Designation: E 424 – 71 (Reapproved 2001)

Standard Test Methods for

Solar Energy Transmittance and Reflectance (Terrestrial) of
Sheet Materials1
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.

1. Scope 3.2 solar admittance—solar heat transfer taking into ac-

1.1 These test methods cover the measurement of solar count reradiated and convected energy.
energy transmittance and reflectance (terrestrial) of materials in 3.3 solar energy— for these methods the direct radiation
sheet form. Method A, using a spectrophotometer, is applicable from the sun at sea level over the solar spectrum as defined in
for both transmittance and reflectance and is the referee 3.2, its intensity being expressed in watts per unit area.
method. Method B is applicable only for measurement of 3.4 solar reflectance—the percent of solar radiation (watts/
transmittance using a pyranometer in an enclosure and the sun unit area) reflected by a material.
as the energy source. Specimens for Method A are limited in 3.5 solar spectrum— for the purposes of these methods the
size by the geometry of the spectrophotometer while Method B solar spectrum at sea level extending from 350 to 2500 nm.
requires a specimen 0.61 m2 (2 ft2). For the materials studied 3.6 solar transmittance—the percent of solar radiation
by the drafting task group, both test methods give essentially (watts/unit area) transmitted by a material.
equivalent results. 4. Summary of Methods
1.2 This standard does not purport to address all of the
safety problems, if any, associated with its use. It is the 4.1 Method A—Measurements of spectral transmittance, or
responsibility of the user of this standard to establish appro- reflectance versus a magnesium oxide standard, are made using
priate safety and health practices and determine the applica- an integrating sphere spectrophotometer over the spectral range
bility of regulatory limitations prior to use. from 350 to 2500 nm. The illumination and viewing mode shall
be normal-diffuse or diffuse-normal. The solar energy trans-
2. Referenced Documents mitted or reflected is obtained by integrating over a standard
2.1 ASTM Standards: solar energy distribution curve using weighted or selected
E 259 Practice for Preparation of Pressed Powder White ordinates for the appropriate solar-energy distribution. The
Reflectance Factor Transfer Standards for Hemispherical distribution at sea level, air mass 2, is used.
Geometry and Bi-Directional Geometries2 4.2 Method B—Using the sun as the source and a pyranom-
E 275 Practice for Describing and Measuring Performance eter as a detector the specimen is made the cover of an
of Ultraviolet, Visible, and Near-Infrared Spectrophotom- enclosure with the plane of the specimen perpendicular to the
eters3 incident radiation; transmittance is measured as the ratio of the
E 284 Terminology of Appearance2 energy transmitted to the incident energy. (The apparatus of
E 308 Practice for Computing the Colors of Objects by Method B has been used for the measurement of solar-energy
Using the CIE System2 reflectance but there is insufficient experience with this tech-
nique for standardization at present.)
3. Terminology Definitions
5. Significance and Use
3.1 solar absorptance—the ratio of absorbed to incident
radiant solar energy (equal to unity minus the reflectance and 5.1 Solar-energy transmittance and reflectance are important
transmittance). factors in the heat admission through fenestration, most com-
monly through glass or plastics. (See Appendix Appendix X3.)
These methods provide a means of measuring these factors
1
These test methods are under the jurisdiction of ASTM Committee E44 on under fixed conditions of incidence and viewing. While the
Solar, Geothermal, and Other Alternative Energy Sources and is the direct data may be of assistance to designers in the selection and
responsibility of Subcommittee E44.05 on Solar Heating and Cooling Subsystems specification of glazing materials, the solar-energy transmit-
and Systems.
Current edition approved April 15, 1971. Published June 1971.
tance and reflectance are not sufficient to define the rate of heat
2
Annual Book of ASTM Standards, Vol 06.01. transfer without information on other important factors. The
3
Annual Book of ASTM Standards, Vol 03.06. methods have been found practical for both transparent and

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.

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 E 424
translucent materials as well as for those with transmittances It is recommended that a freshly coated sphere be used
reduced by highly reflective coatings. Method B is particularly especially when measuring translucent or specularly reflecting
suitable for the measurement of transmittance of inhomoge- specimens.
neous, patterned, or corrugated materials since the transmit- 6.3 Calibration:
tance is averaged over a large area. 6.3.1 Photometric— The calibration of the photometric
scale shall be done as recommended by the manufacturer. It
6. Method A—Spectrophotometric Method shall be carefully executed at reasonable time intervals to
6.1 Apparatus: ensure accuracy over the entire range.
6.1.1 Spectrophotometer—An integrating sphere spectro- 6.3.2 Wavelength—Periodic calibrations should be made of
photometer, by means of which the spectral characteristics of the wavelength scales. Procedures for wavelength calibration
the test specimen or material may be determined throughout may be found in Recommended Practice E 275. A didymium
the solar spectrum. For some materials the spectrum region filter has also been used for this purpose. Although the
from 350 to 1800 nm may be sufficient. The design shall be absorption peaks have been defined for specific resolution in
such that the specimen may be placed in direct contact with the the visible spectrum it also has peaks in the near infrared;
sphere aperture for both transmission and reflection, so that the however, the wavelength of the peaks must be agreed upon,
incident radiation is within 6° of perpendicularity to the plane using a specific instrument.
of the specimen.4 6.4 Procedure:
6.1.2 Standards: 6.4.1 Transmittance— Obtain spectral transmittance data
6.1.2.1 For transmitting specimens, incident radiation shall relative to air. For measurement of transmittance of translucent
be used as the standard relative to which the transmitted light specimens, place freshly prepared matched smoked MgO
is evaluated. Paired reflecting standards are used, prepared in surfaces at the specimen and reference ports at the rear of the
duplicate as described below. sphere (Note 1). The interior of the sphere should be freshly
coated with MgO and in good condition.
6.1.2.2 For reflecting specimens, use smoked magnesium
oxide (MgO) as a standard as the closest practicable approxi- NOTE 1—Magnesium oxide standards may be considered matched if on
mation of the completely reflecting, completely diffusing interchanging them the percent reflectance is altered by no more than 1 %
surface for the region from 300 to 2100 nm. The preferred at any wavelength between 350 and 1800 nm.
standard is a layer (at least 2.0 mm in thickness) freshly 6.4.2 Reflectance— Obtain spectral directional reflectance
prepared from collected smoke of burning magnesium (Rec- data relative to MgO. Include the specular component in the
ommend Practice E 259). Pressed barium sulfate (BaSO4) or reflectance measurement. Back the test specimen with a black
MgO are not recommended because of poor reflecting proper- diffuse surface if it is not opaque. Depending on the required
ties beyond 1000 nm. accuracy, use the measured values directly or make corrections
6.1.3 Specimen Backing for Reflectance Measurement— for instrumental 0 and 100 % lines (see Method E 308).
Transparent and translucent specimens shall be backed by a 6.5 Calculation— Solar energy transmittance or reflectance
light trap or a diffusing black material which is known to is calculated by integration. The distribution of solar energy as
absorb the near infrared. The backing shall reflect no more than reported by Parry Moon6 for sea level and air mass 2 shall be
1 % at all wavelengths from 350 to 2500 nm as determined used.
using the spectrophotometer.5 6.5.1 Weighted Ordinates—Obtain the total solar energy
6.2 Test Specimens: transmittance, T se, and reflectance, Rse, in percent, by integrat-
6.2.1 Opaque specimens shall have at least one plane ing the spectral transmittance (reflectance) over the standard
surface; transparent and translucent specimens shall have two solar energy distribution as follows:
surfaces that are essentially plane and parallel. Tse or Rse 5 (ll 5 2100 nm
5 350nm Tl ~or R l ! 3 El (1)
6.2.2 Comparison of translucent materials is highly depen-
dent on the geometry of the specific instrument being used. It El for air mass 2, at 50-nm intervals, normalized to 100, is
is recommended that the specimen be placed in direct contact given in Appendix X1.
with the sphere to minimize and control loss of scattered 6.5.1.1 This integration is easily programmed for automatic
radiation. computation.
6.2.3 For specularly reflecting specimens the sphere condi- 6.5.2 Selected Ordinates—Integration is done by reading
tions, especially where the reflected beam strikes the sphere the transmittance or reflectance at selected wavelengths and
wall, shall be known to be highly reflecting (95 % or higher). calculating their average. Appendix X2 lists 20 selected ordi-
nates for integration.7
6.6 Report—The report shall include the following:
6.6.1 Complete identification of the material tested, and
4
The Beckman DK-2 Recording Spectrophotometer available from Beckman whether translucent, clear, or specularly reflecting,
Instruments, Inc., Fullerton, CA and the Cary 14 and 17 Recording Spectrophotom-
eter available from Varian Assoc., Palo Alto, CA, have been found satisfactory for
this purpose. For additional apparatus specifications see Recommended Practice
6
E 308. Journal of the Franklin Institute, Vol 230, 1940, p. 583, or Smithsonian
5
A piece of velvet sprayed with Nextel velvet coating 101-C10 Black available Physical Tables, Table 1, Vol 815, 1954, p. 273.
7
from 3M Company; Parson’s Black available from Eppley Laboratories, Newport, Olson, O. H., “Selected Ordinates for Solar Absorptivity Calculations,” Applied
RI or Krylon Flat Black have been found satisfactory for this purpose. Optics, Vol 2, No. 1, January 1963.

2
 E 424
6.6.2 Solar T percent or Solar R percent, or both, to the 7.1.2.2 The pyranometer has a viewing area of 180°. An
nearest 0.1 %, Eppley pyranometer with its 25-mm (1-in.) diameter sensing
6.6.3 Specimen thickness, disk, when placed in the center of the box, views the midpoint
6.6.4 Identification of the instrument used, and of the edges of the test specimen as a cone of 160°; the
6.6.5 Integration method. diagonal of the specimen is viewed as a cone of 166° when the
thermopile is 50 mm (2 in.) below the bottom of the specimen.
7. Method B—Pyranometer Method 7.1.2.3 Read-Out Instrumentation—A recorder, or a nonre-
NOTE 2—The pyranometer is used to measure total global (sun and sky) cording meter capable of indicating in the 0.2 to 15-mV range
radiation (previously designated a 180° pyroheliometer; presently the are permissible for use. The output voltage of the pyranometer
latter word refers to a normal incidence measurement of direct solar will be affected by the input impedance of the meter to which
radiation). See IGY Instruction Manual, Part VI, Radiation Instruments, it is connected. Thus, the meter used to indicate solar intensity
Pergamon Press, New York, NY. should have a very high input impedance, such as a precision
7.1 Apparatus: vacuum-tube voltmeter, or a meter which has been calibrated
7.1.1 Enclosure—The apparatus that has been used success- for one particular sensing element, thus compensating for any
fully is a box capable of supporting a 0.61-m2 (24-in.2) loading effects on that element.
specimen. The box, which would normally be about 0.66-m2 7.2 Specimens—The test specimens should be not less than
(26-in.2) outside, should be capable of being faced in any 0.61 by 0.61 m (24 by 24 in.). If the cross-sectional shape of
direction, as on a universal mount. The inside of the box should the specimen is not flat, care must be taken to prevent the
be painted flat black.4 A typical unit is shown in Fig. 1. possibility of light leaks at the edges such as are caused by the
7.1.2 Sensor: use of oversize specimens.
7.1.2.1 The sensing element of this instrument is a pyra- 7.3 Procedure:
nometer consisting of concentric rings, or wedges of thermo- 7.3.1 Conduct the tests on a clear sunny day with no cloud
piles, colored alternately black and white.8 The voltage output cover interruptions during the individual tests. Conduct testing
of this sensor is proportional to the intensity of the total between the hours of 9 a.m. and 3 p.m. local standard time; this
incident solar irradiation. The spectral sensitivity of this is when the solar radiation is at least 80 % of the value obtained
instrument extends from the ultraviolet to infrared wavelengths at solar noon for that day. In the Northern hemisphere take
(280 to 2800 nm), thus encompassing all the solar spectrum. readings between November and February only between 10
The pyranometer should be located inside the box so that the a.m. and 2 p.m. Expose the test specimen approximately
sensing thermopile is approximately 50 mm (2 in.) from the normal to the sun for 15 min prior to testing. Next, align the
center of the bottom plane of the sample. box normal to the sun’s rays and take the average incident
solar-energy reading over a period of time (normally several
minutes) until a steady trace, or reading is obtained. Then place
8
An Eppley 50-Junction Pyranometer, Serial No. 9624 (6.66 mV/cal·cm 2·min) the test specimen on the box and again record the average solar
and an Eppley 10-Junction Pyranometer, Serial No. 8553 (2.21 mV/cal·cm2·min) energy reaching the sensor. When the test specimen has a
available from Eppley Laboratories, Inc., Newport, RI, have been found satisfactory corrugated or irregular surface move it across the sensing
for this purpose. Various other Eppley Pyranometers have also been successfully
used. element, and take readings at 10-mm (1⁄2-in.) intervals for the
width of one corrugation or irregularity, and average the
readings. Also measure corrugated specimens with the corru-
gations in the North-South direction and in the East-West
direction.
7.3.2 The solar energy transmittance of the test specimens is
the ratio of the energy measured when the test specimen is
placed between the sun and the sensor and the energy measured
by the sensor with no test specimen in place.
7.4 Report—The report shall include the following:
7.4.1 The source and identity of the test specimen,
7.4.2 A complete description of the test specimen, that is,
thickness, cross-sectional shape, color, size, translucent or
transparent, type of material,
7.4.3 The percent solar energy transmittance to the nearest
1 %,
7.4.4 The place, date, and time of the test,
7.4.5 The intensity of the solar radiation,
7.4.6 Type of sensing unit used, and
7.4.7 Ambient air temperature.
8. Keywords
8.1 pyranometer; reflectance; solar energy; spectrophotom-
FIG. 1 Typical Unit with Pyranometer Mounted in Black Box eter; terrestrial reflectance; transmittance

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 E 424

APPENDIXES

(Nonmandatory Information)

X1. SOLAR ENERGY TRANSMITTANCE OR REFLECTANCE USING WEIGHTED ORDINATES

(NORMALIZED TO ( = 100.00)

Tse ~%! 5 (ll 5

5 2100nm
350nm Tl 3 El

Wavelength, Relative Wavelength, Relative Wavelength, Relative

nm Energy nm Energy nm Energy
350 1.27 950 3.29 1550 1.49
400 3.18 1000 4.25 1600 1.36
450 6.79 1050 3.72 1650 1.17
500 8.20 1100 1.70 1700 0.89
550 8.03 1150 1.46 1750 0.54
600 7.88 1200 2.52 1800 0.01
650 7.92 1250 2.21 1850 0.00
700 7.48 1300 1.78 1900 0.00
750 5.85 1350 0.12 1950 0.12
800 5.79 1400 0.00 2000 0.02
850 5.66 1450 0.16 2050 0.26
900 3.24 1500 1.06 2100 0.58

Total 100.00

X2. TWENTY SELECTED ORDINATES FOR EVALUATION OF SOLAR TRANSMITTANCE OR

REFLECTANCE AT SEA LEVEL

Table X2.1

Wavelength, Wavelength,
No. No.
nm nm
1 390 11 745
2 444 12 786
3 481 13 831
4 511 14 877
5 543 15 959
6 574 16 1026
7 606 17 1105
8 639 18 1228
9 669 19 1497
10 705 20 1722

X3. SOLAR ADMITTANCE PARAMETERS

X3.1 Solar energy poses a complex problem to architects tance (TSER) of the materials surrounding the space.
and engineers concerned with maintaining a comfortable
indoor space condition. The problem exists when solar energy X3.3 With homogenous materials the percent of solar
is admitted into a space which must be thermally and optically energy reflected, R, absorbed, A, and transmitted, T, can be
controlled, that is, temperature, humidity, and brightness. determined by the following equation:
100 % 5 R 1 A 1 T (X3.1)
X3.2 The amount of solar-energy admitted into a space can
be calculated with the solar-admittance parameters, total solar- X3.4 For transparent materials, such as glass and clear
energy transmittance (TSET), and total solar-energy reflec- plastics, the total solar energy transmittance is significient and

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 E 424
environmental control systems must be designed to handle the 1.46/@~1.46 1 4.0! ~0.48 3 230!#
changing solar load. ~ASHRAE Handbook, 1967, p. 480! Eq 19 (X3.3)

X3.5 Space environmental engineers use the total solar-

energy transmittance and total solar-energy reflectance param- = portion of absorbed solar energy reradiated and con-
eters of materials to determine the solar energy admitted into a vected indoors
space. = 39 Btu (Note X3.1)]
Total solar energy admitted indoors 5 136 Btu ~Note X3.2!
X3.5.1 For example:
(X3.4)
A 1⁄4-in. bronze-tinted glass has the following typical solar
energy admittance properties: X3.5.2 The 1967 ASHRAE Handbook of Fundamentals9
TSET = 46 % = T reviews this procedure on pages 477 through 480.
TSER = 6 % = R
TSEA = 48 % = A
NOTE X3.1—The amount of absorbed solar energy which is reradiated
and convected indoors is a direct function of the air movement over the
For the following conditions: indoor and outdoor glazing surfaces.
Design Day—Sept. 21, 40° North latitude, 4 p.m., West NOTE X3.2—Cooling loads used for design also include conduction
elevation (ASHRAE Handbook of Fundamentals, 1967, Table resulting from out-in temperature differences; heat capacity of building
4, p. 472).9 (All solar energy rates are per hour, square foot of materials may introduce a delay in peak load timing.
glazing area.) X3.6 The Handbook9 also illustrates a more commonly
Direct normal solar irradiation: 230 Btu used method on pages 470 through 476 with the use of shading
Recommended outdoor wind velocity: 7.5 mph
(Table 9, Item 3, p. 477)9 coefficients for the glazing under consideration and solar heat
[Corresponding outdoor surface coefficient: 4.0 Btu/°F gain factors (SHGF) for 1⁄8-in. clear glass (Tables 2 to 6, pp.
Recommended indoor air velocity: Still 470–474).
(Table 9, Item 3, p. 477)9
[Corresponding indoor surface coefficient: 1.46 Btu/°F X3.6.1 For example:
Total solar energy admitted indoors: A 1⁄4-in. bronze-tinted glass typical shading coeffi-
cient = 0.67.
Total solar heat gain indoors:
If, SHGF = 205 Btu (Table 4, Sept. 21, p. 472, West
5 0.46 ~230! 1 1.46/@~1.46 1 4! ~0.48 elevation, 4 p.m.),
3 230!# (X3.2) Solar energy admitted indoors = 0.67 3 205 = 137 Btu.

where: X3.7 Double glazing computations become more complex

0.46 (230) = transmitted solar energy and the solar energy admitted indoors requires considerably
= 106 Btu more calculation as the R A T formula does not apply directly.
and: For this type of glazing, the shading coefficient technique is
more applicable
9
ASHRAE Handbook of Fundamentals, American Society of Heating, Refriger-
ating, and Air Conditioning Engineers, 345 E. 47th St., New York, NY 10017, 1967, X3.8 Representative shading coefficients are available from
pp. 470–480. glass and plastic manufacturers.

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