ASTM G173-03(2020)

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.

1.1 These tables contain terrestrial solar spectral irradiance distributions for use in terrestrial applications that require a standard reference spectral irradiance for hemispherical solar irradiance (consisting of both direct and diffuse components) incident on a sun-facing, 37° tilted surface or the direct normal spectral irradiance. The data contained in these tables reflect reference spectra with uniform wavelength interval (0.5 nanometer (nm) below 400 nm, 1 nm between 400 and 1700 nm, an intermediate wavelength at 1702 nm, and 5 nm intervals from 1705 to 4000 nm). The data tables represent reasonable cloudless atmospheric conditions favorable for photovoltaic (PV) energy production, as well as weathering and durability exposure applications.

1.2 The 37° slope of the sun-facing tilted surface was chosen to represent the average latitude of the 48 contiguous United States. A wide variety of orientations is possible for exposed surfaces. The availability of the SMARTS model (as an adjunct, ADJG173CD3) to this standard) used to generate the standard spectra allows users to evaluate differences relative to the surface specified here.

1.3 The air mass and atmospheric extinction parameters are chosen to provide (1) historical continuity with respect to previous standard spectra, (2) reasonable cloudless atmospheric conditions favorable for photovoltaic (PV) energy production or weathering and durability exposure, based upon modern broadband solar radiation data, atmospheric profiles, and improved knowledge of aerosol optical depth profiles. In nature, an extremely large range of atmospheric conditions can be encountered even under cloudless skies. Considerable departure from the reference spectra may be observed depending on time of day, geographical location, and changing atmospheric conditions. The availability of the SMARTS model (as an adjunct (ADJG173CD3)to this standard) used to generate the standard spectra allows users to evaluate spectral differences relative to the spectra specified here.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.