18 June 2004

Monitoring light from the Sun and sky

Forrest M. Mims III

Shown here is the business portion of a Yankee Shadowband Radiometer installed at Geronimo Creek Observatory in September 1998. The instrument, which was donated by a power company, has since been moved to the new United States Department of Agriculture UV-B monitoring station on a roof at nearby Texas Lutheran University.

Shadowband radiometers measure light from the Sun and the entire sky when the shadow band is down. When the band casts a shadow over the light-sensitive diffuser atop the temperature-regulated canister at right, the instrument measures only the light from the sky. Subtracting the latter from the former gives the amount of light arriving directly from the Sun.

The instrument shown here measures seven wavelengths of light, from the blue to the near infrared. Originally, all the wavelengths but one were detected by filtered silicon photodiodes. The seventh wavelength was monitored by a red light-emitting diode (LED) that I asked to be installed. The stability of this LED proved so good that the six photodiodes were replaced by LEDs.

This instrument is the only known shadowband radiometer that uses LEDs as detectors. It has been compared with filter shadowband radiometers at Mauna Loa Observatory, Hawaii, in 2002 and 2003.