ASTM G197-14(R2021) pdf free download

ASTM G197-14(R2021) pdf free download.Standard Table for Reference Solar Spectral Distributions: Direct and Diffuse on 20° Tilted and Vertical Surfaces
1. Scope
1.1 This table provides terrestrial solar spectral irradiance distributions that may be employed as weighting functions to (1) calculate the broadband solar or light transmittance of fenestration from its spectral properties; or (2) evaluate the performance of building-integrated technologies such as pho- tovoltaic electricity generators. Most of these systems are installed on vertical walls, but some are also installed on pitched roofs or on other tilted structures, such as sunspaces. Glazing transmittance calculations or measurements require information on both the direct and diffuse components of irradiance. The table provides separate information for direct and diffuse irradiance, and for two different tilt angles, 20° and 90° relative to the horizontal. All distributions are provided at 2002 wavelengths within the spectral range 280–4000 nm. The data contained in this table reflect reference spectra with uniform wavelength interval (0.5 nanometer (nm) below 400 nm, 1 nm between 400 and 1700 nm, an intermediate wave- length at 1702 nm, and 5 nm intervals from 1705 to 4000 nm). The data table represents reasonable cloudless atmospheric conditions favorable for the computerized simulation, com- parative rating, or experimental testing offenestration systems. 1.2 The data contained in this table were generated using the SMARTS version 2.9.2 atmospheric transmission model de- veloped by Gueymard (1, 2). 1.3 The selection of the SMARTS radiative model to generate the spectral distributions is chosen for compatibility with previous standards (ASTM G173 and G177). The atmo- spheric and climatic conditions are identical to those in ASTM G173. The environmental conditions are also identical, with only one exception (see sections 4.3 and X1.2).1.4.2 Separate direct and diffuse solar spectral irradiance incident on a sun-facing, 90° (vertical) tilted surface in the wavelength region from 280–4000 nm for air mass 1.5, at sea level. 1.5 The diffuse spectral distribution on a vertical surface facing away from the sun (i.e., shaded), or at any prescribed azimuth away from the sun, may be computed using the model to obtain representative results (i.e., results that fall within an acceptable range of variance). 1.6 The climatic, atmospheric, and geometric parameters selected reflect the conditions to provide a realistic set of spectral distributions appropriate for building applications under very clear-sky conditions, representative of near- maximum solar heat gains in buildings. 1.7 A wide variety of orientations or local environmental conditions is possible for exposed surfaces. The availability of the SMARTS model (as an adjunct to this standard) used to generate the standard spectra allows users to evaluate spectral differences relative to the spectra specified here. 1.8 This international standard was developed in accor- dance with internationally recognized principles on standard- ization established in the Decision on Principles for the Development of International Standards, Guides and Recom- mendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
5. Significance and Use
5.1 This standard does not purport to address the mean spectral irradiance incident on tilted or vertical fenestration or building-integrated systems over a day, a season, or a year. The spectral irradiance distributions have been chosen to represent a reasonable near-upper limit for solar radiation when these systems are exposed to clear-sky conditions similar to those used to calculate solar heat loads of buildings. The diffuse spectral irradiance distributions can also be used to represent conditions when these systems are shaded from the direct sun. 5.2 Absorptance, reflectance, and transmittance of solar radiation are important factors in studies of light transmission through semi-transparent plates. These properties are normally functions of wavelength, which require that the spectral distri- bution of the solar flux be known before the solar-weighted property can be calculated. 5.3 To compare the relative performance of competitive products by computerized simulations, or to compare the performance of products subjected to experimental tests in laboratory conditions, a reference standard solar spectral dis- tribution for both direct and diffuse irradiance is desirable. 5.4 The table provides appropriate standard spectral irradi- ance distributions for determining the relative optical perfor- mance of semi-transparent materials and other systems. The table may be used to evaluate components and materials for the purpose of solar simulation where the direct and the diffuse spectral solar irradiances are needed separately. 5.5 The selected air mass value of 1.5 for a plane-parallel atmosphere above a flat earth corresponds to a zenith angle of 48.19°. The SMARTS2 computation of air mass accounts for atmospheric curvature and the vertical density profile of molecules, which results in a solar zenith angle of 48.236°, or an equivalent plane-parallel-atmosphere air mass of 1.50136.