4.1 Absorptance, reflectance, and transmittance of solar energy are important factors in material degradation studies, solar thermal system performance, solar photovoltaic system performance, biological studies, and solar simulation activities. These optical properties are normally functions of wavelength, which require the spectral distribution of the solar flux be known before the solar-weighted property can be calculated. To compare the relative performance of competitive products, or to compare the performance of products before and after being subjected to weathering or other exposure conditions, a reference standard solar spectral distribution is desirable.

4.2 These tables provide appropriate standard spectral irradiance distributions for determining the relative optical performance of materials, solar thermal, solar photovoltaic, and other systems. The tables may be used to evaluate components and materials for the purpose of solar simulation where either the direct or the hemispherical (that is, direct beam plus diffuse sky) spectral solar irradiance is desired. However, these tables are not intended to be used as a benchmark for ultraviolet radiation used in indoor exposure testing of materials using manufactured light sources.

4.3 The total integrated irradiances for the direct and hemispherical tilted spectra are 900.1 W·m^-2 and 1000.4 W·m^-2, respectively. Note that, in PV applications, no amplitude adjustments are required to match standard reporting condition irradiances of 1000 W·m^-2 for hemispherical irradiance.

4.4 Previously defined global hemispherical reference spectrum (G159) for a sun-facing 37°-tilted surface served well to meet the needs of the flat plate photovoltaic research, development, and industrial community. Investigation of prevailing conditions and measured spectra shows that this global hemispherical reference spectrum can be attained in practice under a variety of conditions, and that these conditions can be interpreted as representative for many combinations of atmospheric parameters. Earlier global hemispherical reference spectrum may be closely, but not exactly, reproduced with improved spectral wavelength range, uniform spectral interval, and spectral resolution equivalent to the spectral interval, using inputs in X1.4.

4.5 Reference spectra generated by the SMARTS Version 2.9.2 model for the indicated conditions are shown in Fig. 1. The exact input file structure required to generate the reference spectra is shown in Table 1.

FIG. 1 Plot of Direct Normal Spectral Irradiance (Solid Line) and Hemispherical Spectral Irradiance on 37° Tilted Sun-Facing Surface (Dotted Line) Computed Using Smarts Version 2.9.2 Model With Input File in Table 1TABLE 1 SMARTS Version 2.9.2 Input File to Generate the Reference Spectra

4.6 The availability of the adjunct (ADJG173CD³) standard computer software for SMARTS allows one to (1) reproduce the reference spectra, using the above input parameters; (2) compute test spectra to attempt to match measured data at a specified FWHM, and evaluate atmospheric conditions; and (3) compute test spectra representing specific conditions for analysis vis-à-vis any one or all of the reference spectra.

4.7 Differences from the previous standard spectra (G159) can be summarized as follows:

4.7.1 Extended spectral interval in the ultraviolet (down to 280 nm, rather than 305 nm),

4.7.2 Better resolution (2002 wavelengths, as compared to 120),

4.7.3 Constant intervals (0.5 nm below 400 nm, 1 nm between 400 and 1700 nm, and 5 nm above),

4.7.4 Better definition of atmospheric scattering and gaseous absorption, with more species considered,

4.7.5 Better defined extraterrestrial spectrum,

4.7.6 More realistic spectral ground reflectance, and

4.7.7 Lower aerosol optical depth, yielding significantly larger direct normal irradiance.