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 distribution 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 distribution for both direct and diffuse irradiance is desirable.
5.4 The table provides appropriate standard spectral irradiance distributions for determining the relative optical performance 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.
| Card ID | Value | Parameter/Description/Variable Name |
| 1 | 'ASTM_Tilted_Std_Spectra' | Header |
| 2 | 1 | Pressure input mode (1 = pressure and altitude): ISPR |
| 2a | 1013.25 0.0. | Station Pressure (mb) & altitude (km): SPR, ALT |
| 3 | 1 | Standard Atmosphere Profile Selection (1 = use default atmosphere): IATM1 |
| 3a | 'USSA' | Default Standard Atmosphere Profile: ATM |
| 4 | 1 | Water Vapor Input (1 = default from Atmospheric Profile): IH2O |
| 5 | 1 | Ozone Calculation (1 = default from Atmospheric Profile): IO3 |
| 6 | 1 | Pollution level mode (1 = standard conditions/no pollution): IGAS (see Appendix C) |
| 7 | 370 | Carbon Dioxide volume mixing ratio (ppm): qCO2 (see Appendix C) |
| 7a | 1 | Extraterrestrial Spectrum (1 = SMARTS/Gueymard): ISPCTR |
| 8 | 'S&F_RURAL' | Aerosol Profile to Use: AEROS |
| 9 | 0 | Specification for aerosol optical depth/turbidity input (0 = AOD at 500 nm): ITURB |
| 9a | 0.084 | Aerosol Optical Depth @ 500 nm: TAU5 |
| 10 | 37 | Far field Spectral Albedo file to use (38= Light Sandy Soil): IALBDX |
| 10b | 1 | Specify tilt calculation (1 = yes): ITILT |
| 10c | 37 20 180 | Albedo and Tilt variables—Albedo file to use for near field, Tilt, and Azimuth: IALBDG, TILT, WAZIM |
| 11 | 280 4000 1.0 1367.0 | Wavelength Range—start, stop, mean radius vector correction, integrated solar spectrum irradiance: WLMN, WLMX, SUNCOR, SOLARC |
| 12 | 2 | Separate spectral output file print mode (2 = yes): IPRT |
| 12a | 280 4000 0.5 | Output file wavelength—Print limits, start, stop, minimum step size: WPMN, WPMX, INTVL |
| 12b | 2 | Number of output variables to print: IOTOT |
| 12c | 6 7 | Code relating output variables to print [6 = Direct tilt, OUT(6); 7 = Diffuse tilt, OUT(7)] |
| 13 | 0 | Circumsolar calculation mode (1 = yes): ICIRC |
| 14 | 0 | Smooth function mode (0 = none): ISCAN |
| 15 | 0 | Illuminance calculation mode (0 = none): ILLUM |
| 16 | 0 | UV calculation mode (0 = none): IUV |
| 17 | 2 | Solar Geometry mode (2 = Air Mass): IMASS |
| 17a | 1.5 | Air mass value: AMASS |
| Card ID | Value | Parameter/Description/Variable Name |
| 10c | 37 90 180 | Albedo and Tilt variables—Albedo file to use for near field, Tilt, and Azimuth:IALBDG, TILT, WAZIM |
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 Fig. 1. A similar plot, but for diffuse radiation, is shown in Fig. 2.
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 Table 2.
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.
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 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).
