An Instrument Design Concept for Measuring Solar Diffuse Irradiance
| Rutledge, Charles | NASA Langley Research Center |
| Schuster, Greg | NASA Langley Research Center |
Category: Instruments
Recent effort towards the development of a diffuse horizontal solar irradiance standard group [Michalsky et.al. 2005] using well calibrated pyranometers suggested that inter-instrument differences in cosine response characteristics may be problematic. They showed a calibration method using overcast skies (an approximately uniform radiance distribution) produced superior diffuse radiometer performance when compared to a calibration method using clear skies (which have a radiance distribution systematically varying from horizon to the near vicinity of the sun). The proposed instrument offers a significantly different basic design compared to pyranometers and specifically responds to the cosine response problem. The key components of the proposed instrument includes dual integrating spheres as fore-optics, an electrically calibrated pyroelectric absolute radiometer and a monochromator. A near ideal cosine response may be accomplished using a tapered edge to make up the circular input port of the primary integrating sphere. The dual sphere design offers an optimally homogeneous radiance target for the detector. The electrically calibrated pyroelectric detector is an absolute radiometer using uncertainties associated with electrical not optical standards. A dynasil window located in the exit port of the secondary sphere will have platinum resistance thermistor to allow for the correct removal of longwave radiation from the registered signal. The monochromator will provide spectral shape only to allow for frequency dependent “unfiltering” of the reflectance and transmittance curves associated with the integrating spheres and the optical windows. Optical modeling is presently beginning to determine size parameters and optical feasibility.
This poster will be displayed at the ARM Science Team Meeting.
