Rapid advances in various spectrally-selective technologies, such as PV and multi-coated glazings, have put a lot of pressure on the solar radiation community to provide demanding measurements of the solar spectrum, as well as appropriate models to predict its variations. Only a very few institutions, such as NREL in Colorado, are actually measuring spectral irradiance on a permanent basis. Most other institutions measuring the solar spectrum do it for experimental reasons, and therefore on a sporadic basis. Indeed, obtaining good-quality spectra is not easy because considerable resources are required: costly instrumentation, frequent and expensive recalibrations, and highly skilled personnel. All these conditions greatly limit the availability of the reference spectral irradiance databanks that are necessary to serve the development of spectrally-selective technologies.
This lack of measurements can be compensated for in large part by the use of appropriate radiative transfer models. A variety of such models have been developed for the needs of atmospheric scientists and of the remote sensing and climate change communities. Examples of such models include MODTRAN, SBDART, libRadtran, and 6S. There are many reasons, however, why such models are not convenient for the engineering applications envisioned here. One essential drawback of these models, besides their complexity and considerable execution time, is that they do not address the essential case of spectral irradiance incident on tilted surfaces.
Since its development began many years ago, the SMARTS model's conceptual idea has always been to offer fast and accurate predictions of spectral irradiance on any tilted surface without the difficulties and limitations associated with the atmospheric models mentioned above. The SMARTS model is now used by an estimated 6000 scientists worldwide, for a large variety of applications. This is actually made possible by the model's versatility and accuracy, which have been discussed in recent scientific papers , . Although the model currently accommodates the case of cloudless skies only, it is hoped that funding will become eventually available to expand its scope through the development of an all-sky version, which could be used to simulate the yearly performance of spectrally-selective devices, for instance.
Version 2.9.2 of the model is freely available in two different packages (PC and Mac-Classic) from http://www.nrel.gov/rredc/smarts/.
A newer and much expanded version, 2.9.5, is also available for three platforms (Linux, Mac-OSX and PC-Windows). It
can be downloaded here.
Among other refinements and improvements, this latest version has the option to use an ISO-sanctioned
extraterrestrial spectrum, which can de obtained here. Windows-only graphic interfaces are also available for both version 2.9.2 and 2.9.5, making them easier to use by novice users.
SMARTS v2.9.2 has been used to produce Reference terrestrial spectra for standardization purposes, including ASTM Standards G173, G177 and G197.
Many applications involve the prediction of the components (direct, diffuse, or global) of the solar spectrum under ideal or realistic conditions. In the latter case, the accurracy of the modeled spectra is essentially constrained by that of the input data (e.g., aerosol optical depth, precipitable water, or ozone amount). Such realistic predictions can be compared to actual measurements, as shown in the figures above.
More Technical Details
 C.A. Gueymard, Interdisciplinary applications of a versatile spectral solar irradiance model: A review. Energy, vol. 30, 1551-1576 (2005).