This is a gas discharge device that emits light from a plasma of vaporized metal when it is excited by a discharge. Some products employ a fused silica arc tube that is excited to peak emission at 185nm UV-C wavelength (along with some UV-A and UV-B emissions) for disinfection and sterilization purposes (Figure 2).
Figure 2: Before the advent of UV-C LEDs, low-pressure mercury vapor lamps were the most practical source of UV light. (Image source: JKL components)
Compared with traditional incandescent lamps, mercury vapor lamps are highly efficient and have a long lifespan; the main disadvantage is that if the bulb ruptures or is discarded during normal use, it releases toxic mercury into the environment.
On the other hand, the key advantages of UV-C LEDs for disinfection and sterilization applications are what LEDs are for general lighting, specifically energy efficiency, higher light output, longer lifetime and lower lifetime cost. Additionally, while LEDs must still be handled with care, they do not pose the same environmental hazard as mercury-based light sources.
UV-C LEDs are manufactured based on blue LED technology. These products use an aluminum gallium nitride (AlGaN) substrate as an emitter platform with a wider bandgap (shorter wavelength) than red LEDs. However, UV-C LEDs are less efficient and cost more than blue LEDs, mainly because UV-C radiation cannot pass through gallium nitride. Therefore, relatively few UV-C photons escape from the chip.
Many recent developments, including reflective p-contact metallization, patterned substrates, textured surfaces, microcavity effects, and volumetric shaping, are now being used to enhance the efficacy of UV LEDs and are now available in commercial products. Reasonable performance. But engineers should be aware that such devices exhibit lower levels of efficacy than visible-light LEDs, and not only increase the complexity of extracting photons, but also drive up costs. Manufacturers often avoid efficacy numbers in their data sheets, instead specifying the luminous flux in milliwatts (mW) at a given drive current and voltage.
Examples of UV-C LED Solutions
There are several commercial UV-C LEDs on the market that are specifically designed to radiate at optimal wavelengths to inactivate pathogens. For example, the SU CULDN1.VC-MAMP-67-4E4F-350-R18 OSLON UV 3636 from OSRAM Opto Semiconductors, Inc. is a UV-C LED that emits at 275 nm. The LED delivers 35 to 100 mW of total radiant flux at 350 milliamps (mA), 5 to 6 V forward current/voltage (depending on the choice of bin) (Figure 3).
Figure 3: UV-C LEDs achieve peak radiation in the 100 – 280 nm range. For inactivation of SARS-CoV-2, the ideal peak is between 250 – 280 nm. The radiant flux of the OSRAM OSLON UV-C LED shown here peaks at 277 nm. (Image credit: OSRAM)
Another example device is Everlight Electronics’ ELUC3535NUB, a 270 to 285 nm UV-C LED. The device is ceramic-based and radiates 10 mW of power at 100 mA, 5 to 7 V forward current/voltage (Figure 4).
Figure 4: Everlight Electronics’ 270 to 285 nm UV-C LEDs are housed in a ceramic substrate. The LED dimensions are 3.45 x 3.45 mm. (Image credit: Everlight Electronics)
For SETi/Seoul Viosys, CUD5GF1B is provided. This LED is a 255 nm emitter housed in a ceramic package for surface mount, featuring low thermal resistance. The device has a radiated power of 7 mW with a drive current/voltage of 200 mA/7.5 V. As the temperature increases, the emission wavelength of the LED has only a very small deviation: within the 50°C chip temperature range, the deviation from its 255nm peak output is only 1nm. This is an important consideration for a device that requires tightly controlled output to ensure good virus inactivation (Figure 5).
Figure 5: SETi/Seoul Viosys’ CUD5GF1B UV-C LED deviates only 1nm from its 255nm peak output over a 50˚C die temperature range. (Image credit: SETi/Seoul Viosys)
Designing with UV-C LEDs
LEDs themselves also present design challenges, so it is impractical to attempt to adapt products based on mercury vapor light source designs to accommodate UV-C LEDs. So, in disinfection or sterilization applications, replacing mercury vapor lamps with UV-C LEDs is not just a simple light source replacement.
When choosing UV-C LEDs for disinfection or sterilization, the area where UV-C light needs to be used, and the radiation flux (“irradiance” required to inactivate target pathogens within the irradiated area) should be determined first during design. ) in watts per square meter (W/m2).
For example, we consider the application of disinfection of air coming out of air conditioning ducts. Based on the above requirement of 17 J/m2, for an area of 0.25 m2, inactivating all viruses in the airflow within 5 seconds requires a system with an irradiance of around 4 W/m2 (total power of 1 W).
Once the required irradiance is calculated, engineers know how to design and manufacture. A rule of thumb is to consider the radiant flux per LED, then divide the total radiant flux by this number to get the number of LEDs required per product.
This is a crude simplification because it does not take into account how these radiant fluxes are distributed. Two factors determine how the radiation flux affects the target surface. The first is the distance from the LED to the object, and the second is the “beam angle” of the LED.
If an LED is treated as a point light source, its irradiance decreases according to the inverse square law. For example, if the irradiance is 10 milliwatts per square centimeter (mW/cm2) at 1 cm from the emission point, then the irradiance at 10 cm will drop to 0.1 mW/cm2. However, this calculation assumes that the LED radiates the same in all directions, which is not the case. In contrast, LEDs feature primary optics that direct the radiant flux in a specific direction. Manufacturers typically list the LED beam angle in their datasheet, which is defined as the angle at which the light reaches a 50% peak irradiance on both sides of the origin.
The aforementioned UV-C LEDs from OSRAM, Everlight Electronics, and SETi/Seoul Viosys have beam angles of 120, 120, and 125 degrees, respectively. Figure 6 shows the radiation pattern of OSRAM’s SU CULDN1.VC-MAMP-67-4E4F-350-R18 UV-C LED. In the figure, the dashed line between 0,4 and 0,6 indicates where 50% of the peak irradiance is reached, defining the beam angle (60+60 degrees).
Figure 6: Irradiation pattern for OSRAM’s SU CULDN1.VC-MAMP-67-4E4F-350-R18 UV-C LED, the dashed line between 0,4 and 0,6 represents the 50% peak irradiance place, defines the beam angle (60+60 degrees). (Image credit: OSRAM)
The key characteristic that determines the beam angle is the size ratio of the LED chip to the primary optics. Therefore, producing a narrower beam requires a smaller emitter or larger optics (or an appropriate balance between the two). The trade-off in design is that the smaller the chip, the lower the emissions, and the larger and more difficult optics to manufacture, driving up the price and limiting beam angle control.
Commercial LEDs are often shipped with primary optics, so sizing the chip/optics ratio is beyond the control of the design engineer. Therefore, the review of the beam angle of the selected product is very important, because two devices with the same output from different suppliers may have completely different emission patterns.
While the distance and beam angle of the LED to the object being irradiated is a good initial guideline for the radiation pattern, there are also sources of variance. For example, LEDs from the same manufacturer, with theoretically the same output and beam angle, can vary widely in radiant intensity and quality for different primary optical designs. Only by testing the output of the selected product can the actual radiation pattern be determined.
Knowing the LED output, the distance between the LED and the surface of the item to be sanitized, the beam angle and actual emission data, the engineer can calculate how many LEDs are needed and how they should be positioned to produce the desired irradiance in the active area .
The final choice for LEDs comes down to the required trade-offs between cost, energy efficiency, and complexity. UV-C LEDs are expensive, so one approach is to use fewer, higher-power devices instead of a lot of lower-power devices. The benefit of this approach is that the cost of LED components may be reduced and the driver complexity will be reduced. The downside is that due to low efficiency, more powerful devices require better thermal management to ensure their long lifespan (high temperatures can drastically shorten LED lifespan). This necessitated larger heat sinks, which resulted in some of the expected cost savings goals not being achieved.
Design introduction of secondary optics
Another option for adding LEDs and/or increasing LED power is to consider secondary optics. These devices collimate the UV-C beam output from the LED (producing a parallel beam of equal intensity), effectively eliminating any beam angle effects. In theory, when using collimation, the irradiance should be uniform across the entire target surface (regardless of the LED arrangement), and a given irradiance should be achieved with fewer LEDs, since a small fraction of the output will be wasted. Additionally, higher irradiance can be achieved with the same number of LEDs as in the design without secondary optics (350 mW/m2 vs. 175 mW/m2) (Figure 7).
Figure 7: Collimation of UV-C emission using secondary optics (left) increases irradiance in the target area compared to a system with the same LED output but using (uncollimated) primary optics. (Image credit: LEDiL)
In practice, irradiance using secondary optics is less uniform because even the best products have imperfect collimation due to diffraction (although the smaller the LED, the better the collimation). Also, lengthy experimentation with the placement of the LEDs and secondary optics is often required to ensure the desired irradiance is obtained from fewer components than a similar design without secondary optics.
Note that the secondary optics of UV-C LEDs are fabricated from a different material than visible light LEDs. A common solution is injection-molded silicone parts, which reflect UV-C wavelengths well and allow the production of lenses with complex designs. Aluminum reflectors can also be used to collimate UV-C. There are two trade-offs when using secondary optics, the cost savings from using fewer LEDs and the added complexity from the collimator design.
Although UV rays cannot penetrate deeply into human skin, they can cause short-term damage such as burns when absorbed, and long-term damage such as wrinkles and premature skin aging. In extreme cases, UV exposure can cause skin cancer. Ultraviolet rays are particularly harmful to the eyes and can damage the retina and cornea. Ozone is also produced when UV radiation interacts with the air, and high concentrations of ozone are thought to pose a health risk.
To avoid these hazards, it is good practice to design products that limit exposure to UV-C light and keep users from looking directly at the LED. Since UV-C is invisible, it’s also a good idea to deliberately add some visible blue light emission when choosing LEDs. Doing this makes the UV-C LEDs visible when turned on.
For SARS-CoV-2 in particular, incorporating disinfection into HVAC units can quickly inactivate airborne viruses while keeping UV-C out of sight. As for other locations, research is underway on LEDs that can be mounted on lamps to not only irradiate surfaces with very low levels of UV-C, which is harmless to humans, but also provide enough radiation for a long period of time to inactivate tables, chairs, Any viruses on surfaces like floors and doorknobs.
UV-C radiation can be used to inactivate pathogens like SARS-CoV-2 in disinfection and sterilization products. However, the common artificial UV-C light source is the mercury vapor lamp, which needs to overcome various difficulties during disposal due to the heavy metal content. UV-C LEDs offer a more efficient and longer-lasting alternative, alleviating disposal issues. Several UV-C LEDs are already on the market with emission peak wavelengths well suited for pathogen inactivation.
However, these LEDs are not drop-in replacements and require careful design to take full advantage of them. As mentioned above, the designer must cut in from the irradiance required for the active surface, and in turn calculate the number and arrangement of UV-C LEDs required to achieve that irradiance. The designer must also decide whether to rely on the primary optics of the LED to produce uniform irradiance, or use the secondary optics to collimate the UV-C output for the best form factor, taking into account the resulting higher complexity higher cost.
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