ASTM G197-08

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: G197 − 08
StandardTable for
Reference Solar Spectral Distributions: Direct and Diffuse
on 20° Tilted and Vertical Surfaces
This standard is issued under the fixed designation G197; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
This table of solar spectral irradiance distributions has been developed to meet the need for
weighting functions to be used in evaluating the broadband transmittance of fenestration systems or
the performance of technologies such as building-integrated photovoltaic devices. To compare the
relative optical performance of spectrally sensitive products by theoretical simulation, or to compare
the performance of products by actual testing under laboratory conditions, separate reference standard
solar spectral distributions for direct and diffuse irradiance are required.This table was prepared using
version 2.9.2 of the Simple Model of the Atmospheric Radiative Transfer of Sunshine (SMARTS)
atmospheric transmission code (1, 2). SMARTS uses parameterizations of version 4.0 of the Air
ForceGeophysicalLaboratory(AFGL)ModerateResolutionTransmissionmodel,MODTRAN (3, 4).
An extraterrestrial spectrum differing only slightly from the extraterrestrial spectrum in ASTM E490
is used to calculate the resultant spectra.The directional beam and diffuse hemispherical (2π steradian
acceptance angle) spectral irradiances on planes tilted 20° (representative of a pitched roof slope) and
90° (tilt of a typical wall) to the horizontal are tabulated.The wavelength range for the spectra extends
from 280 nm to 4000 nm, covering the spectral range for which windows or solar collectors respond
to shortwave energy. The input parameters used in conjunction with SMARTS for each set of
conditions are tabulated. The SMARTS model and documentation are available as an adjunct to this
standard.
1. Scope data contained in this table reflect reference spectra with
uniform wavelength interval (0.5 nanometer (nm) below 400
1.1 This table provides terrestrial solar spectral irradiance
nm, 1 nm between 400 and 1700 nm, an intermediate wave-
distributions that may be employed as weighting functions to
length at 1702 nm, and 5 nm intervals from 1705 to 4000 nm).
(1) calculate the broadband solar or light transmittance of
The data table represents reasonable cloudless atmospheric
fenestration from its spectral properties; or (2) evaluate the
conditions favorable for the computerized simulation, com-
performance of building-integrated technologies such as pho-
parative rating, or experimental testing of fenestration systems.
tovoltaic electricity generators. Most of these systems are
installed on vertical walls, but some are also installed on
1.2 Thedatacontainedinthistableweregeneratedusingthe
pitched roofs or on other tilted structures, such as sunspaces.
SMARTS version 2.9.2 atmospheric transmission model de-
Glazing transmittance calculations or measurements require
veloped by Gueymard (1, 2).
information on both the direct and diffuse components of
1.3 The selection of the SMARTS radiative model to
irradiance. The table provides separate information for direct
generate the spectral distributions is chosen for compatibility
and diffuse irradiance, and for two different tilt angles, 20° and
with previous standards (ASTM G173 and G177). The atmo-
90° relative to the horizontal. All distributions are provided at
spheric and climatic conditions are identical to those inASTM
2002 wavelengths within the spectral range 280–4000 nm. The
G173. The environmental conditions are also identical, with
only one exception (see sections 4.3 and X1.2).
ThesetablesareunderthejurisdictionofASTMCommitteeG03onWeathering
1.4 The table defines four solar spectral irradiance distribu-
and Durability and is the direct responsibility of Subcommittee G03.09 on
tions:
Radiometry.
Current edition approved June 1, 2008. Published July 2008. DOI: 10.1520/
1.4.1 Separate direct and diffuse solar spectral irradiance
G0197-08.
incident on a sun-facing, 20° tilted surface in the wavelength
The boldface numbers in parentheses refer to the list of references at the end of
this standard. region from 280–4000 nm for air mass 1.5, at sea level.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G197 − 08
1.4.2 Separate direct and diffuse solar spectral irradiance 3.2.3.1 Discussion—For this standard, the albedo refers to
incident on a sun-facing, 90° (vertical) tilted surface in the the spectral reflectance of the ground relative to hemispherical
wavelength region from 280–4000 nm for air mass 1.5, at sea irradiance. Two different albedos are considered by the model,
level. and both only affect diffuse irradiance. The first albedo
corresponds to the average surface reflectance of the far-field
1.5 The diffuse spectral distribution on a vertical surface
area around the site, within a radius of 5–50 km. The second
facing away from the sun (i.e., shaded), or at any prescribed
albedo is that of the foreground (or near-field) immediately
azimuth away from the sun, may be computed using the model
adjacent to the tilted surface, to a distance of 10–100 m. The
to obtain representative results (i.e., results that fall within an
two albedos can be identical or different, but the foreground
acceptable range of variance).
albedo’s effect significantly increases with tilt angle, whereas
1.6 The climatic, atmospheric, and geometric parameters
the far-field albedo’s effect on diffuse irradiance decreases with
selected reflect the conditions to provide a realistic set of
tilt angle.
spectral distributions appropriate for building applications
3.2.4 integrated irradiance E —spectral irradiance inte-
λ1-λ2
under very clear-sky conditions, representative of near-
grated over a specific wavelength interval from λ to λ ,
maximum solar heat gains in buildings. 1 2
-2
measured in W·m ; mathematically:
1.7 A wide variety of orientations or local environmental
λ2
conditions is possible for exposed surfaces. The availability of
*
E 5 E dλ (1)
the SMARTS model (as an adjunct to this standard) used to λ12λ2 λ
λ1
generate the standard spectra allows users to evaluate spectral
3.2.5 shading—condition that results in partial obscuration
differences relative to the spectra specified here.
of the sky, including the sun or not.
2. Referenced Documents
3.2.5.1 Discussion—For this standard, no shading of the sky
is considered. However, the standard also applies to cases
2.1 ASTM Standards:
when the 20° or 90° tilted surfaces are shaded from the sun
E490 Standard Solar Constant and Zero Air Mass Solar
only, due to their relative geometry or other circumstances.
Spectral Irradiance Tables
This cancels the direct irradiance component, so that the total
E772 Terminology of Solar Energy Conversion
hemispherical irradiance reduces to the diffuse component.
G173 TablesforReferenceSolarSpectralIrradiances:Direct
Normal and Hemispherical on 37° Tilted Surface
3.2.6 solar irradiance, spectral E —solar irradiance E per
λ
G177 Tables for Reference Solar Ultraviolet Spectral Distri-
unit wavelength interval at a given wavelength λ (unit: Watts
-2 -1
butions: Hemispherical on 37° Tilted Surface
per square meter per nanometer, W·m ·nm )
2.2 ASTM Adjuncts:
dE
E 5 (2)
ADJG173CD—SMARTS, Simple Model of the Atmo- λ
dλ
spheric Radiative Transfer of Sunshine, Terrestrial Solar
4 3.2.7 spectral interval—the distance in wavelength units
Spectral Modeling Code
between adjacent spectral irradiance data points.
3. Terminology
3.2.8 spectral passband—the effective wavelength interval
within which spectral irradiance is allowed to pass, as through
3.1 Definitions—Definitions of terms used in this specifica-
a filter or monochromator. The convolution integral of the
tion not otherwise described below may be found in Terminol-
spectral passband (normalized to unity at maximum) and the
ogy E772.
incident spectral irradiance produces the effective transmitted
3.2 Definitions of Terms Specific to This Standard:
irradiance.
3.2.1 aerosol optical depth (AOD)—the wavelength-
3.2.8.1 Discussion—Spectral passband may also be referred
dependent total extinction (scattering and absorption) by aero-
to as the spectral bandwidth of a filter or device. Passbands are
sols in the atmosphere. This optical depth (also called “optical
usually specified as the interval between wavelengths at which
thickness”) is defined here at 500 nm.
one half of the maximum transmission of the filter or device
3.2.2 air mass zero (AM0)—describes solar radiation quan-
occurs, or as full-width at half-maximum, FWHM.
tities outside the Earth’s atmosphere at the mean Earth-Sun
3.2.9 spectral resolution—the minimum wavelength differ-
distance (1 Astronomical Unit). See ASTM E490.
ence between two wavelengths that can be identified unam-
3.2.3 albedo—also called reflectance, a measure of the
biguously.
reflective characteristics of a surface relative to incident
3.2.9.1 Discussion—In the context of this standard, the
irradiance.
spectral resolution is simply the interval, ∆λ, between spectral
data points, or the spectral interval.
3.2.10 spectral solar irradiance, diffuse E —on a given
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
dλ
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
plane, the solar radiant flux at wavelength λ received from
Standards volume information, refer to the standard’s Document Summary page on
within the 2π steradian field of view of a tilted plane from the
the ASTM website.
portion of the sky dome and the foreground included in the
Available on CD-ROM fromASTM International Headquarters. OrderAdjunct
No. ADJG173CD. plane’s field of view, excluding direct solar radiation.
G197 − 08
3.2.11 spectral solar irradiance, direct E —on a given 4.7 The terrestrial solar spectral in the tables have been
Dλ
plane, the solar radiant flux at wavelength λ received directly computed with a spectral bandwidth equivalent to the spectral
from the sun, excluding diffuse solar radiation. resolution of the tables, namely 0.5 nm to 5 nm (see 1.1).
3.2.11.1 Discussion—In practice, instruments measuring di-
4.8 The SMARTS model code and documentation is avail-
rect solar irradiance have a larger acceptance angle than the
able as an adjunct standard. Request ASTM Stock number
apparent diameter of the sun. Consequently, it is virtually
ADJG173CD.
impossible to measure E without the contribution of diffuse
Dλ
radiation emanating from the sun’s aureole. This contribution 5. Significance and Use
is referred to as “circumsolar radiation.” Contrarily to ASTM
5.1 This standard does not purport to address the mean
G173, the data in Table 3 use the ideal definition of direct
spectral irradiance incident on tilted or vertical fenestration or
irradiance, and therefore ignore the circumsolar contribution.
building-integrated systems over a day, a season, or a year.The
The sum of direct and diffuse irradiance is the hemispherical
spectral irradiance distributions have been chosen to represent
solar irradiance, E .
Hλ
a reasonable near-upper limit for solar radiation when these
3.2.12 total ozone—the depth of a column of pure ozone
systems are exposed to clear-sky conditions similar to those
equivalent to the total of the ozone in a vertical column from
used to calculate solar heat loads of buildings. The diffuse
the ground to the top of the atmosphere. (unit: atmosphere-cm)
spectral irradiance distributions can also be used to represent
conditions when these systems are shaded from the direct sun.
3.2.13 total precipitable water—the depth of a column of
water (with a section of 1 cm ) equivalent to the condensed
5.2 Absorptance, reflectance, and transmittance of solar
water vapor in a vertical column from the ground to the top of
radiation are important factors in studies of light transmission
the atmosphere. (unit: cm or g/cm )
through semi-transparent plates. These properties are normally
3.2.14 wavenumber—a unit of frequency, í, in units of functions of wavelength, which require that the spectral distri-
-1
reciprocal centimeters (symbol cm ) commonly used in place bution of the solar flux be known before the solar-weighted
property can be calculated.
of wavelength, λ (units of length, typically nanometers). To
7 -1
convert wavenumbers to nanometers, λ nm = 1·10 /í cm .
5.3 To compare the relative performance of competitive
products by computerized simulations, or to compare the
4. Technical Bases for the Tables
performance of products subjected to experimental tests in
4.1 These tables are modeled data generated using an air laboratory conditions, a reference standard solar spectral dis-
mass zero (AM0) spectrum based on the extraterrestrial spec- tribution for both direct and diffuse irradiance is desirable.
trum of Gueymard (1, 2) derived from Kurucz (5), the United
5.4 The table provides appropriate standard spectral irradi-
States Standard Atmosphere of 1976 (USSA) reference Atmo-
ance distributions for determining the relative optical perfor-
sphere (6), the Shettle and Fenn rural aerosol profile (7), and
mance of semi-transparent materials and other systems. The
the SMARTS version 2.9.2 radiative transfer code. Further
tablemaybeusedtoevaluatecomponentsandmaterialsforthe
details are provided in ASTM G173.
purpose of solar simulation where the direct and the diffuse
4.2 The 20° tilted surface closely represents the geometry of spectral solar irradiances are needed separately.
pitched roofs or tilted glazed structures, which are common
5.5 The selected air mass value of 1.5 for a plane-parallel
elements of buildings. The 90° (vertical) surface represents the
atmosphere above a flat earth corresponds to a zenith angle of
norm for building walls.
48.19°. The SMARTS2 computation of air mass accounts for
4.3 The tabulated diffuse irradiance is a combination of atmospheric curvature and the vertical density profile of
scattered irradiance from the sky and reflected irradiance from molecules, which results in a solar zenith angle of 48.236°, or
far-field and near-field ground surfaces. Dry soil conditions
an equivalent plane-parallel-atmosphere air mass of 1.50136.
have been chosen for both surfaces. This is a darker, less The angle of incidence computed by SMARTS for the direct
reflective surface than what was used inASTM G173 or G177. beam irradiance incident on a 20°-tilted plane facing the sun is
thus 28.236°. It is 41.764° for a 90°-
...