ASTM E424-71(1993)e1

Standard Test Methods for Solar Energy Transmittance and Reflectance (Terrestrial) of Sheet Materials

General Information

• Status: Historical
• Publication Date: 31-Dec-1992
• Technical Committee: E44 - Solar, Geothermal and Other Alternative Energy Sources

Standards Content (Sample)

ASTM E424 71 0759530 0529b03 503
f#]b Designation: E 424 - 71 (Reapproved 1993)” AMERICAN SOCIEPI FOR TESTING AND MATERIALS
191 6 Race St Philadelphia, Pa 191 03
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
If not listed in the current combined inde? will appear in the neai edition.
Standard Test Methods for
Solar Energy Transmittance and Reflectance (Terrestrial) of
Sheet Materials’
This standard is issued under the fixed designation E 424; 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 (e) indicates an editorial change since the last revision or reapproval.
‘lNOTE-Section 8, Keywords, was added editorially in June 1993.
3.5 solar spectrum-for the purposes of these methods the
1. Scope
solar spectrum at sea level extending from 350 to 2500 nm.
1.1 These test methods cover the measurement of solar
3.6 solar transmittance-the percent of solar radiation
energy transmittance and reflectance (terrestrial) of materials
(watts/unit area) transmitted by a material.
in sheet form. Method A, using a spectrophotometer, is
applicable for both transmittance and reflectance and is the
4. Summary of Methods
referee method. Method B is applicable only for measure-
4.1 Method A-Measurements of spectral transmittance,
ment of transmittance using a Pyranometer in an enclosure
or reflectance versus a magnesium oxide standard, are made
and the sun as the energy source. Specimens for Method A
using an integrating sphere spectrophotometer over the
are limited in size by the geometry of the spectrophotometer
spectral range from 350 to 2500 nm. The illumination and
B requires a specimen 0.61 m2 (2 ft2). For the
while Method
viewing mode shall be normal-diffuse or diffuse-normal. The
both test
materials studied by the drafting task group,
solar energy transmitted or reflected is obtained by inte-
methods give essentially equivalent results.
grating over a standard solar energy distribution curve using
1.2 This standard does not purport to address all ofthe
weighted or selected ordinates for the appropriate solar-
safety problems, if any, associated with its use. It is the
energy distribution. The distribution at sea level, air mass 2,
responsibility of the user of this standard to establish appro-
is used.
priate safety and health practices and determine the applica-
4.2 Method B-Using the sun as the source and a
bility of regulatory limitations prior to use.
Pyranometer as a detector the specimen is made the cover of
2. Referenced Documents an enclosure with the plane of the specimen perpendicular to
the incident radiation; transmittance is measured as the ratio
2.1 ASTM Standards:
of the energy transmitted to the incident energy. (The
E 259 Practice for Preparation of Reference White Reflec-
apparatus of Method B has been used for the measurement
tance Standards2
of solar-energy reflectance but there is insufficient experience
E 275 Practice for Describing and Measuring Performance
with this technique for standardization at present.)
of Ultraviolet, Visible, and Near Infrared Spectro-
photometers3
5. Significance and Use
E 284 Terminology of Appearance2
5.1 Solar-energy transmittance and reflectance are impor-
E 308 Test Method for Computing the Colors of Objects
tant factors in the heat admission through fenestration, most
by Using the CIE System2
commonly through glass or plastics. (See Appendix X3.)
These methods provide a means of measuring these factors
3. Definitions
under fixed conditions of incidence and viewing. While the
3.1 solar absorptance-the ratio of absorbed to incident
data may be of assistance to designers in the selection and
radiant solar energy (equal to unity minus the reflectance
specification of glazing materials, the solar-energy transmit-
and transmittance).
tance and reflectance are not sufficient to define the rate of
3.2 solar admittance-solar heat transfer taking into ac-
heat transfer without information on other important fac-
count reradiated and convected energy.
tors. The methods have been found practical for both
3.3 solar energy-for these methods the direct radiation
transparent and translucent materials as well as for those
from the sun at sea level over the solar spectrum as defined
with transmittances reduced by highly reflective coatings.
in 3.2, its intensity being expressed in watts per unit area.
Method B is particularly suitable for the measurement of
3.4 solar reflectance-the percent of solar radiation
transmittance of inhomogeneous, patterned, or corrugated
(watts/unit area) reflected by a material.
materials since the transmittance is averaged over a large
area.
These test methods are under the jurisdiction of ASTM Committee E-44 on
6. Method A-Spectrophotometric Method
Solar, Geothermal, and Other Alternative Energy Sources and is the direct
responsibility of Subcommittee E 44.04 on Materials Performance.
6.1 Apparatus:
Current edition approved April 15, 1971. Published June 1971.
6.1.1 Spectrophotometer-An integrating sphere spectro-
Annual Book of ASTM Siandards. Vol 06.0 I.
Annual Book of ASTM Siandards. Vol 14.0 I, photometer, by means of which the spectral characteristics of

ASTM E424 71 = 0759510 0527602 448
(im E424
the test specimen or material may be determined throughout infrared; however, the wavelength of the peaks must be
the solar spectrum. For some materials the spectrum region agreed upon, using a specific instrument.
from 350 to 1800 nm may be sufficient. The design shall be 6.4 Procedure:
6.4.1 Transmittance-Obtain spectral transmittance data
such that the specimen may be placed in direct contact with
so relative to air. For measurement of transmittance of translu-
the sphere aperture for both transmission and reflection,
cent specimens, place freshly prepared matched smoked
that the incident radiation is within 6” of perpendicularity to
MgO surfaces at the specimen and reference ports at the rear
the plane of the specimen.,
be
of the sphere (Note 1). The interior of the sphere should
6.1.2 Standards:
freshly coated with MgO and in good condition.
6.1.2.1 For transmitting specimens, incident radiation
shall be used as the standard relative to which the trans-
NOTE 1-Magnesium oxide standards may be considered matched if
mitted light is evaluated. Paired reflecting standards are used,
on interchanging them the percent reflectance is altered by no more than
i % at any wavelength between 350 and 1800 nm.
prepared in duplicate as described below.
6.1.2.2 For reflecting specimens, use smoked magnesium
6.4.2 Reflecrance-Obtain spectral directional reflectance
oxide (MgO) as a standard as the closest practicable approx-
data relative to MgO. Include the specular component in the
imation of the completely reflecting, completely diffusing
reflectance measurement. Back the test specimen with a
surface for the region from 300 to 2100 nm. The preferred
black diffuse surface if it is not opaque. Depending on the
standard is a layer (at least 2.0 mm in thickness) freshly
required accuracy, use the measured values directly or make
prepared from collected smoke of burning magnesium (Rec-
corrections for instrumental O and 100 % lines (see Method
ommend Practice E 259). Pressed barium sulfate (BaSO,) or
E 308).
MgO are not recommended because of poor reflecting
6.5 Calculation-Solar energy transmittance or reflec-
properties beyond 1000 nm.
tance is calculated by integration. The distribution of solar
6.1.3 Specimen Backing for Reflectance Measurement-
energy as reported by Parry Moon6 for sea level and air mass
Transparent and translucent specimens shall be backed by a
2 shall be used.
light trap or a diffusing black material which is known to
6.5.1 Weighted Ordinates-Obtain the total solar energy
absorb the near infrared. The backing shall reflect no more
transmittance, T,, and reflectance, R,, in percent, by
than 1 % at all wavelengths from 350 to 2500 nm as
integrating the spectral transmittance (reflectance) over the
determined using the ~pectrophotometer.~
standard solar energy distribution as follows:
6.2 Test Specimens:
T, or R, = LX~:~~$m T, (or R,) x E,
6.2.1 Opaque specimens shall have at least one plane
EX for air mass 2, at 50-nm intervals, normalized to 100, is
surface; transparent and translucent specimens shall have
given in Appendix XI.
two surfaces that are essentially plane and parallel.
6.5.1.1 This integration is easily programmed for auto-
6.2.2 Comparison of translucent materials is highly de-
matic computation.
pendent on the geometw of the specific instrument being
6.5.2 Selected Ordinates-Integration is done by reading
used. It is recommended that the specimen be placed in
the transmittance or reflectance at selected wavelengths and
direct contact with the sphere to minimize and control loss
calculating their average. Appendix X2 lists 20 selected
of scattered radiation.
ordinates for integration.’
6.2.3 For specularly reflecting specimens the sphere con-
6.6 Report-The report shall include the following:
ditions, especially where the reflected beam strikes the sphere
6.6.1 Complete identification of the material tested, and
wall, shall be known to be highly reflecting (95 96 or higher).
whether translucent, clear, or specularly reflecting,
be used
It is recommended that a freshly coated sphere
6.6.2 Solar T percent or Solar R percent, or both, to the
especially when measuring translucent or specularly re-
nearest 0.1 %,
flecting specimens.
6.6.3 Specimen thickness,
6.3 Calibration:
6.6.4 Identification of the instrument used, and
6.3.1 Photometric-The calibration of the photometric
6.6.5 Integration method.
scale shall be done as recommended by the manufacturer. It
shall be carefully executed at reasonable time intervals to
7. Method B-Pyranometer Method
ensure accuracy over the entire range.
6.3.2 Wavelength-Periodic calibrations should be made
NOTE 2-The Pyranometer is used to measure total global (sun and
of the wavelength scales. Procedures for wavelength calibra- sky) radiation (previously designated a 180” pyroheliometer; presently
the latter word refers to a normal incidence measurement of direct solar
tion may be found in Recommended Practice E275. A
radiation). See ICY Instruction Manual, Part VI, Radiation Instru-
didymium filter has also been used for this purpose. Al-
ments, Pergamon Press, New York, NY.
though the absorption peaks have been defined for specific
resolution in the visible spectrum it also has peaks in the near
7.1 Apparatus:
7.1.1 Enclosure-The apparatus that has been used suc-
cessfully is a box capable of supporting a 0.61-m2 (24-in.2)
The Beckman DK-2 Recording Spectrophotometer available from Beckman
specimen. T
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