ASTM E424-71(2007)
(Test Method)Standard Test Methods for Solar Energy Transmittance and Reflectance (Terrestrial) of Sheet Materials
Standard Test Methods for Solar Energy Transmittance and Reflectance (Terrestrial) of Sheet Materials
SIGNIFICANCE AND USE
Solar-energy transmittance and reflectance are important factors in the heat admission through fenestration, most commonly through glass or plastics. (See Appendix X3.) These methods provide a means of measuring these factors under fixed conditions of incidence and viewing. While the data may be of assistance to designers in the selection and specification of glazing materials, the solar-energy transmittance and reflectance are not sufficient to define the rate of heat transfer without information on other important factors. The methods have been found practical for both transparent and translucent materials as well as for those with transmittances reduced by highly reflective coatings. Method B is particularly suitable for the measurement of transmittance of inhomogeneous, patterned, or corrugated materials since the transmittance is averaged over a large area.
SCOPE
1.1 These test methods cover the measurement of solar energy transmittance and reflectance (terrestrial) of materials in sheet form. Method A, using a spectrophotometer, is applicable for both transmittance and reflectance and is the referee method. Method B is applicable only for measurement of transmittance using a pyranometer in an enclosure and the sun as the energy source. Specimens for Method A are limited in size by the geometry of the spectrophotometer while Method B requires a specimen 0.61 m2 (2 ft2). For the materials studied by the drafting task group, both test methods give essentially equivalent results.
1.2 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:E424 −71(Reapproved 2007)
Standard Test Methods for
Solar Energy Transmittance and Reflectance (Terrestrial) of
Sheet Materials
This standard is issued under the fixed designation E424; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope 3. Definitions
1.1 These test methods cover the measurement of solar
3.1 solar absorptance—the ratio of absorbed to incident
energytransmittanceandreflectance(terrestrial)ofmaterialsin
radiant solar energy (equal to unity minus the reflectance and
sheetform.MethodA,usingaspectrophotometer,isapplicable
transmittance).
for both transmittance and reflectance and is the referee
3.2 solar admittance—solar heat transfer taking into ac-
method. Method B is applicable only for measurement of
count reradiated and convected energy.
transmittance using a pyranometer in an enclosure and the sun
as the energy source. Specimens for Method A are limited in
3.3 solar energy—for these methods the direct radiation
sizebythegeometryofthespectrophotometerwhileMethodB from the sun at sea level over the solar spectrum as defined in
2 2
requires a specimen 0.61 m (2 ft ). For the materials studied
3.2, its intensity being expressed in watts per unit area.
by the drafting task group, both test methods give essentially
3.4 solar reflectance—the percent of solar radiation (watts/
equivalent results.
unit area) reflected by a material.
1.2 This standard does not purport to address all of the
3.5 solar spectrum—for the purposes of these methods the
safety problems, if any, associated with its use. It is the
solar spectrum at sea level extending from 350 to 2500 nm.
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
3.6 solar transmittance—the percent of solar radiation
bility of regulatory limitations prior to use.
(watts/unit area) transmitted by a material.
2. Referenced Documents
4. Summary of Methods
2.1 ASTM Standards:
4.1 Method A—Measurements of spectral transmittance, or
E259Practice for Preparation of Pressed Powder White
reflectance versusamagnesiumoxidestandard,aremadeusing
Reflectance Factor Transfer Standards for Hemispherical
anintegratingspherespectrophotometeroverthespectralrange
and Bi-Directional Geometries
from350to2500nm.Theilluminationandviewingmodeshall
E275PracticeforDescribingandMeasuringPerformanceof
be normal-diffuse or diffuse-normal. The solar energy trans-
Ultraviolet and Visible Spectrophotometers
mitted or reflected is obtained by integrating over a standard
E308PracticeforComputingtheColorsofObjectsbyUsing
solar energy distribution curve using weighted or selected
the CIE System
ordinates for the appropriate solar-energy distribution. The
distribution at sea level, air mass 2, is used.
These test methods are under the jurisdiction of ASTM Committee E44 on
Solar, Geothermal and Other Alternative Energy Sources and is the direct respon- 4.2 Method B—Using the sun as the source and a pyranom-
sibility of Subcommittee E44.05 on Solar Heating and Cooling Systems and
eter as a detector the specimen is made the cover of an
Materials.
enclosure with the plane of the specimen perpendicular to the
Current edition approved March 1, 2007. Published April 2007. Originally
incidentradiation;transmittanceismeasuredastheratioofthe
approved in 1971. Last previous edition approved in 2001 as E424-71(2001). DOI:
10.1520/E0424-71R07.
energy transmitted to the incident energy. (The apparatus of
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Method B has been used for the measurement of solar-energy
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
reflectance but there is insufficient experience with this tech-
Standards volume information, refer to the standard’s Document Summary page on
nique for standardization at present.)
the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E424−71 (2007)
5. Significance and Use is recommended that the specimen be placed in direct contact
with the sphere to minimize and control loss of scattered
5.1 Solar-energytransmittanceandreflectanceareimportant
radiation.
factors in the heat admission through fenestration, most com-
6.2.3 For specularly reflecting specimens the sphere
monly through glass or plastics. (See Appendix X3.) These
conditions, especially where the reflected beam strikes the
methods provide a means of measuring these factors under
sphere wall, shall be known to be highly reflecting (95% or
fixed conditions of incidence and viewing.While the data may
higher).Itisrecommendedthatafreshlycoatedspherebeused
be of assistance to designers in the selection and specification
especially when measuring translucent or specularly reflecting
of glazing materials, the solar-energy transmittance and reflec-
specimens.
tance are not sufficient to define the rate of heat transfer
without information on other important factors. The methods
6.3 Calibration:
have been found practical for both transparent and translucent
6.3.1 Photometric—Thecalibrationofthephotometricscale
materials as well as for those with transmittances reduced by
shall be done as recommended by the manufacturer. It shall be
highlyreflectivecoatings.MethodBisparticularlysuitablefor
carefully executed at reasonable time intervals to ensure
the measurement of transmittance of inhomogeneous,
accuracy over the entire range.
patterned, or corrugated materials since the transmittance is
6.3.2 Wavelength—Periodic calibrations should be made of
averaged over a large area.
the wavelength scales. Procedures for wavelength calibration
may be found in Recommended Practice E275. A didymium
6. Method A—Spectrophotometric Method
filter has also been used for this purpose. Although the
absorption peaks have been defined for specific resolution in
6.1 Apparatus:
the visible spectrum it also has peaks in the near infrared;
6.1.1 Spectrophotometer—An integrating sphere
however, the wavelength of the peaks must be agreed upon,
spectrophotometer, by means of which the spectral character-
using a specific instrument.
istics of the test specimen or material may be determined
throughout the solar spectrum. For some materials the spec-
6.4 Procedure:
trumregionfrom350to1800nmmaybesufficient.Thedesign
6.4.1 Transmittance—Obtain spectral transmittance data
shallbesuchthatthespecimenmaybeplacedindirectcontact
relativetoair.Formeasurementoftransmittanceoftranslucent
with the sphere aperture for both transmission and reflection,
specimens, place freshly prepared matched smoked MgO
sothattheincidentradiationiswithin6°ofperpendicularityto
surfaces at the specimen and reference ports at the rear of the
the plane of the specimen.
sphere (Note 1). The interior of the sphere should be freshly
6.1.2 Standards:
coated with MgO and in good condition.
6.1.2.1 For transmitting specimens, incident radiation shall
NOTE1—Magnesiumoxidestandardsmaybeconsideredmatchedifon
be used as the standard relative to which the transmitted light
interchanging them the percent reflectance is altered by no more than 1%
is evaluated. Paired reflecting standards are used, prepared in
at any wavelength between 350 and 1800 nm.
duplicate as described below.
6.4.2 Reflectance—Obtain spectral directional reflectance
6.1.2.2 For reflecting specimens, use smoked magnesium
data relative to MgO. Include the specular component in the
oxide (MgO) as a standard as the closest practicable approxi-
reflectance measurement. Back the test specimen with a black
mation of the completely reflecting, completely diffusing
diffuse surface if it is not opaque. Depending on the required
surface for the region from 300 to 2100 nm. The preferred
accuracy,usethemeasuredvaluesdirectlyormakecorrections
standard is a layer (at least 2.0 mm in thickness) freshly
for instrumental 0 and 100% lines (see Method E308).
prepared from collected smoke of burning magnesium (Rec-
ommend Practice E259). Pressed barium sulfate (BaSO)or
6.5 Calculation—Solar energy transmittance or reflectance
MgO are not recommended because of poor reflecting proper-
is calculated by integration.The distribution of solar energy as
ties beyond 1000 nm.
reported by Parry Moon for sea level and air mass 2 shall be
6.1.3 Specimen Backing for Reflectance Measurement—
used.
Transparent and translucent specimens shall be backed by a
6.5.1 Weighted Ordinates—Obtain the total solar energy
light trap or a diffusing black material which is known to
transmittance, T , and reflectance, R , in percent, by integrat-
se se
absorbthenearinfrared.Thebackingshallreflectnomorethan
ing the spectral transmittance (reflectance) over the standard
1% at all wavelengths from 350 to 2500 nm as determined
solar energy distribution as follows:
using the spectrophotometer.
λ52100 nm
T or R 5 T or R 3 E (1)
~ !
se se λ λ λ
(λ5350nm
6.2 Test Specimens:
6.2.1 Opaque specimens shall have at least one plane
Eλ for air mass 2, at 50-nm intervals, normalized to 100, is
surface; transparent and translucent specimens shall have two
given in Appendix X1.
surfaces that are essentially plane and parallel.
6.5.1.1 This integration is easily programmed for automatic
6.2.2 Comparison of translucent materials is highly depen-
computation.
dent on the geometry of the specific instrument being used. It
Journal of the Franklin Institute, Vol 230, 1940, p. 583, or Smithsonian
For additional apparatus specifications see Recommended Practice E308. Physical Tables, Table 1, Vol 815, 1954, p. 273.
E424−71 (2007)
6.5.2 Selected Ordinates—Integrationisdonebyreadingthe (280 to 2800 nm), thus encompassing all the solar spectrum.
transmittance or reflectance at selected wavelengths and cal- The pyranometer should be located inside the box so that the
culating their average. Appendix X2 lists 20 selected ordinates sensing thermopile is approximately 50 mm (2 in.) from the
for integration. center of the bottom plane of the sample.
7.1.2.2 The pyranometer has a viewing area of 180°. An
6.6 Report—The report shall include the following:
Eppley pyranometer wi
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