LEDs are enabling UV disinfection to be integrated in ways never before possible—from water disinfection right at the point-of-dispense to portable point-of-care medical devices that reduce HAIs. For these applications, it’s not the first glass of water or the first patient that is the primary concern - it’s the last. To ensure disinfection performance in your application is as good at the end of the product's life as it is out of the box, you need to understand the relationship between UVC LED lifetime and reliability.
Lifetime is a variable, not a specification. We talk about the approach to defining required lifetime for both point-of-use water and healthcare applications in other articles on our website. When selecting a UVC LED to meet application requirements, understanding role of device reliability and its relationship to lifetime is an important distinction to understand.
What do we mean by reliability?
When we’re talking about product reliability, we are referring to the percentage of the LED population operating outside specifications. For a given forward current and operating temperature, there will be a natural statistical distribution of light degradation (or lifetime). The percentage of devices in a population which exhibit light output below a specified L value is known as the B value.
Figure 1 (below) provides different LEDs behaviors based on B values of a population. The blue line shows that at 500 hours, under the specific operating conditions 50 percent of a sample population (the average) will emit 45 mW while 50 percent of the sample population will emit something less than 45 mW.
Figure 1: UVC output for a population of devices operated at a drive current of 700 mA with an ambient temperature of 35°C. The graph shows the population data for reliability values of B50 and B10.
The red line in Figure 1 shows data for B10 of this group of devices. The B10 line shows that at 500 hours only 10 percent of devices emit less than 36 mW of their initial output—which is the L60B10 value.
Why should you care about reliability?
Engineers need to consider end of life performance when designing disinfection applications. As stated before—it’s the safety of the last glass of water or the last patient that is the most important. To determine this, designers need to first understand the UVC power required for that last dose, and then calculate the cumulative on time for their product lifetime. Because of the instant on/off nature of LEDs, it’s this cumulative on time that is the required device lifetime.
Generically, designers are typically either designing to the lowest cost solution—like in consumer goods, or the highest level of disinfection confidence—like in healthcare. Lowest cost solutions often require the fewest number of LEDs possible while there is no room for error in critical infection prevention equipment. If half the LEDs fail then you risk delivering less than the required disinfection dose so, understanding how 50 percent (B50) of an LED population will behave is not enough.
Design engineers can increase their end product quality by looking at lifetime performance at lower B values, like the red line in Figure 1. By understanding this data, the total UVC power required at the end of life, and the cumulative on time in an application, design engineers can ensure disinfection performance that offers the highest level of confidence for the lowest cost in their products.