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UVC LED technology

UV-C LED structural and operational differences UV-C LEDs are both structurally and operationally different than mercury lamps. From a structural standpoint, UV-C LEDs are smaller and offer higher ultraviolet power densities than mercury-based UV lamps [Figure 1], meaning you can concentrate more light in a given area without the aid of external optical components and at a very small footprint. This is the primary reason UV-C LEDs were first used in analytical products despite their low output  power. Figure 1: Optical Power Density comparisons between a 30mW UV-C LED and typicallow-pressure mercury lamp (LP-Hg), typical medium pressure mercury lamp (MP-Hg), and typical amalgam lamp. Emitting surface area is given by the glass envelope (no ballast) in the case of the lamps and by the semiconductor chip in the case of the LED. While the vast differences in package size between a UV-C LED and a mercury lamp are easily identifiable, the optical and thermal differences can have an even larger impact on system design. UV-C LEDs theoretically emit in all directions; however, they are limited by material and package restrictions to be directional point sources. A typical beam profile from a packaged UV-C LED will have most of the light fall within a 100- to 120-degree beam. This is usually defined by the viewing angle where the light intensity has fallen to 50% of the maximum. Such specific optical output characteristics allow a wide variety of possible intensity distributions in a LED based disinfection system.  For instance, spot disinfection at a critical process point could be achieved with a single device, or alternately, a broad homogeneous intensity could be distributed over a specified area. The intensity could also be tailored to a specific pattern of lower intensity spots and higher intensity spots directly adjacent to each other. The optical output characteristics of UV-C LEDs raise the question of how to best exploit these features at the system level. Thermal considerations are important in LED systems as well as mercury systems. UV-C LEDs generate heat from the solid piece of semiconductor (the LED chip). This heat is extracted from the backside of the package holding the chip. Because the footprint of the packaged LED is small, cooling techniques can be used such as Peltier coolers or fans. These are located on the back of the package and can be as small as a centimeter. Active cooling techniques make it relatively easy to cool the LED to room temperature under hotter ambient conditions. This is fortunate because UV-C LEDs have an opposite reaction to ambient temperatures than mercury sources. Unlike mercury sources, UV-C LEDs increase their optical output at cold ambient temperatures with decreasing outputs occurring at higher temperatures [Figure 2]. This makes UV-C LEDs ideal for refrigerated/cold applications where they require less heat management and no warm up time. Figure 2: Output power of a low-pressure mercury lamp and a 265nm LED as a function of temperature. The data is normalized at 26 degrees C. From an operating standpoint, UV-C LEDs offer key design freedoms over traditional Hg technology. UV-C LEDs operate off of direct current (DC), which allows for portable, battery-powered system operation, and in general minimizes system electronics. One of the more neglected aspects of system design using UV-C LEDs may be in the area of back-end electronics. Back-end electronics can be used in ways similar to those of mercury lamps, by providing power to the LEDs as well as adding functionality to the system. Back-end electronics can be used to drive LEDs in pulsed or continuous modes of operation; they can monitor changes in the LED itself, such as changes in forward voltage or intensity, which can be used in feedback modes to bring the LED back within specified operating criteria or to signal failure. Back-end electronics can incorporate temperature control feedback or even perform wireless data logging, and because the entire system is DC, the entire circuit can co-locate with the uv LED source in a very small footprint.

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