# ASTM G197-14

Designation: G197 − 14Standard Table forReference Solar Spectral Distributions: Direct and Diffuseon 20° Tilted and Vertical Surfaces1This standard is issued under the fixed designation G197; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.INTRODUCTIONThis table of solar spectral irradiance distributions has been developed to meet the need forweighting functions to be used in evaluating the broadband transmittance of fenestration systems orthe performance of technologies such as building-integrated photovoltaic devices. To compare therelative optical performance of spectrally sensitive products by theoretical simulation, or to comparethe performance of products by actual testing under laboratory conditions, separate reference standardsolar spectral distributions for direct and diffuse irradiance are required. This table was prepared usingversion 2.9.2 of the Simple Model of the Atmospheric Radiative Transfer of Sunshine (SMARTS)atmospheric transmission code (1, 2).2SMARTS uses parameterizations of version 4.0 of the AirForce Geophysical Laboratory (AFGL) Moderate Resolution Transmission model, MODTRAN (3, 4).An extraterrestrial spectrum differing only slightly from the extraterrestrial spectrum in ASTM E490is used to calculate the resultant spectra. The directional beam and diffuse hemispherical (2π steradianacceptance angle) spectral irradiances on planes tilted 20° (representative of a pitched roof slope) and90° (tilt of a typical wall) to the horizontal are tabulated. The wavelength range for the spectra extendsfrom 280 nm to 4000 nm, covering the spectral range for which windows or solar collectors respondto shortwave energy. The input parameters used in conjunction with SMARTS for each set ofconditions are tabulated. The SMARTS model and documentation are available as an adjunct to thisstandard.1. Scope1.1 This table provides terrestrial solar spectral irradiancedistributions that may be employed as weighting functions to(1) calculate the broadband solar or light transmittance offenestration from its spectral properties; or (2) evaluate theperformance of building-integrated technologies such as pho-tovoltaic electricity generators. Most of these systems areinstalled on vertical walls, but some are also installed onpitched roofs or on other tilted structures, such as sunspaces.Glazing transmittance calculations or measurements requireinformation on both the direct and diffuse components ofirradiance. The table provides separate information for directand diffuse irradiance, and for two different tilt angles, 20° and90° relative to the horizontal. All distributions are provided at2002 wavelengths within the spectral range 280–4000 nm. Thedata contained in this table reflect reference spectra withuniform wavelength interval (0.5 nanometer (nm) below 400nm, 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 atmosphericconditions favorable for the computerized simulation, com-parative rating, or experimental testing of fenestration systems.1.2 The data contained in this table were generated using theSMARTS version 2.9.2 atmospheric transmission model de-veloped by Gueymard (1, 2).1.3 The selection of the SMARTS radiative model togenerate the spectral distributions is chosen for compatibilitywith previous standards (ASTM G173 and G177). The atmo-spheric and climatic conditions are identical to those in ASTMG173. The environmental conditions are also identical, withonly one exception (see sections 4.3 and X1.2).1.4 The table defines four solar spectral irradiance distribu-tions:1.4.1 Separate direct and diffuse solar spectral irradianceincident on a sun-facing, 20° tilted surface in the wavelengthregion from 280–4000 nm for air mass 1.5, at sea level.1These tables are under the jurisdiction ofASTM Committee G03 on Weatheringand Durability and is the direct responsibility of Subcommittee G03.09 onRadiometry.Current edition approved Dec. 1, 2014. Published December 2014 Last previousedition published 2012 as G197–08(2012). DOI: 10.1520/G0197-14.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States11.4.2 Separate direct and diffuse solar spectral irradianceincident on a sun-facing, 90° (vertical) tilted surface in thewavelength region from 280–4000 nm for air mass 1.5, at sealevel.1.5 The diffuse spectral distribution on a vertical surfacefacing away from the sun (i.e., shaded), or at any prescribedazimuth away from the sun, may be computed using the modelto obtain representative results (i.e., results that fall within anacceptable range of variance).1.6 The climatic, atmospheric, and geometric parametersselected reflect the conditions to provide a realistic set ofspectral distributions appropriate for building applicationsunder very clear-sky conditions, representative of near-maximum solar heat gains in buildings.1.7 A wide variety of orientations or local environmentalconditions is possible for exposed surfaces. The availability ofthe SMARTS model (as an adjunct to this standard) used togenerate the standard spectra allows users to evaluate spectraldifferences relative to the spectra specified here.2. Referenced Documents2.1 ASTM Standards:3E490 Standard Solar Constant and Zero Air Mass SolarSpectral Irradiance TablesE772 Terminology of Solar Energy ConversionG173 Tables for Reference Solar Spectral Irradiances: DirectNormal and Hemispherical on 37° Tilted SurfaceG177 Tables for Reference Solar Ultraviolet Spectral Distri-butions: Hemispherical on 37° Tilted Surface2.2 ASTM Adjuncts:ADJG173CD—SMARTS, Simple Model of the Atmo-spheric Radiative Transfer of Sunshine, Terrestrial SolarSpectral Modeling Code43. Terminology3.1 Definitions—Definitions of terms used in this specifica-tion not otherwise described below may be found in Terminol-ogy E772.3.2 Definitions of Terms Specific to This Standard:3.2.1 aerosol optical depth (AOD)—the wavelength-dependent total extinction (scattering and absorption) by aero-sols in the atmosphere. This optical depth (also called “opticalthickness”) is defined here at 500 nm.3.2.2 air mass zero (AM0)—describes solar radiation quan-tities outside the Earth’s atmosphere at the mean Earth-Sundistance (1 Astronomical Unit). See ASTM E490.3.2.3 albedo—also called reflectance, a measure of thereflective characteristics of a surface relative to incidentirradiance.3.2.3.1 Discussion—For this standard, the albedo refers tothe spectral reflectance of the ground relative to hemisphericalirradiance. Two different albedos are considered by the model,and both only affect diffuse irradiance. The first albedocorresponds to the average surface reflectance of the far-fieldarea around the site, within a radius of 5–50 km. The secondalbedo is that of the foreground (or near-field) immediatelyadjacent to the tilted surface, to a distance of 10–100 m. Thetwo albedos can be identical or different, but the foregroundalbedo’s effect significantly increases with tilt angle, whereasthe far-field albedo’s effect on diffuse irradiance decreases withtilt angle.3.2.4 integrated irradiance Eλ1-λ2—spectral irradiance inte-grated over a specific wavelength interval from λ1to λ2,measured in W·m-2; mathematically:Eλ12λ25λ2*λ1Eλdλ (1)3.2.5 shading—condition that results in partial obscurationof the sky, including the sun or not.3.2.5.1 Discussion—For this standard, no shading of the skyis considered. However, the standard also applies to caseswhen the 20° or 90° tilted surfaces are shaded from the sunonly, due to their relative geometry or other circumstances.This cancels the direct irradiance component, so that the totalhemispherical irradiance reduces to the diffuse component.3.2.6 solar irradiance, spectral Eλ—solar irradiance E perunit wavelength interval at a given wavelength λ (unit: Wattsper square meter per nanometer, W·m-2·nm-1)Eλ5dEdλ(2)3.2.7 spectral interval—the distance in wavelength unitsbetween adjacent spectral irradiance data points.3.2.8 spectral passband—the effective wavelength intervalwithin which spectral irradiance is allowed to pass, as througha filter or monochromator. The convolution integral of thespectral passband (normalized to unity at maximum) and theincident spectral irradiance produces the effective transmittedirradiance.3.2.8.1 Discussion—Spectral passband may also be referredto as the spectral bandwidth of a filter or device. Passbands areusually specified as the interval between wavelengths at whichone half of the maximum transmission of the filter or deviceoccurs, or as full-width at half-maximum, FWHM.3.2.9 spectral resolution—the minimum wavelength differ-ence between two wavelengths that can be identified unam-biguously.3.2.9.1 Discussion—In the context of this standard, thespectral resolution is simply the interval, ∆λ, between spectraldata points, or the spectral interval.3.2.10 spectral solar irradiance, diffuse Edλ—on a givenplane, the solar radiant flux at wavelength λ received fromwithin the 2π steradian field of view of a tilted plane from theportion of the sky dome and the foreground included in theplane’s field of view, excluding direct solar radiation.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.4Available on CD-ROM from ASTM International Headquarters. Order AdjunctNo. ADJG173CD.G197 − 1423.2.11 spectral solar irradiance, direct EDλ—on a givenplane, the solar radiant flux at wavelength λ received directlyfrom the sun, excluding diffuse solar radiation.3.2.11.1 Discussion—In practice, instruments measuring di-rect solar irradiance have a larger acceptance angle than theapparent diameter of the sun. Consequently, it is virtuallyimpossible to measure EDλwithout the contribution of diffuseradiation emanating from the sun’s aureole. This contributionis referred to as “circumsolar radiation.” Contrarily to ASTMG173, the data in Table 3 use the ideal definition of directirradiance, and therefore ignore the circumsolar contribution.The sum of direct and diffuse irradiance is the hemisphericalsolar irradiance, EHλ.3.2.12 total ozone—the depth of a column of pure ozoneequivalent to the total of the ozone in a vertical column fromthe ground to the top of the atmosphere. (unit: atmosphere-cm)3.2.13 total precipitable water—the depth of a column ofwater (with a section of 1 cm2) equivalent to the condensedwater vapor in a vertical column from the ground to the top ofthe atmosphere. (unit: cm or g/cm2)3.2.14 wavenumber—a unit of frequency, í, in units ofreciprocal centimeters (symbol cm-1) commonly used in placeof wavelength, λ (units of length, typically nanometers). Toconvert wavenumbers to nanometers, λ nm = 1·107/í cm-1.4. Technical Bases for the Tables4.1 These tables are modeled data generated using an airmass zero (AM0) spectrum based on the extraterrestrial spec-trum of Gueymard (1, 2) derived from Kurucz (5), the UnitedStates Standard Atmosphere of 1976 (USSA) reference Atmo-sphere (6), the Shettle and Fenn rural aerosol profile (7), andthe SMARTS version 2.9.2 radiative transfer code. Furtherdetails are provided in ASTM G173.4.2 The 20° tilted surface closely represents the geometry ofpitched roofs or tilted glazed structures, which are commonelements of buildings. The 90° (vertical) surface represents thenorm for building walls.4.3 The tabulated diffuse irradiance is a combination ofscattered irradiance from the sky and reflected irradiance fromfar-field and near-field ground surfaces. Dry soil conditionshave been chosen for both surfaces. This is a darker, lessreflective surface than what was used in ASTM G173 or G177.4.4 The documented USSA atmospheric profiles utilized inthe MODTRAN spectral transmission model (6) have beenused to provide atmospheric properties and concentrations ofabsorbers.4.5 To provide spectral data with a uniform spectral stepsize, the AM0 spectrum used in conjunction with SMARTS togenerate the terrestrial spectrum is slightly different from theASTM extraterrestrial spectrum, ASTM E490. Because ASTME490 and SMARTS both use the data of Kurucz (5), theSMARTS and E490 spectra are in excellent agreement al-though they do not have the same spectral resolution.4.6 The current spectra reflect improved knowledge ofatmospheric aerosol optical properties, transmission properties,and radiative transfer modeling (8).4.7 The terrestrial solar spectral in the tables have beencomputed with a spectral bandwidth equivalent to the spectralresolution of the tables, namely 0.5 nm to 5 nm (see 1.1).4.8 The SMARTS model code and documentation is avail-able as an adjunct standard. Request ASTM Stock numberADJG173CD.5. Significance and Use5.1 This standard does not purport to address the meanspectral irradiance incident on tilted or vertical fenestration orbuilding-integrated systems over a day, a season, or a year. Thespectral irradiance distributions have been chosen to representa reasonable near-upper limit for solar radiation when thesesystems are exposed to clear-sky conditions similar to thoseused to calculate solar heat loads of buildings. The diffusespectral irradiance distributions can also be used to representconditions when these systems are shaded from the direct sun.5.2 Absorptance, reflectance, and transmittance of solarradiation are important factors in studies of light transmissionthrough semi-transparent plates. These properties are normallyfunctions of wavelength, which require that the spectral distri-bution of the solar flux be known before the solar-weightedproperty can be calculated.5.3 To compare the relative performance of competitiveproducts by computerized simulations, or to compare theperformance of products subjected to experimental tests inlaboratory 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. Thetable may be used to evaluate components and materials for thepurpose of solar simulation where the direct and the diffusespectral solar irradiances are needed separately.5.5 The selected air mass value of 1.5 for a plane-parallelatmosphere above a flat earth corresponds to a zenith angle of48.19°. The SMARTS2 computation of air mass accounts foratmospheric curvature and the vertical density profile ofmolecules