As the performance of UVC LEDs (short-wave UV LEDs) continues to increase, this new technology is gaining momentum from life sciences and environmental monitoring instrumentation applications. As with all emerging technologies, designers must understand some of the fundamental differences between them and existing solutions, rather than taking it for granted that they can be “ready to use”. This allows designers to fully utilize the full benefits of UVC LEDs. After careful trade-offs, the UVCLED design reduces product size, reduces power consumption, and lowers cost of ownership for end users.

UVC LED instrumentation application

As UVCLEDs meet market demands for miniaturization, cost reduction and real-time measurement, interest in introducing them into spectroscopy applications continues to heat up. Compared to xenon or xenon flash lamps, LEDs output narrower spectral sources, and all of the component's light output can be used for measurements. Users can select specific peak wavelengths of interest based on application needs. Standardized measurement methods for 254 nm wavelength mercury lamps have been developed for specific applications. For example, when testing water and air quality in accordance with EPA standards, an LED source with a peak wavelength of approximately 254 nm is required. Table 1 summarizes some important organic compounds that can be identified by spectrum in applications such as life science research, pharmaceutical production, and environmental monitoring.

Table 1: Common organic compounds and their peak absorption wavelengths

In instrumentation applications, another major criterion for selecting a source is the light output at the peak wavelength. Since LEDs have only a single peak, the light output is concentrated at a specific wavelength, which is not the same as other ultraviolet (UV) lamps. Applications for absorption spectroscopy typically require low levels of light output - 1 mW or less. However, in the case where the flowcell is isolated from the light source, higher optical output power is required because the optical signal is severely attenuated before reaching the flow cell. This can require the LED's light output to be well over 1mW.

In the fluorescence spectral domain, the signal intensity is proportional to the intensity of the light. The excitation power of the LED depends on the level of tracer required to be detected, so in these applications, the light output required for a single LED may be greater than 2 mW. Figure 1 compares the irradiance of UV sources commonly used in instruments. Although the input power of the LED is much smaller, the irradiance in the desired UVA wavelength band is higher than other sources, making it a more efficient source of light in a particular measurement.

Figure 1: Comparison of irradiance of UVC LEDs, Xenon flash lamps and xenon lamps

After selecting the wavelength and light output, another important parameter is the viewing angle, which affects the entire optical system of the instrument. Broadly speaking, it has two options: a narrow viewing angle or a wide viewing angle. The narrow viewing angle is achieved via a ball lens and the wide viewing angle is a flat window. A narrow viewing angle provides high intensity light in a small range, while its package is typically used in applications where light is directly focused onto the instrument.

Planar windows have a wider radiation pattern and have better tolerance when used with the fiber for remote coupling. This applies in particular to applications where the flow cell must be isolated from the source and electronic circuitry (eg monitoring high temperature chemical processes or chromatographic analysis with highly volatile solvents). In practical applications: narrow-angle ball lenses minimize the number of components in the instrument; wide-angle flat windows make the design more flexible.

Optimized drive currents allow designers to balance light output and application life requirements. Driving the LED at a rated current lower than the manufacturer's specifications will reduce the light output, but will also extend the life of the source. In applications that require high LED output power, some end users choose to drive the LEDs at a higher current than the data sheet specifications. Increasing the drive current in this way increases the light output, but it also carries the risk of sacrificing performance.

Overheating is a common problem that can have a negative impact on the LED's light output and life cycle. Due to the transient switching characteristics of the LEDs, the LEDs can be turned on and off quickly in a periodic manner. In fluorescent applications where higher light output is typically required, duty cycle operations are often employed to increase LED current more safely.

The duty cycle is defined as the percentage of the LED's turn-on time in one cycle; where cycle is the total time taken to complete an LED on-off cycle. For example, LEDs that operate at 50% duty cycle have exactly half the turn-on and turn-off times. Figure 2 shows the normalized light output for different drive currents and duty cycles.

Figure 2: Here we see the effect of different duty cycles on the normalized light output, while the LED turn-on time lasts for 500 μs. The normalized power is the relative optical output power of the light output relative to the maximum rated operating current of 100 mA (with a suitable heat sink). Driving the LED with a large current affects the junction temperature of the LED, which in turn affects its lifetime and light output.

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