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Addressing design concerns when using UVC LEDs

Posted: 04 Dec 2015 ?? ?Print Version ?Bookmark and Share

Keywords:UVC LEDs? spectroscopy? instrumentation? Overheating? duty cycle?

As the performance of UVC LEDs increases, adoption of this relatively new technology is gaining momentum in life sciences and environmental monitoring instrumentation. As in all emerging technologies, the designer must be aware of some fundamental differences with respect to the incumbent solution and not assume "drop-in" replacement. This allows designers to realise the full benefits of UVC LEDs. With careful consideration, UVC LEDs can decrease footprint and power consumptionimproving cost of ownership for the end user.

UVC LEDs in instrumentation
Interest in UVC LEDs for spectroscopy is increasing as they can address market trends around miniaturisation, decreasing costs, and real-time measurements. Unlike deuterium or xenon flash lamps, LEDs emit a narrow spectrum where all the light output from the device is useful for measurement. Users can select the specific peak wavelength of interest based on their application requirements. In specific applications, standardised measurement methods have been developed with a mercury lamp emission line at 254 nm. For instance, water and air quality as measured to EPA standards requires an LED closely matched to the 254 nm peak wavelength. The table illustrates some of the important organic compounds in life sciences research, pharmaceutical production, and environmental monitoring that can be identified with spectroscopy.

Table: Common organic compounds with peak absorption wavelength.

The other primary criterion for light source selection in instrumentation is light output at the peak wavelength. Because LEDs have a single peak, the light output is concentrated at a particular wavelength, unlike other UV lamps. Absorption spectroscopy applications generally require a low level of light output C 1 mW or less. However, in cases where the flow cell is isolated from the light source, higher output is required due to significant light attenuation before the signal reaches the cell. This can raise the required light output from the LED to well over 1 mW. In fluorescence spectroscopy, signal strength is directly proportional to light intensity. The excitation power depends on the trace concentration levels that need to be detected, so in these applications the light output required from a single LED can be greater than 2 mW. Figure 1 shows the irradiance comparison between common UV light sources in instrumentation. Although the input power is much less for the LED, the irradiance at the desired UVC wavelength is higher than the other sources making it a more efficient light source for the specific measurement.

Figure 1: This graph compares the irradiance of a UVC LED, xenon flash lamp, and deuterium lamp.

After wavelength and light output are selected, another important parameter is the viewing angle as it impacts the instrument optical train. Broadly, there are two options C narrow or wide angle. The former is achieved with a ball lens, the latter with a flat window. The narrow viewing angle allows for high intensity of light available over a small area. This package type is typically used when directly focusing the light into the instrument.

A flat window package has a wider radiation pattern that has a greater tolerance in alignment with fibre for remote coupling. It is particularly useful in applications where the flow cell must be isolated from the light source and electronics, like in monitoring high temperature chemical processes or in chromatography with highly volatile solvents. When practical, a narrow angle ball lens can keep components in the instrument to a minimum, while the flat window provides enhanced flexibility in design.

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