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 – 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 fiber 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.
Optimizing the drive current allows the designer to balance the light output with the lifetime requirements of the application. Driving an LED below the manufacturers rated current will decrease the light output, but it will also increase the lifetime of the light source. In applications that require a high LED output power, some end users choose to operate LEDs at an elevated current above