Peculiarity when measuring UVC LEDs
Ultraviolet optical radiation (UV radiation) is used in a variety of tasks. It can be used to modify the physical properties of materials - radiation curing - or to treat skin diseases such as psoriasis. Water, air and solid surfaces are disinfected with UV radiation. A wide variety of gases and biomolecules can be detected with UV radiation. Among artificial radiation sources of UV radiation, UV LEDs are becoming a serious alternative to the hitherto dominant gas discharge lamps. Although the technology of UV LEDs is still young, it is making fast progress particularly in the UVB and UVC spectral ranges. The classic instrument for measuring the total radiant power of UV LEDs is a spectroradiometer with an integrating sphere.
A peculiarity of quasi monochromatic LED radiation must be considered when designing integrating sphere spectroradiometers for UV LEDs. The monochromatic UV radiation of the LED causes fluorescence of the integrating sphere coating which leads to significant measurement errors that cannot be compensated by conventional correction methods. Research at the Gigahertz-Optik laboratory has confirmed that barium sulfate coatings and synthetic coatings for integrating spheres both fluoresce when exposed to UVC LEDs < 250 nm. This fluorescence is caused by organic molecules which deposit over time on the integrating sphere’s coating. When stimulated with UV-LED radiation <= 230 nm, the radiant flux of the fluorescence can even reach the magnitude of the radiant power of UV-LEDs themselves. The fluorescence level can be reduced by irradiating the organic molecules using a sufficient UV radiation dose.