17 December 2004

The end of a long day measuring
the sun and sky

Forrest M. Mims III

In 1998 a local power company donated a Yankee Multifilter Rotating Shadowband Radiometer (MFR-7) to my research site, Geronimo Creek Observatory. The MFR-7 measures the full sky irradiance at seven wavelengths of blue, green red and near infrared light every 15 seconds. The shadowband blocks the direct sun at each measuring cycle. By subtracting the signal when the detector array in the canister is shaded from the full sky irradiance, the direct sunlight can be calculated. All this is done automatically, and the data are stored by the computer that controls the positional rotation of the shadowband. Careful analysis of the data enable the determination of the daily cycle of photosynthetic sunlight and the total column amount of water vapor in the atmosphere. Total ozone can be estimated by comparing the signals in the green and red, near the Chappuis ozone absorption band peaks. (Ultraviolet wavelengths are much better than visible light for measuring ozone, and I have developed a variety of instruments for that purpose. Click here for details.)

Since 1990, I have measured sunlight using a new kind of sun photometer that replaces traditional filtered detectors with ordinary light-emitting diodes (LEDs), which serve as very stable, spectrally selective light sensors. The development of this method to accurately measure sunlight is described in F. M. Mims III, Sun Photometer with Light-Emitting Diodes as Spectrally Selective Detectors, Applied Optics, 31, 6965-6967, 1992. The publications page of my web site lists various other peer-reviewed papers on this topic.

When the MFR-7 was ordered in 1998, Yankee agreed to replace one of the detectors with a red LED. Because the LED drifted less than the filtered detectors, Yankee agreed to replace the remaining six filtered detectors with LEDs. This resulted in the first all-LED shadowband radiometer, which is shown in the photograph after a long day of measuring the sun and sky.

This instrument was taken to Mauna Loa Observatory for calibration in the summer of 2002 and 2003. In March 2004, it was moved from Geronimo Creek Observatory to the newly installed United States Department of Agriculture UV-B and visible sunlight monitoring site on the roof of the Moody Science Building at Texas Lutheran University in Seguin, Texas. I maintain this site for Colorado State University, and after the instrument has been in place for a few years, Dr. James Slusser and I plan to write a paper that describes in detail the operation of this unique kind of sunlight monitoring instrument.

Dr. Slusser heads the USDA UV-B Monitoring and Research Program, a program of the US Department of Agriculture's Cooperative State Research, Education and Extension Service (CSREES). The program, which was begun in 1992, is administered by Colorado State University. It operates a national network of instruments that provide data about the geographical distribution and temporal trends of UV-B (ultraviolet-B) radiation in the United States. For current data at any of the USDA sites, including the one I maintain in South Texas, click here and follow the instructions.