ASTM G197-08(2012)

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. The angle of incidence computed by SMARTS for the direct beam irradiance incident on a 20°-tilted plane facing the sun is thus 28.236°. It is 41.764° for a 90°-tilted surface facing the sun.

5.6 A plot of the SMARTS model output for the reference direct radiation on a 20° and 90° tilted surfaces is shown in 5.7 The input needed by SMARTS to generate the spectra for the prescribed conditions and the 20°-tilted surface is provided in Table 1. The input file for the 90°-tilted surface differs only by one line. This modified line appears in 5.8 The total irradiance, integrated over the spectral range 280–4000 nm, is 791.07 and 97.96 W·m^-2 for direct and diffuse radiation incident on the 20° tilted surface, respectively. It is 669.74 and 140.56 W·m^-2 for direct and diffuse radiation incident on the 90° tilted surface, respectively.

5.9 The availability of the adjunct standard computer software for SMARTS allows one to (a) reproduce the reference spectra, using the above input parameters; (b) compute test spectra to attempt to match measured data at a specified FWHM, and evaluate atmospheric conditions; (c) compute test spectra representing specific conditions for analysis vis-à-vis any one or all of the reference spectra; (d) obtain the sky diffuse and the ground-reflected diffuse spectra (whose sum appears in the table) separately; and (e) smooth the spectral results to different resolution and wavelength step by using the postprocessing options.

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 photovoltaic 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 wavelength 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, comparative rating, or experimental testing of fenestration systems.

1.2 The data contained in this table were generated using the SMARTS version 2.9.2 atmospheric transmission model developed 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 atmospheric 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 The table defines four solar spectral irradiance distributions:

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.