Measuring UV-A Radiation

Acknowledgment and Disclaimer:
The material on this Web page is based upon work supported by the GLOBE Program through the National Aeronautics and Space Administration, Goddard Space Flight Center, Grant No. NAG5-8929. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of NASA, Goddard Space Flight Center, or the EOS-Aura project.

The GLOBE/GSFC UV-A radiometer/sun photometer.
In collaboration with NASA's Goddard Space Flight Center, in Greenbelt, Maryland, USA, The GLOBE Aerosol Monitoring Project Science Team has developed an instrument for measuring UV radiation at about 370 nm. This is in the UV-A part of the solar spectrum. Like other atmospheric monitoring instruments developed by this Team for GLOBE, the GLOBE/GSFC UV-A instrument uses a light emitting diode (LED) as a spectrally selective detector.

Unlike the GLOBE sun photometer, the UV-A instrument operates both as a full-sky instrument (a radiometer) and as a sun photometer, with a removable collimating tube. The design of this instrument is more complicated than the sun photometer because the spectral response of the LED used as a detector. This LED, which emits blue light, has a strong spectral response peak at about 370 nm. However, it also has a response "shoulder" that extends into the visible part of the spectrum, beyond 400 nm. Thus, we have included a cutoff filter to remove the visible response.

For a full-sky instrument, the cosine response of the detector is important. For an instrument with perfect cosine response, the signal received by a detector from direct sunlight should be proportional to the zenith angle of the sun. Real detectors do not have perfect cosine response. In this case, we have covered the LED/filter assembly with a Teflon disk to diffuse the incoming signal and improve the cosine response.

The UV-A instrument is designed specifically to support measurements made from the EOS/Aura spacecraft. There is good news if you would like to participate in the UV-A Protocol. Unlike most GLOBE equipment, which must be purchased by participating schools, the GLOBE/GSFC UV-A vapor instrument is sponsored by a grant from NASA/GSFC. These instruments are available at no cost, in return for an agreement to collect and report data. Please contact the PI for this project, Dr. David Brooks, at