ASTM E1918-21

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Designation: E1918 − 16 E1918 − 21
Standard Test Method for
Measuring Solar Reflectance of Horizontal and Low-Sloped
Surfaces in the Field
This standard is issued under the fixed designation E1918; 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.
1. Scope
1.1 This test method covers the measurement of solar reflectance of various horizontal and low-sloped surfaces and materials in
the field, using a an albedometer or pyranometer. The test method is intended for use when the sun angle to the normal from a
surface is less than 45°.
1.2 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
3. Terminology
3.1 Definitions:
3.1.1 albedometer—an instrument consisting of two anti-parallel (back-to-back) pyranometers, where the upper pyranometer
measures incoming solar radiation and the lower pyranometer measures solar radiation reflected from the test surface.
3.1.2 inhomogeneous test site—a test site of nonuniform solar reflectance.
3.1.3 low-sloped surfaces—surface—surfaces a surface with a slope smaller than tilt angle not exceeding 9.5°. The roofing
industry has widely accepted a slope of 2:12 or less as a definition of low-sloped roofs.less than 2:12 (16.7 %) as characteristic
of a low-sloped roof. This corresponds to a slope tilt angle of approximately 9.5° (16.7 %).9.5°.
3.1.4 pyranometer—an a radiometric instrument (radiometer) used to measure the total hemispherical (beam plus diffuse) solar
radiant energy incident upon a surface per unit time and unit surface area.
3.1.5 solar energy—the radiant energy originating from the sun. Approximately 99 % of terrestrial solar energy lies betweenarrives
at wavelengths ofbetween 0.3 to 3.5and 2.5 μm.
This test method is under the jurisdiction of ASTM Committee D08 on Roofing and Waterproofing and is the direct responsibility of Subcommittee D08.18 on
Nonbituminous Organic Roof Coverings.
Current edition approved Nov. 1, 2016Oct. 1, 2021. Published January 2017October 2021. Originally approved in 1997. Last previous edition approved in 20152016 as
E1918 – 06 (2015).E1918 – 16. DOI: 10.1520/E1918-16.10.1520/E1918-21.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
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E1918 − 21
3.1.6 solar flux—for these measurements, the directbeam and diffuse radiation radiance (radiative power per unit area) from the
sun received at ground level over the solar spectrum, level, expressed in watts per square metre.meter.
3.1.7 solar reflectance—the fraction of solar flux reflected by a surface.
3.1.8 test site—a location that contains one or more test surfaces.
3.1.9 test surface—a surface whose solar reflectance is to be measured with a pyranometer.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 solar spectrum—the solar spectrum at ground level extending from wavelength 0.3 to 3.5 μm.
4. Summary of Test Method
4.1 A An albedometer or pyranometer is used to measure incoming and reflected solar radiation for a uniform horizontal or
low-sloped surface. The solar reflectance is the ratio of the reflected radiation to the incoming radiation.
5. Significance and Use
5.1 Solar reflectance is an important factor affecting surface and the temperature of a sunlit surface and that of the near-surface
ambient air temperature. Surfaces with low solar reflectance (typically 30 % or lower), absorb a high fraction of the incoming solar
energy which is either conducted into buildings or convected to air (leading to higher air temperatures). Use of materials with high
solar reflectance may result in lower air-conditioning energy use and cooler cities and communities. The test method described
hereherein measures the solar reflectance of surfaces in the field.natural sunlight.
6. Apparatus
6.1 Sensor—A precision spectral pyranometer (PSP) sensitive to radiant energy in the 0.28–2.8 0.28 to 2.8 μm band is
–2
recommended. A typical pyranometer yields a linear output of 60.5 % between 0 and 1400 W·m and a response time of one s.
1 s. Specific characteristics can be obtained based on calibration by the manufacturer of the pyranometer. Other suitable
pyranometers are discussed in Zerlaut. The double-dome design of the PSP minimizes the effects of internal convection resulting
from tilting the pyranometer at different angles. For this reason, the PSP is especially suitable for this test, since measurement of
solar reflectivity requires the apparatus to alternatively face up and down.
6.2 Read-OutReadout Instrument—The analog output from the pyranometer is converted to digital output with a readout meter
(such as EPLAB Model 455 Instantaneous Solar Radiation Meter) that has an accuracy of better than 60.5 % and a resolution of
–2
1 W·m1 W·m . The meter shall be scaled to the sensitivity of the specific PSP by the manufacturer of the pyranometer.
Alternatively, a precision voltmeter can be used.
6.3 Albedometer or Pyranometer Stand—Support—The albedometer (Fig. 1) or pyranometer (Fig. 2) shall be mounted on an arm
and a stand that placescenters the sensor at a height of 50 cm500 mm above the target surface to minimize the effect of the shadow
effects of sensor, arm, and stand shadows on measured reflected radiation. The horizontal distance from the center of the
albedometer or pyranometer to the edge of the test surface shall be at least 2 m. The arm and stand shall be strong, strong and cast
the smallest possible shadow, and shadow. If using a pyranometer, the support must allow the pyranometer to be turned upward
and downward easily as shown in Fig. 12.
7. Sampling, Test Specimens, and Test Units
7.1 TheThis test method described here applies to large (circles with low-sloped test surfaces that are at least four metres 4 m in
diameter or squares four metres on a side), homogeneous, low-sloped surfaces, such as roofs, streets, and parking lots. The
measurements shall be performed on dry surfaces.(if circular) or at least 4 m on each side (if rectangular). Examples of sites that
may contain test surfaces include roofs, streets, and parking lots.
Zerlaut, G., “Solar Radiation Instrumentation,” Solar Resources, R.L. R. L. Hulstrom, ed., MIT Press, Cambridge, MA, 1989, pp. 173–308.
E1918 − 21
FIG. 1 Schematic of the PyranometerAlbedometer and its StandIts Support
FIG. 2 Schematic of the Pyranometer and Its Support
8. Calibration and Standardization
8.1 The albedometer or pyranometer shall be checked to ensure its accuracy. Most pyranometers are precalibrated by
manufacturers. A radiometric instrument is usually pre-calibrated by its manufacturer. It is a good practice to recalibrate the
pyranometer as specified by the manufacturer (typically instrument at the manufacturer-specified interval, typically once every year
or two years).years. Recalibration is doneperformed by the manufacturer of the pyranometer.manufacturer.
9. Procedure
9.1 Cloud cover and haze significantly affect the measurements. The tests shall be conducted on a clear, sunny day with no cloud
cover or haze during the individual measurements. See Annex A1 for guidelines on determination of the suitability of the
atmospheric conditions for conducting the tests.
9.2 The test shall be done in conditions where the angle of the sun to the normal from the test surface of interest is less than 45°.
45° (cosine of solar incidence angle is greater than ≈ 0.707). For flathorizontal and low-sloped surfaces, this generally limits
=2
the test to between the hours of 9 a.m. and 3 p.m. local standard time; this is when solar radiation is at least 70 % of the value
obtained at solar noon for that day. In winter months (when solar incidence angle is low), perform the tests between hours of 10
a.m. and 2 p.m.
NOTE 1—Solar elevation angle, solar azimuth angle, and the cosine of the solar incidence angle based on time and location may be obtained from a tool
such as the NREL Solar Position and Intensity Calculator (https://midcdmz.nrel.gov/solpos/solpos.html).
E1918 − 21
9.3 Align the standsupport such that the arm points toward the sun (thissun; this eliminates the shadow of the people conducting
the test and minimizes the effect of the shadow from equipment).equipment. There shall be no other shadow on the measurement
area other than the minimal shadow cast by the pyranometer and the stand.its support. The pyranometer shall be parallel to the test
surface where measurement is conducted. The horizontal distance from the pyranometer sensor to the nearest edge of the test
surface must be at least 2 m.
9.4 Face the pyranometer upward (that is, looking directly away from the surface) to read incoming solar radiation. Flip the
pyranometer downward to read reflected solar radiation. Make sure the readings are constant for at least 10 s. The measurements
of incoming and reflected radiation shall be performed in a time interval notThe measurement practices for a dual-sensor
albedometer (9.4.1 to exceed 2 min. Solar reflectance is the ratio of the reflected radiation to incoming radiation. Repeat the pairs
of incoming and reflected measurements at least three times. The calculated solar reflectance from all the measurements shall agree
within 0.01 in a reflectivity scale of) and a single-sensor pyranometer (9.4.2 0.00 to 1.00.) are as follows:
9.4.1 Albedometers will read both the incoming solar radiation and the reflected solar radiation simultaneously. Make sure each
reading is constant for at least 10 s before recording its value. The albedometer shall be parallel to the test surface. Measure at least
three pairs of incoming and reflected radiation within 2 min. The calculated solar reflectance (the ratio of the reflected radiation
to incoming radiation) from all the measurements shall agree to within 0.01 in a reflectivity scale of 0.00 to 1.00.
NOTE 2—Since it is challenging to align the albedometer parallel to a tilted test surface, it is simpler and preferable to characterize a horizontal test surface.
9.4.2 To use a pyranometer, face the pyranometer upward and parallel to the test surface to read incoming solar radiation. Flip
the pyranometer downward and parallel to the test surface to read reflected solar radiation. Make sure each reading is constant for
at least 10 s before recording its value. Measure at least three pairs of incoming and reflected radiat
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