How to Interpret UVC LED Data Sheet Values and Figures
Although deep ultraviolet (UVC) LEDs have been around for quite a while now, they’re still considered an emerging technology. As is typically the case with any emerging technology, standards are still trying to catch up with the industry needs. This leads to minor (and sometimes major) variations in product characteristics presented on data sheets. We often find design engineers asking, “What information is actually necessary to design a product?” or “How do I translate the values presented in a data sheet to my system requirements?”
This article aims to address those questions and bring clarity to the relevant information from a UVC LED data sheet. Typically, a data sheet has three key sections:
What bins are available and how are they categorized?
The product nomenclature section of a UVC LED data sheet includes the list of specific part numbers available within a given UVC LED product line - or “bins.” This table provides information concerning peak wavelength and power output at a defined operating condition.
The peak wavelength is the first specification that engineers look at: for disinfection, a change in wavelength will impact the germicidal efficacy and therefore the power requirements (which cascades to a whole range of design variables) to obtain the desired disinfection levels.
A parameter which is often overlooked is the binning tolerances, both for the peak wavelength and power output. As explained above, a change in wavelength will affect the product design or may not suit the application, and therefore it is essential to consider all wavelength specified within the tolerances and design for the worst case scenario.
This means for disinfection applications, to take the wavelength with the lowest germicidal efficacy as taken from convolution with absorption spectra of the target pathogen. For example, if you’re looking at an LED with a peak output at 265 nm and a binning tolerance of +/- 5 nm, you need to consider the germicidal efficacy from 260 nm to 270 nm. For this specific range, most microbes will have a negligible relative change in germicidal efficacy between 260 and 270 nm - but engineers should consider the germicidal effect at the worst case being 270 nm, and therefore use that value in modeling. For an LED at 278 nm with a +/- of 5 nm, the worst-case scenario is 283 nm, where the change in relative germicidal efficacy is significant (>50 percent for E. coli for example).
Equally, design engineers should consider the lowest rated power in order to ensure that no matter which LED from the bin is used, the resulting design will perform as expected.
What are the fixed specifications of the specific LEDs?
The viewing angle is defined as twice the angle between the axial direction and the direction in which the light intensity value is half of the axial intensity. The intensity and surface area irradiated by an LED are all functions of the viewing angle. Optical engineers use this spec to design optical elements, define the number of LEDs and exposure time required for required disinfection levels. A wider viewing angle will increase the area irradiated but decrease the intensity across the area - this is typically used for large surface disinfection as it reduces the number of LEDs needed while providing a uniform irradiation.
The forward voltage, as specified for an operating current and solder temperature, is intrinsically related to the materials properties and is particularly useful to choose or design a power supply. It is also an important value to design thermal management because a higher voltage means a higher thermal power to be dissipated.
The forward current provides the minimum and maximum values specified for the LED operation as well as the typical value that the LED was tested at by the manufacturer. Operating the LEDs above the maximum rated forward current may result in permanent damage of the LEDs, while operation below the minimum specified current may not be sufficient to turn-on the LEDs.
The thermal resistance, of the LED package is a fixed variable when designing thermal management systems and will impact the range of PCB materials and type chosen (FR4 vs. metal core, vias) for the final design.
The maximum junction temperature should not be exceeded as permanent damage may be caused to the LED and the output degradation over time will not be predictable anymore. This temperature, although essential, cannot be measured directly and therefore can only be estimated by measuring the solder temperature (temperature at the solder point between the LED and the PCB) and considering the case thermal resistivity.
What are your design levers?
The curve showing light output vs current is important to find the expected relationship between current and output, and the linear trend (or deviation) between the highest and lowest rated forward currents. This information allows design engineers to control power with current, which is essential to meet end-of-life requirements (i.e. have the right level of power output at a certain wavelength and a defined point in time).
The typical radiation pattern shows how useful the power emitted will be for a defined application. The ability to focus light depends on its emission pattern, and the irradiance on a surface or in a product such as a water reactor is highly dependent on the way light is being emitted. For instance, sapphire-based UVC LEDs are typically associated with butterfly radiation patterns with side lobes that augment the etendue and can make it more difficult to steer the light, while aluminum nitride-based UVC LEDs have a Lambertian emission pattern which reduces the etendue.
When designing to a specific disinfection requirement, it is essential to understand the performance of UVC LEDs under a range of conditions and how these specifications are related to each other. Wavelength and power are the first information a design engineer looks for, but they are not the only considerations. The wavelength, viewing angle and radiation pattern provide insights on the usefulness of the power specified, while current related information allow for the control and design of the system for end-of-life requirements.
Finally, thermal related information such as maximum junction temperature and thermal resistance are key specifications for the design of an efficient and application-specific thermal management.