ASTM E424-71(2023)

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

SIGNIFICANCE AND USE
5.1 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 concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.3 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.

General Information

• Status: Published
• Publication Date: 30-Apr-2023
• ICS: 81.040.20 - Glass in building
• ICS: 81.040.30 - Glass products
• Technical Committee: E44 - Solar, Geothermal and Other Alternative Energy Sources
• Drafting Committee: E44.20 - Optical Materials for Solar Applications

Relations

• Refers: ASTM E308-17 - Standard Practice for Computing the Colors of Objects by Using the CIE System
• Effective Date: 01-May-2017
• Refers: ASTM E259-06(2015) - Standard Practice for Preparation of Pressed Powder White Reflectance Factor Transfer Standards for Hemispherical and Bi-Directional Geometries (Withdrawn 2024)
• Effective Date: 01-Nov-2015
• Refers: ASTM E308-15 - Standard Practice for Computing the Colors of Objects by Using the CIE System
• Effective Date: 01-Apr-2015
• Refers: ASTM E308-12 - Standard Practice for Computing the Colors of Objects by Using the CIE System
• Effective Date: 01-Jul-2012
• Refers: ASTM E259-06(2011) - Standard Practice for Preparation of Pressed Powder White Reflectance Factor Transfer Standards for Hemispherical and Bi-Directional Geometries
• Effective Date: 01-Nov-2011
• Refers: ASTM E308-08 - Standard Practice for Computing the Colors of Objects by Using the CIE System
• Effective Date: 01-Dec-2008
• Refers: ASTM E275-08 - Describing and Measuring Performance of Ultraviolet and Visible Spectrophotometers
• Effective Date: 15-Oct-2008
• Refers: ASTM E308-06 - Standard Practice for Computing the Colors of Objects by Using the CIE System
• Effective Date: 01-Dec-2006
• Refers: ASTM E259-06 - Standard Practice for Preparation of Pressed Powder White Reflectance Factor Transfer Standards for Hemispherical and Bi-Directional Geometries
• Effective Date: 01-Sep-2006
• Refers: ASTM E259-98(2003) - Standard Practice for Preparation of Pressed Powder White Reflectance Factor Transfer Standards for Hemispherical and Bi-Directional Geometries
• Effective Date: 10-Jul-2003
• Refers: ASTM E308-01 - Standard Practice for Computing the Colors of Objects by Using the CIE System
• Effective Date: 10-Aug-2001
• Refers: ASTM E308-99 - Standard Practice for Computing the Colors of Objects by Using the CIE System
• Effective Date: 10-Aug-2001
• Refers: ASTM E275-93 - Standard Practice for Describing and Measuring Performance of Ultraviolet, Visible, and Near-Infrared Spectrophotometers
• Effective Date: 10-Feb-2001
• Refers: ASTM E275-01 - Standard Practice for Describing and Measuring Performance of Ultraviolet, Visible, and Near-Infrared Spectrophotometers
• Effective Date: 10-Feb-2001

Overview

ASTM E424-71(2023): Standard Test Methods for Solar Energy Transmittance and Reflectance (Terrestrial) of Sheet Materials provides established procedures for measuring how much solar energy passes through or is reflected by materials in sheet form, such as glass and plastics. Developed by ASTM International, these methods are critical in assessing the suitability of materials for use in terrestrial settings, particularly where energy efficiency, thermal management, and daylighting are important design considerations.

Solar energy transmittance and reflectance are vital parameters for understanding solar heat gain through fenestration and building envelopes. This standard outlines two primary test methods - one utilizing a spectrophotometer (Method A) and another employing a pyranometer under natural sunlight (Method B). Each approach is suitable for different material types and application scenarios.

Key Topics

• Solar Energy Transmittance and Reflectance
  The document defines metrics for:

  • Transmittance: Percent of incident solar energy transmitted through a material.
  • Reflectance: Percent of solar radiation reflected by the surface.

• Test Methods

  • Method A (Spectrophotometer): Measures spectral transmittance and reflectance across a range of 350 to 2500 nm. Suitable for both reflection and transmission analysis. Ideal for transparent, translucent, and coated materials, but specimen size is dictated by instrument geometry.
  • Method B (Pyranometer with Solar Source): Measures overall transmittance using natural solar radiation. Well-suited for large or patterned specimens, including inhomogeneous, corrugated, or non-flat materials.

• Material Applicability

  • Transparent and translucent sheet materials, such as architectural glass and plastics.
  • Materials with reflective coatings aimed at reducing solar heat gain.
  • Both homogeneous and patterned products are addressed.

• Limitations
  While critical for material selection, solar energy transmittance and reflectance alone do not determine full heat-transfer rates, as other factors (such as absorption, reradiation, convection, and environmental influences) must also be considered for complete thermal performance analysis.

Applications

ASTM E424 is widely used by:

• Architects and Building Designers

  • To select glazing and sheet materials for windows, skylights, and facades based on their solar performance characteristics
  • Enhancing occupant comfort and energy efficiency by effectively controlling solar heat gain and daylighting

• Material Manufacturers

  • In quality control, R&D, and product specification to ensure compliance with energy codes and industry benchmarks
  • For development and certification of solar control and energy-efficient glazing materials

• Engineers and Energy Consultants

  • Incorporating measured solar transmittance and reflectance data in energy modeling and load calculations
  • Supporting compliance with building standards and green building certifications

• Regulatory and Standards Bodies

  • Referencing robust measurement practices for product labeling or regulatory frameworks in the energy performance of building materials

The methods are applicable in laboratory environments and, with Method B, in field conditions using natural sunlight.

Related Standards

For comprehensive analysis of solar energy properties in building materials, ASTM E424 is frequently used with or referenced alongside:

• ASTM E308: Practice for Computing the Colors of Objects by Using the CIE System
• ASTM E275: Practice for Describing and Measuring Performance of Ultraviolet and Visible Spectrophotometers
• ASTM E259: Practice for Preparation of Pressed Powder White Reflectance Factor Transfer Standards
• Energy modeling standards and codes, such as those issued by ASHRAE or International Energy Conservation Code (IECC)

For professionals evaluating solar optical properties of sheet materials, ASTM E424 establishes standardized, reliable procedures supporting energy-efficient building design and material innovation.

Standards Content (Sample)

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.
Designation: E424 − 71 (Reapproved 2023)
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. A number 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 U.S. Department of Defense.
1. Scope E308 Practice for Computing the Colors of Objects by Using
the CIE System
1.1 These test methods cover the measurement of solar
energy transmittance and reflectance (terrestrial) of materials in
3. Definitions
sheet form. Method A, using a spectrophotometer, is applicable
for both transmittance and reflectance and is the referee
3.1 solar absorptance—the ratio of absorbed to incident
method. Method B is applicable only for measurement of
radiant solar energy (equal to unity minus the reflectance and
transmittance using a pyranometer in an enclosure and the sun
transmittance).
as the energy source. Specimens for Method A are limited in
3.2 solar admittance—solar heat transfer taking into ac-
size by the geometry of the spectrophotometer while Method B
2 2
count reradiated and convected energy.
requires a specimen 0.61 m (2 ft ). For the materials studied
by the drafting task group, both test methods give essentially
3.3 solar energy—for these methods the direct radiation
equivalent results.
from the sun at sea level over the solar spectrum as defined in
1.2 This standard does not purport to address all of the
3.2, its intensity being expressed in watts per unit area.
safety concerns, if any, associated with its use. It is the
3.4 solar reflectance—the percent of solar radiation (watts/
responsibility of the user of this standard to establish appro-
unit area) reflected by a material.
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
3.5 solar spectrum—for the purposes of these methods the
1.3 This international standard was developed in accor-
solar spectrum at sea level extending from 350 to 2500 nm.
dance with internationally recognized principles on standard-
3.6 solar transmittance—the percent of solar radiation
ization established in the Decision on Principles for the
(watts/unit area) transmitted by a material.
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
4. Summary of Methods
Barriers to Trade (TBT) Committee.
4.1 Method A—Measurements of spectral transmittance, or
2. Referenced Documents
reflectance versus a magnesium oxide standard, are made using
2.1 ASTM Standards:
an integrating sphere spectrophotometer over the spectral range
E259 Practice for Preparation of Pressed Powder White
from 350 to 2500 nm. The illumination and viewing mode shall
Reflectance Factor Transfer Standards for Hemispherical
be normal-diffuse or diffuse-normal. The solar energy trans-
and Bi-Directional Geometries
mitted or reflected is obtained by integrating over a standard
E275 Practice for Describing and Measuring Performance of
solar energy distribution curve using weighted or selected
Ultraviolet and Visible Spectrophotometers
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 4.2 Method B—Using the sun as the source and a pyranom-
Solar, Geothermal and Other Alternative Energy Sources and are the direct
eter as a detector the specimen is made the cover of an
responsibility of Subcommittee E44.20 on Optical Materials for Solar Applications.
enclosure with the plane of the specimen perpendicular to the
Current edition approved May 1, 2023. Published May 2023. Originally
incident radiation; transmittance is measured as the ratio of the
approved in 1971. Last previous edition approved in 2015 as E424 – 71 (2015).
DOI: 10.1520/E0424-71R23.
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 [e-mail]. 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
the ASTM website. nique for standardization at present.)
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E424 − 71 (2023)
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-energy transmittance and reflectance are important
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). It is recommended that a freshly coated sphere be used
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—The calibration of the photometric scale
materials as well as for those with transmittances reduced by
shall be done as recommended by the manufacturer. It shall be
highly reflective coatings. Method B is particularly suitable for
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 Practice E275. A didymium filter has also
6. Method A—Spectrophotometric Method
been used for this purpose. Although the absorption peaks have
6.1 Apparatus:
been defined for specific resolution in the visible spectrum it
6.1.1 Spectrophotometer—An integrating sphere
also has peaks in the near infrared; however, the wavelength of
spectrophotometer, by means of which the spectral character-
the peaks must be agreed upon, using a specific instrument.
istics of the test specimen or material may be determined
6.4 Procedure:
throughout the solar spectrum. For some materials the spec-
trum region from 350 to 1800 nm may be sufficient. The design 6.4.1 Transmittance—Obtain spectral transmittance data
shall be such that the specimen may be placed in direct contact
relative to air. For measurement of transmittance of translucent
with the sphere aperture for both transmission and reflection, specimens, place freshly prepared matched smoked MgO
so that the incident radiation is within 6° of perpendicularity to
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
NOTE 1—Magnesium oxide standards may be considered matched if on
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, use the measured values directly or make corrections
standard is a layer (at least 2.0 mm in thickness) freshly
for instrumental 0 and 100 % lines (see Practice E308).
prepared from collected smoke of burning magnesium (Prac-
tice E259). Pressed barium sulfate (BaSO ) or MgO are not
6.5 Calculation—Solar energy transmittance or reflectance
recommended because of poor reflecting properties beyond
is calculated by integration. The distribution of solar energy as
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
absorb the near infrared. The backing shall reflect no more than
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 × E (1)
~ !
se se (λ5350nm λ λ λ
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 Practice E308. Physical Tables, Table 1, Vol 815, 1954, p. 273.
E424 − 71 (2023)
6.5.2 Selected Ordinates—Integration is done by reading the (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
...