UV Depolarizer Based on Integrating Sphere for Irradiance Calibration at SURF III
National Institue of Standards and Technology
Gaithersburg, MD 20899 USA
The calculability nature of synchrotron radiation makes electron storage rings wonderful light sources for radiometry especially in UV and VUV spectral region. Facility for Irradiance Calibration Using Synchrotrons (FICUS) is under construction on beamline 3 at the Synchrotron Ultraviolet Radiation Facility (SURF III). This facility will be built up as a spectral irradiance calibration station over UV to VUV range (150 nm ~ 400 nm) particularly suitable for customer transfer standard light sources such as deuterium lamps, the widely used in industrial and scientific applications. Currently we could fulfill the irradiance calibration in the air UV from 200 nm to 400 nm wavelengths range. The measurement uncertainty is 1.2 % (k=2), much better than the past uncertainty using a wall-stabilized hydrogen arc, transfer standard FEL lamp and high-temperature blackbody.
In this facility, we used an UV integrating sphere with PTFE internal coating as depolarizer and diffuser to uniform either the radiation from electron storage ring or testing user light sources because the highly polarization of synchrotron radiation. During our measurements, the significant throughput signal changes under UV irradiance from same deuterium lamp. For integrating spheres, whose throughput are highly sensitive to material reflectance, it was suggested that throughput changes under UV irradiation could be attributed to the UV-produced fluorescence from some kind of contamination. The fluorescence could be an additional issue for accurate radiometric measurements. In our investigation of the performance of different integrating spheres, we set up two experimental systems to measure and compare the spectral throughput for a variety of integrating spheres, observe relative UV absorption spectra after integrating sphere and study the change in spectral throughput of integrating spheres after prolonged irradiation by the deuterium lamp. We used two light sources for two systems, one is deuterium lamp and another is tunable laser. The integrating spheres studied are made with PTFE both in pressed and sintered forms. The pressed PTFE integrating spheres are made at NIST and the sintered PTFE are obtained from commercial products.
Our measurement results clearly show distinct UV absorption and fluorescence features by PTFE integrating spheres. In accordance with previous studies, we found contaminants are mostly responsible for the observed absorption and fluorescence as well as the degradation of integrating spheres. Fluorescence from laser excitation and measurement of the UV throughput of an integrating sphere are very useful in characterizing the condition of an integrating sphere. PTFE Integrating spheres can be contaminated from environment such as air pollution and cause absorption and fluorescence in UV. Baking in vacuum is much more effective than baking in air condition to remove some volatile contaminants. The fluorescence could be effectively decreased by baking in vacuum but not all. The pressed NIST PTFE integrating sphere showed the best throughput stability and the smallest fluorescence effect under the prolonged UV exposure.