Modeling a Solar Source
How can I model a solar source?
The method for defining a solar source in FRED varies depending on the required accuracy of the model. Parameterizing the source depends on the time of year, time of day, longitude and latitude, weather conditions, etc. In its simplest form, the procedure below can be effective:
- Create a plane source with randomized ray positions and directions into an angular range (the subtense of the solar disk, about 0.5o full angle).
- Add the wavelengths and weighting appropriate for a blackbody source such as the sun. This can be accomplished by creating a blackbody spectrum. See FRED’s Online Help topic, “Spectrum – Blackbody” for details.
- Scale the wavelength weights by the atmospheric transmission values to account for weather or regional conditions.
- Set the power of the source in the Power tab appropriately for the solar irradiance value and the size of the system aperture.
More complex models can be achieved by using theory such as the Bird Model ( www.nrel.gov). The diffuse radiance from the atmosphere can be considered isotropic and specific data can be obtained regarding the wavelength spectrum.
As of version v18.11 there is a Source Primitive that defines a simple solar source, in much the same was as discussed above.
The Solar Source (simple) type Source Primitive model represents an aperture that is being irradiated by direct solar illumination. A cone of rays with a 0.267 degree semi-angle is emitted from each point within the aperture and represents the angular subtense of the solar disk. The radiometric power illuminating the aperture is indirectly specified by providing the irradiance at the source aperture in units of W/m2. A common, but not universal, value for the direct solar irradiance at the top of the Earth atmosphere is 1360 W/m2, assuming an idealized blackbody at 5770K over a spectral band from 0.1 - 5.0 microns.