ASTM E772-15(2021)

Standard Terminology of Solar Energy Conversion

SCOPE
1.1 This terminology pertains to the conversion of solar energy into other forms of energy by various means, including thermal absorption (i.e., solar thermal) and the photovoltaic effect (i.e., photovoltaics).  
1.2 This terminology also pertains to instrumentation used to measure solar radiation.  
1.3 This terminology also pertains to glass for solar energy applications.  
1.4 Fundamental terms associated with electromagnetic radiation that are indicates as derived units in Standard IEEE/ASTM SI 10 are not repeated in this terminology.  
1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.6 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: 31-Oct-2021
• ICS: 01.040.27 - Energy and heat transfer engineering (Vocabularies)
• ICS: 27.160 - Solar energy engineering
• Technical Committee: E44 - Solar, Geothermal and Other Alternative Energy Sources
• Drafting Committee: E44.01 - Terminology and Editorial

Relations

• Refers: ASTM D5544-16(2023) - Standard Test Method for On-Line Measurement of Residue After Evaporation of High-Purity Water
• Effective Date: 01-Nov-2023
• Refers: ASTM D4865-23 - Standard Guide for Generation and Dissipation of Static Electricity in Petroleum Fuel Systems
• Effective Date: 01-Nov-2023
• Refers: ASTM C162-23 - Standard Terminology of Glass and Glass Products
• Effective Date: 01-Oct-2023
• Refers: ASTM E1125-16(2020) - Standard Test Method for Calibration of Primary Non-Concentrator Terrestrial Photovoltaic Reference Cells Using a Tabular Spectrum
• Effective Date: 01-Jun-2020
• Refers: ASTM G130-12(2020) - Standard Test Method for Calibration of Narrow- and Broad-Band Ultraviolet Radiometers Using a Spectroradiometer
• Effective Date: 01-Jun-2020
• Refers: ASTM E948-16(2020) - Standard Test Method for Electrical Performance of Photovoltaic Cells Using Reference Cells Under Simulated Sunlight
• Effective Date: 01-Jun-2020
• Refers: ASTM G138-12(2020)e1 - Standard Test Method for Calibration of a Spectroradiometer Using a Standard Source of Irradiance
• Effective Date: 01-Jun-2020
• Refers: ASTM E349-06(2019)e1 - Standard Terminology Relating to Space Simulation
• Effective Date: 01-Oct-2019
• Refers: ASTM E1171-15(2019) - Standard Test Methods for Photovoltaic Modules in Cyclic Temperature and Humidity Environments
• Effective Date: 01-Apr-2019
• Refers: ASTM E1036-15(2019) - Standard Test Methods for Electrical Performance of Nonconcentrator Terrestrial Photovoltaic Modules and Arrays Using Reference Cells
• Effective Date: 01-Apr-2019
• Refers: ASTM E2527-15(2019) - Standard Test Method for Electrical Performance of Concentrator Terrestrial Photovoltaic Modules and Systems Under Natural Sunlight
• Effective Date: 01-Apr-2019
• Refers: ASTM E1362-15(2019) - Standard Test Methods for Calibration of Non-Concentrator Photovoltaic Non-Primary Reference Cells
• Effective Date: 01-Apr-2019
• Refers: ASTM E2236-10(2019) - Standard Test Methods for Measurement of Electrical Performance and Spectral Response of Nonconcentrator Multijunction Photovoltaic Cells and Modules
• Effective Date: 01-Apr-2019
• Refers: ASTM E1021-15(2019) - Standard Test Method for Spectral Responsivity Measurements of Photovoltaic Devices
• Effective Date: 01-Apr-2019
• Refers: ASTM E927-19 - Standard Classification for Solar Simulators for Electrical Performance Testing of Photovoltaic Devices
• Effective Date: 01-Feb-2019

Overview

ASTM E772-15(2021), Standard Terminology of Solar Energy Conversion, is an internationally recognized reference published by ASTM International. It establishes a comprehensive set of terminology for the conversion of solar energy into other forms, such as thermal energy (solar thermal) and electricity via the photovoltaic effect (photovoltaics). This standard also covers instrumentation for measuring solar radiation and terminology relevant to glass in solar energy applications. Clear, consistent language is essential for effective communication across the solar energy industry, from research and testing to product development and deployment.

The vocabulary defined in this standard promotes clarity in technical discussions, facilitates global collaboration, and supports the development, testing, and certification of solar technology. The terms adhere to SI units and follow World Trade Organization (WTO) principles for standards development, ensuring international compatibility.

Key Topics

1. Solar Energy Conversion Methods

• Definitions relevant to the absorption of solar radiation for heating (solar thermal) and electricity generation (photovoltaics).
• Terms describing radiative energy transformations such as absorption, emission, and transmission.

2. Measurement and Instrumentation

• Terminology for devices such as pyranometers, pyrheliometers, radiometers, and sunphotometers, vital for accurate solar radiation measurement.
• Definitions related to calibration, sensitivity, spectral ranges, and field/primary/secondary standard instruments.
• Concepts around measurement conditions like air mass, optical depth, and radiometric quantities.

3. Glass for Solar Applications

• Terms highlighting the importance of glass properties in solar collectors and photovoltaic modules.
• Definitions for optical properties: transmittance, reflectance, and optical distortion.
• Reference to glass testing and quality assurance methods.

4. Electromagnetic Radiation & Physical Quantities

• Standardized units and descriptive modifiers (e.g., diffuse, direct, spectral, hemispherical).
• Distinctions between radiometric and photometric measurements.
• Consideration of atmospheric interactions, scattering, albedo, and related solar irradiance terms.

Applications

Practical Value Across the Solar Sector:

• Product Design and Development: Manufacturers of solar panels, collectors, and glass use these definitions to ensure product specifications and test reports are consistent and comparable worldwide.
• Testing and Calibration Laboratories: Precision in terminology is crucial for proper calibration of instruments like pyranometers and reference cells, supporting reliable performance verification.
• System Installation and Operation: Installers and engineers reference these terms when planning solar energy systems, selecting measurement tools, and interpreting site conditions.
• Research and Standards Development: Researchers and policy-makers apply standardized language to advance innovation, policy, and regulation in renewable energy.
• Compliance and Certification: Ensures compliance with international requirements for product certification, safety, and trade.

Related Standards

• ASTM E490 – Solar Constant and Zero Air Mass Solar Spectral Irradiance Tables
• ASTM E1036 – Electrical Performance of Nonconcentrator Terrestrial Photovoltaic Modules
• ASTM E927 – Classification for Solar Simulators for Photovoltaic Devices
• ASTM E816 – Calibration of Pyrheliometers by Comparison to Reference Pyrheliometers
• ISO 9060 – Specification and Classification of Instruments for Measuring Solar Radiation
• WMO-No. 8 – Guide to Meteorological Instruments and Methods of Observation

Summary

ASTM E772-15(2021) provides the essential terminology for solar energy conversion and measurement, supporting accurate communication, instrument calibration, system design, and international cooperation across the solar energy industry. Utilizing these standardized terms enhances product quality, measurement reliability, and compliance with international energy standards.

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: E772 − 15 (Reapproved 2021)
Standard Terminology of
Solar Energy Conversion
This standard is issued under the fixed designation E772; 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.
1. Scope by Chemical Microscopy
D4865Guide for Generation and Dissipation of Static Elec-
1.1 This terminology pertains to the conversion of solar
tricity in Petroleum Fuel Systems
energy into other forms of energy by various means, including
D5544Test Method for On-Line Measurement of Residue
thermal absorption (i.e., solar thermal) and the photovoltaic
After Evaporation of High-Purity Water
effect (i.e., photovoltaics).
D7236Test Method for Flash Point by Small Scale Closed
1.2 This terminology also pertains to instrumentation used
Cup Tester (Ramp Method)
to measure solar radiation.
E349Terminology Relating to Space Simulation
1.3 This terminology also pertains to glass for solar energy
E490Standard Solar Constant and Zero Air Mass Solar
applications.
Spectral Irradiance Tables
1.4 Fundamental terms associated with electromagnetic ra- E491Practice for Solar Simulation for Thermal Balance
diation that are indicates as derived units in Standard IEEE/ Testing of Spacecraft
ASTM SI 10 are not repeated in this terminology.
E927Classification for Solar Simulators for Electrical Per-
formance Testing of Photovoltaic Devices
1.5 The values stated in SI units are to be regarded as
E948Test Method for Electrical Performance of Photovol-
standard. No other units of measurement are included in this
taic Cells Using Reference Cells Under Simulated Sun-
standard.
light
1.6 This international standard was developed in accor-
E816Test Method for Calibration of Pyrheliometers by
dance with internationally recognized principles on standard-
Comparison to Reference Pyrheliometers
ization established in the Decision on Principles for the
E1021TestMethodforSpectralResponsivityMeasurements
Development of International Standards, Guides and Recom-
of Photovoltaic Devices
mendations issued by the World Trade Organization Technical
E1036Test Methods for Electrical Performance of Noncon-
Barriers to Trade (TBT) Committee.
centrator Terrestrial Photovoltaic Modules and Arrays
Using Reference Cells
2. Referenced Documents
E1125 Test Method for Calibration of Primary Non-
2.1 ASTM Standards:
Concentrator Terrestrial Photovoltaic Reference Cells Us-
C162Terminology of Glass and Glass Products
ing a Tabular Spectrum
C1048Specification for Heat-Strengthened and Fully Tem-
E1171Test Methods for Photovoltaic Modules in Cyclic
pered Flat Glass
Temperature and Humidity Environments
C1651Test Method for Measurement of Roll Wave Optical
E1362Test Methods for Calibration of Non-Concentrator
Distortion in Heat-Treated Flat Glass
Photovoltaic Non-Primary Reference Cells
D1003Test Method for Haze and Luminous Transmittance
E1462Test Methods for Insulation Integrity and Ground
of Transparent Plastics
Path Continuity of Photovoltaic Modules
D1245Practice for Examination of Water-Formed Deposits
E2236Test Methods for Measurement of Electrical Perfor-
mance and Spectral Response of Nonconcentrator Multi-
junction Photovoltaic Cells and Modules
This terminology is under the jurisdiction ofASTM Committee E44 on Solar,
GeothermalandOtherAlternativeEnergySourcesandisthedirectresponsibilityof
E2527Test Method for Electrical Performance of Concen-
Subcommittee E44.01 on Terminology and Editorial.
trator Terrestrial Photovoltaic Modules and Systems Un-
Current edition approved Nov. 1, 2021. Published November 2021. Originally
der Natural Sunlight
approved in 1980. Last previous edition approved in 2015 as E772–15. DOI:
10.1520/E0772-15R21.
F1863Test Method for Measuring the NightVision Goggle-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Weighted Transmissivity of Transparent Parts
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
G113Terminology Relating to Natural andArtificialWeath-
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. ering Tests of Nonmetallic Materials
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E772 − 15 (2021)
G130Test Method for Calibration of Narrow- and Broad- absorption, n—transformation of radiant energy to a different
Band Ultraviolet Radiometers Using a Spectroradiometer form of energy by interaction with matter. E349
G138Test Method for Calibration of a Spectroradiometer
aerosol, n—any solid or liquid particles, with a nominal size
Using a Standard Source of Irradiance
range from 10 nm to 100 µm, suspended in a gas (usually
G167Test Method for Calibration of a Pyranometer Using a
air). D5544
Pyrheliometer
G173TablesforReferenceSolarSpectralIrradiances:Direct
aerosol optical depth, AOD, n—a measure of the extinction
Normal and Hemispherical on 37° Tilted Surface
caused by aerosols in the atmosphere relative to the zenith
G197Table for Reference Solar Spectral Distributions: Di-
and modeled with Ångstrom’s turbidity formula.
rect and Diffuse on 20° Tilted and Vertical Surfaces
DISCUSSION—Although it varies with wavelength, it is common to
IEEE/ASTM SI 10American National Standard for Metric
report aerosol optical depth at a single wavelength only, especially 0.5
Practice µm.
2.2 ISO Standard:
air mass, AM, n—relative optical mass (see optical mass,
ISO9060SpecificationandClassificationofInstrumentsfor
relative) calculated using the density of air as a function of
Measuring Hemispherical Solar and Direct Solar Radiai-
altitude.
ton
AM'l /l 5 secθ , for θ #1 rad ~60°! (1)
s z z z
2.3 WMO Document:
DISCUSSION—Eq 1 is a simple approximation of the optical mass,
WMO-No. 8Guide to Meteorological Instruments and
relative (see Eq 5) that uses the ratio of the path length along the sun
Methods of Observation, Seventh ed., 2008, World Me-
vector (l ) to the path length along the zenith (l ) (see sun vector,
s z
teorological Organization (WMO), Geneva
zenith,and zenith angle, solar).Othersolutionsaremorecomplicated
and take factors such as refraction and local air pressure into account.
3. Adjectives for Electromagnetic Radiation
DISCUSSION—Theabbreviation AMisalsocommonlyusedtoreferto
3.1 Properties and quantities associated with electromag-
aparticularstandardsolarspectralirradiance,suchasthoseinStandard
netic radiation vary with: E490, Tables G173, and Table G197. Thus, AM0 can indicate the
extraterrestrial spectral irradiance table in Standard E490, and AM1.5
3.1.1 The direction and geometric extent (solid angle) over
the hemispherical spectral irradiance table in Tables G173. Using
which the incident or exitant flux, or both, is evaluated, and
AM1.5 in this way is discouraged because air mass is but one of many
3.1.2 The relative spectral distribution of the incident flux
variables that modify solar spectral irradiance such as clouds, aerosol
and the spectral response of the detector for exitant flux.
scattering, and water vapor absorption; note that bothTables G173 and
Table G197 use an air mass value of 1.5, but differ greatly. The
3.2 Adjective modifiers can be used to indicate the
distinction between a spectral irradiance and a path length ratio should
geometric, spectral, and polarization conditions under which
be made clear whenever these abbreviations are used.
radiometric properties and quantities are evaluated. The adjec-
tives defined in this Terminology are: conical, diffuse, direct,
air mass one, AM1, n—a relative optical mass (see optical
directional, hemispherical, luminous, normal, and spectral.
mass, relative) that is equal to one. Because of the way in
which relative optical mass is defined,AM1 always denotes
3.3 For reflectance and transmittance, the direction and
a vertical path at sea level.
geometric extent of both the incident beam and exitant beam
must be specified.
air mass, optical—see optical mass, relative.
3.4 For emittance, only the exitant beam need be specified,
air mass, pressure corrected, AM , n—an approximation of
and for absorptance, only the incident beam need be specified. p
air mass for locations above sea level that uses the ratio of
3.5 Radiometricpropertiesalsovarywiththepolarizationof
the local barometric pressure P, to the standard sea level
the incident flux and the sensitivity to polarization of the
atmospheric pressure P = 101.325 kPa (see Eq 2).
collector-detector system for flux incident or exitant at angles
P
greater than about 15° from normal.
AMp' AM (2)
P
3.6 An instrument used for solar energy measurements or a
solar energy receiver will usually determine the directions and air mass ratio—see optical mass, relative.
geometric extents, such as a pyranometer, a pyrheliometer, or
air mass, relative optical—see optical mass, relative.
a flat-plate solar thermal collector.
air mass zero, AM0, n—the absence of atmospheric attenua-
4. Terminology
tion of the solar irradiance at one astronomical unit from the
sun. E491
ELECTROMAGNETIC RADIATION AND OPTICS
albedo—discouraged in favor of the preferred term, reflec-
absorptance, n—ratiooftheabsorbedradiantorluminousflux
tance.
to the incident flux. E349
angle of incidence, rad or °, n—the angle between a ray and
the normal vector to the plane on which the ray is incident;
Available from International Organization for Standardization (ISO), 1, ch. de
especially the angle between the sun vector and the normal
la Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http://www.iso.org.
Available from World Meteorological Organization, http://www.wmo.int. vector.
E772 − 15 (2021)
angle of reflection,rador°, n—theanglebetweenthedirection diffusion, n—change of the spatial distribution of a beam of
ofpropagationofareflectedrayandthenormalvectortothe radiation when it is deviated in many directions by a surface
surface of interest at the point of reflection. or a medium. E349
angle of refraction, rad or °, n—the angle between the direct, adj—describing solar radiation, a collimated beam.
direction of propagation of a refracted ray and the normal
directional, adj—of or relating to a direction in space.
vector to the interface of interest at the point of refraction.
DISCUSSION—Foropticalproperties,overaninfinitesimalsolidangle,
thepropertyisassumedconstant.Thevariationinopticalpropertywith
altitude angle, solar—see elevation angle, solar.
respect to changing azimuth (counter-clockwise) and incidence angle
attenuation—see extinction. (from the surface normal), with respect to a reference mark on a
sample, is the directional response.
azimuth angle, solar, ψ[rad or °], n—the angle between the
elevation angle, solar, α [rad or °], n—the complement of the
line of longitude (or geographical meridian) at the location
solar zenith angle, i.e. π/2 – θ radians. See zenith angle,
of interest and the horizontal component of the sun vector. z
solar.
Byconvention,theazimuthangleispositivewhenthesunis
east of the line of longitude and negative when it is west of
emission, n—release of radiant energy. E349
the line of longitude.
emissive power—discouraged in favor of the preferred term
beam, n—of radiant energy, a collection of rays confined to a
radiant exitance.
specific path.
emittance, ε, n— for a sample at a given temperature, ratio of
blackbody, Planckian radiator, n—a thermal radiator which
the radiant flux emitted by a sample to that emitted by a
completely absorbs all incident radiation, whatever the
blackbody radiator at the same temperature, under the same
wavelength, the direction of incidence, or the polarization.
spectral and geometric conditions of measurement.
Thisradiatorhas,foranywavelength,themaximumspectral
concentration of radiant exitance at a given temperature. extinction, n—the attenuation of radiant energy from an
incident beam by the processes of molecular absorption and
E491
scattering caused by atmospheric constituents.
Bouguer’s Law, n—an expression of the extinction of radia-
DISCUSSION—Scattering by air molecules can be modeled with
tion in a medium that states the intensity exponentially
Rayleigh scattering, and scattering by aerosols with Ångstrom’s
decreases due to both scattering and absorption as it passes
turbidity formula. Absorption processes are modeled with tables of
through the medium (see Eq 3), where τ is the wavelength- measured absorption coefficients versus wavelength.
λ
dependent extinction optical thickness. The ratio of I to I
extinction coefficient, monochromatic, k [dimensionless],
iλ
is equal to the atmospheric transmittance, T, and τ is equal
λ
n—a measure of the extinction caused by a particular
to the summation of the extinction optical thicknesses
atmosphericconstituent(see Bouguer’s Lawand extinction
associated with each individual scattering or absorption
optical thickness, monochromatic).
process τ .
iλ
n
extinction optical depth, monochromatic, [dimensionless],
I 5 I exp 2τ 5 I exp 2 τ (3)
~ ! S D
0 λ 0 ( iλ
n—theproductofthe extinction coefficient k foraparticu-
iλ
i51
laratmosphericconstituenttimesthepathlengthtothetopof
DISCUSSION—Bouguer’s Law is also known as Lambert’s Law or
theatmosphere, m ,see extinction optical thickness, mono-
Beer’s Law.
r
chromatic and optical mass, relative.
circumsolar diffuse radiation—see radiant energy, circumso-
DISCUSSION—Optical depth is sometimes used synonymously with
lar.
optical thickness, but the preferred distinction between the two is that
optical thickness refers to the extinction along the entire path through
conical, adj—describing a solid angle larger than an infinitesi-
the atmosphere rather than the vertical path.
malelementandlessthanahemisphere(2πsr);thegeometry
extinction optical thickness, monochromatic, τ
of the solid angle must be described in context.
iλ
[dimensionless], n—theproductofthe extinction coefficient
diffuse, adj—describing radiometric quantities, indicates flux
k for a particular atmospheric constituent times the path
iλ
propagating in many directions, as opposed to a collimated
lengththroughatmosphere,see Bouguer’s LawandEq4,in
beam.
which m is the optical mass, actual.
act
τ 5 k ·m (4)
diffuse, adj—describing solar irradiance, the global hemi-
iλ iλ act
spherical irradiance minus the direct beam irradiance.
hemispherical, adj—describing half of a sphere, i.e. a 2π sr
solid angle.
diffuse, adj—describing reflectance, the directional hemi-
spherical reflectance minus the specular reflectance.
incident angle—see angle of incidence.
DISCUSSION—Diffusehasbeenusedinthepasttorefertohemispheri-
index of refraction, n—the numerical expression of the ratio
cal collection (including the specular component) or irradiation, with
ofthevelocityoflightinavacuumtothevelocityoflightin
equal radiance for all directions over a hemisphere. This use is
deprecated in favor of the more precise term hemispherical. a substance. D1245
E772 − 15 (2021)
infrared radiation, n—radiation for which the wavelengths of optical mass, relative, m [dimensionless], n—the ratio of the
r
the monochromatic components are greater than those for actual optical mass (see optical mass, actual, m )tothe
act
visible radiation, and less than about 1 mm. E349 lineintegralalongthe zenithofthedensityofasubstanceas
a function of altitude (ρ) between a point in the atmosphere
–2
irradiance, E[W·m ], n—at a point on a surface,radiantflux
(0) and the vacuum of space (∞) (see Eq 6).
incident per unit area of the surface; the derived unit heat
`
flux density, irradiance in Standard IEEE/ASTM SI 10.
m 5 m / ρds (6)
*
r act
–2 –1 –2 –1
irradiance, spectral, E or E(λ)[W·m ·nm orW·m ·µm ],
λ
optical thickness—see extinction optical thickness, mono-
n—the irradiation at a specific wavelength over a narrow
chromatic.
bandwidth, or as a function of wavelength; also, the deriva-
tive with respect to wavelength of irradiance. polarization, n— with respect to optical radiation, the restric-
DISCUSSION—Spectral irradiance is commonly reported in tabular tion of the magnetic or electric field vector to a single plane.
formaspairsofwavelengthandirradiancevalues,asinStandardE490,
G138
Tables G173, and Table G197; see spectral.
polarization, parallel, n—a plane of polarization parallel to
DISCUSSION—Following the normal SI rules for compound units (see
the plane of incidence, reflectance, or transmittance.
Standard IEEE/ASTM SI 10), the units for spectral irradiance, the
derivative of irradiance with respect to wavelength dE/dλ, would be
polarization, perpendicular, n—a plane of polarization per-
−3
W·m . However, to avoid possible confusion with a volumetric power
pendicular to the plane of incidence, reflectance, or trans-
density unit and for convenience in numerical calculations, it is
mittance.
common practice to separate the wavelength with a compound unit.
Compound units are used in Standard E490, Tables G173, and Table
polarization, plane of, n—byconvention,theplanecontaining
G197.
an electromagnetic wave’s electric vector.
–2
irradiance, total, E [W·m ], n—the integration over all
T
–2 –1
radiance, W·m ·sr , n—the SI derived unit radiance in
wavelengths of spectral irradiance, or the solar irradiance
Standard IEEE/ASTM SI 10.
measured with a pyranometer or a pyrheliometer.
radiant emissive power—see radiant exitance.
irradiation, n—application of radiation to an object. E349
radiant energy, Q[J], n—energy in the form of photons or
irradiation—at a point on a surface, see radiant exposure.
electromagnetic waves.
isotropic radiant energy— see radiant energy, isotropic.
radiant energy, atmospheric, Q[J], n—the portion of terres-
local zenith—see zenith.
trial radiation that is emitted by the atmosphere.
luminous, adj—referring to a radiometric quantity, weighted
radiant energy, blackbody, J, n—radiant energy emitted by a
accordingtothespectralluminousefficiencyfunctionV()of
(laboratory) blackbody, or radiant energy having that spec-
the CIE (1987). D1003
tral distribution. See Planck’s law in Practice E491.
monochromatic radiation, n—radiation charcterized by a
radiant energy, circumsolar, J, n—radiation scattered by the
single frequency. By extension, radiation of a very small
atmospheresothatitappearstooriginatefromanareaofthe
range of frequency or wavelenght that can be described by
sky immediately adjacent to the sun. Often referred to as the
stating a single frequency or wavelength. E349
solar aureole, its angular extent is generally directly related
normal, adj—describing a direction that is perpendicular to a
to the aerosol optical depth.
surface.
radiant energy, effective nocturnal, J , n—energy transfer
normal vector, n—the upward-pointing vector normal to the
required to maintain a horizontal upward-facing blackbody
plane of a receiver.
surface at the ambient air temperature, in the absence of
solar irradiance.
optical depth—see extinction optical depth, monochromatic.
radiant energy, infrared—see infrared radiation.
optical mass, actual, m [dimensionless], n—thelineintegral
act
along the sun vector of the density (ρ) of a substance as a
radiant energy, isotropic,J, n—diffuseradiantenergythathas
function of altitude between a point in the atmosphere (0)
the same radiance in all directions.
and the vacuum of space (∞); in atmospheric transmittance
calculations, the densities are normalized with units of radiant energy, terrestrial, J, n—radiant energy emitted by
–1
(length) (see Eq 5). the earth, including its atmosphere.
`
–2
radiant exitance at a point on a surface, M[W·m ],
m 5 * ρds (5)
act
n—quotient of the radiant flux leaving an element of the
DISCUSSION—The word “air” has been avoided in this definition
surface containing the point, by the area of that element.
because direct solar radiation is attenuated not only by air molecules
but also by additional constituents such as aerosols and water vapor. E349
Thus, it is possible to calculate water vapor mass as well as air mass
radiant exitance—see radiant exitance at a point on a
usingthisequation.Opticalmassesareoccasionallyreportedwithunits
of km. surface.
E772 − 15 (2021)
DISCUSSION—When applied to a property, spectral is indicated by the
radiant exitance, emitted—see radiant exitance at a point on
subscript λ following the symbol for the quantity, as L = dL/dλ;ata
λ
a surface.
specific wavelength, it is indicated by the subscript λ with the
–2
wavelength in parentheses, as L (500 nm).
radiant exposure, H[J·m ], n—at a point on a surface, time
λ
integral of irradiance.
sun vector, n—thevectorpointingfromthelocationofinterest
(usually a point on the Earth’s surface in solar energy
radiant flux, Φ[J/s], n—the SI derived quantity power, radi-
applications) to the center of the sun’s disk.
ant flux in Standard IEEE/ASTM SI 10.
DISCUSSION—Because of the curvature of the Earth, and because of
radiant flux, net, W, n—difference between downward and
the refraction due to density variations with altitude, the sun vector
upward(totalsolarandterrestrial)radiantflux;netfluxofall varies along the path a beam of solar radiation follows from the top of
atmosphere to the ground.
radiant energy across an imaginary horizontal surface.
total irradiance—see irradiance, total.
radiant flux, net terrestrial, W, n—difference between down-
ward and upward terrestrial radiant fluxes; net flux of
transmission, n—passage of radiation through a medium
terrestrial radiant energy.
without change of frequency of the monochromatic compo-
nents of which the radiation is composed. E349
radiant power—see radiant flux.
radiation, n—(1) emission or transfer of energy in the form of
transmission coeffıcient—see extinction coefficient.
electromagnetic waves or particles. (2) the electromagnetic
waves or particles. E349
transmittance, T [dimensionless], n—ratio of the transmitted
radiant or luminous flux to the incident flux. E349
radiation coefficient, n—the quotient of the net radiant exi-
tance of a blackbody (full radiator), by the temperature
turbidity, n—anempiricalexpressionof aerosol optical depth
difference between the blackbody and the surroundings with thatusesÅngstrom’swavelength-dependentformula(seeEq
which it is exchanging radiation. E349
8).
2α
k 5 β·λ (8)
αλ
Rayleigh scattering, n—amodelofmolecularscatteringinthe
DISCUSSION—In Eq 8, α and β are called the Ångstrom turbidity
atmosphere in which the monochromatic extinction coef-
parameters and λ is the wavelength. The units of α and β are such that
ficient varies as the wavelength raised to the negative fourth
the units of k are dimensionless. With wavelength units of µm, β is
α
power. Eq 7 is an approximation for dry air using wave-
commonly called the “turbidity” because it varies more than α, which
lengths in µm.
tends to stay fairly constant.
24.08
k 50.008735λ (7)
rλ
ultraviolet radiation, n—radiation for which the wavelengths
ofthemonochromaticcomponentsaresmallerthanthosefor
reflectance, n—ratio of the reflected radiant or luminous flux
visible radiation and more than about 1 nm. E349
to the incident flux. E349
visible radiation, n—any radiation capable of causing a visual
reflection, n—return of radiation by a surface without change
sensation. E349
offrequencyofthemonochromaticcomponentsofwhichthe
radiation is composed. E349
zenith, n—the upward-pointing vector normal to the Earth’s
surface at the location of interest (usually a point on the
reflection angle—See angle of reflection.
Earth’s surface in solar energy applications).
reflectivity, n—reflectance of a layer of material of such a
thickness that there is no change of reflectance with in- zenith angle, solar, θ [rad or °], n—the angle between the
z
zenith and the sun vector.
creased thickness. E349
DISCUSSION—Reflectivityisapropertyofamaterialandreflectanceis
INSTRUMENTATION
a property of a sample of the material, with no restriction on thickness
or surface topography.
absolute cavity pyrheliometer—see self-calibrating absolute
refraction, n—change in the direction of propagation of cavity pyrheliometer.
radiation determined by change in the velocity of propaga-
absolute cavity radiometer—see self-calibrating absolute
tion in passing from one medium to another medium with a
cavity pyrheliometer.
different index of refraction.
bolometer, n—instrument for measuring irradiance. Its prin-
refraction angle—see angle of refraction.
ciple is based on the variation of electrical resistance, with
refraction index—see index of refraction.
the incoming radiation of one or both of the resistance
reradiation, n—loss of energy by radiation from a surface
elements which comprise the instrument, as a result of
previously heated by absorption.
temperature changes.
spectral, adj—referring to radiometric quantities, for mono-
cavity radiometer—see self-calibrating absolute cavity
chromaticradiationataspecifiedwavelength(orfrequency),
pyrheliometer.
or, by extension, for radiation within a narrow wavelength
band about a specified wavelength. E349 edge-stress meter—see polarimeter, edge-stress.
E772 − 15 (2021)
field pyrheliometer, n—pyrheliometers that are designed and quality”orbetter(thatis,“GoodQuality”or“HighQuality”)
used for long-term field measurements of direct solar radia- First Class specifications, described in WMO-No. 8, appro-
tion. These pyrheliometers are weatherproof and therefore priate to field use, and typically exposed continuously.
possess windows, usually quartz, at the field aperture that
pyranometer, net, n—an instrument for measuring the differ-
pass all solar radiation in the range from 0.3 to 4-µm
ence between the irradiance falling on the top and bottom of
wavelength. E816
a horizontal surface.
full width at half maximum, FWHM[nm or µm], n—in a
pyranometer, reference, n—a pyranometer (see also ISO
bandpass filter, FWHM is the interval between wavelengths
9060), used as a reference to calibrate other pyranometers,
at which transmittance is 50 % of the peak, frequently
which is well-maintained and carefully selected to possess
referred to as bandwidth. G130
relatively high stability and has been calibrated using a
grazing-angle surface polarimeter—see polarimeter, grazing- pyrheliometer. G167
angle surface.
pyranometer, spherical, n—instrument for measuring the
international pyrheliometric scale—see World Radiometric solar flux falling from a 4π sr solid angle onto a spherical
surface.
Reference.
pyrgeometer, n—an instrument for measuring infrared atmo-
net pyrgeometer—see pyranometer, net.
spheric irradiance at wavelengths greater than 3000 nm on a
net pyrradiometer— see pyranometer, net.
horizontal upward facing black surface at the ambient air
photometer, n—a device that measures luminous intensity or
temperature.
brightness by converting (weighing) the radiant intensity of
an object using the relative sensitivity of the human visual
pyrheliometer, n—a radiometer used to measure the direct or
system as defined by the photopic curve. F1863
beam solar irradiance incident on a surface normal to the
sun’s rays.
polarimeter, n—an instrument used to measure the rotation of
the plane of polarization of polarized light passing through
pyrheliometer, compensated, n—pyrheliometer based on the
an optical structure or sample.
comparison of the heating of two identical metal strips, one
exposed to a solar radiant energy, the other to a joule effect.
polarimeter, edge stress, n—a specialized polarimeter for
measuring residual edge stress in annealed, heat-
pyrheliometer, field—see field pyrheliometer.
strengthened, or thermally tempered flat glass. Used as a
non-destructive method of characterizing strength and rela- pyrheliometer, primary standard—see primary standard
pyrheliomers.
tive frangibility of glass.
pyrheliometer, reference—see reference pyrheliometer.
polarimeter, grazing-angle surface, n—a specialized pola-
rimeter for measuring residual surface stress in annealed,
pyrheliometer, secondary standard—see secondary standard
heat-strengthened, or thermally tempered flat glass. Used as
pyrheliometer.
a non-destructive method of characterizing strength and
relative frangibility of glass. pyrheliometer, self-calibrating absolute cavity—see self-
calibrating absolute cavity pyrheliometer
polarimeter, photoelastic, n—a polariscopeadaptedforquan-
titative measurement of optical retardation, birefringence, or pyrheliometer, secondary reference, n—a pyrheliometer es-
stress and strain using photoelastic analysis techniques. sentially meeting the World Meteorological Organization
(WMO) “High Quality” specifications as described in
polariscope, n—an optical device consisting of a light source,
WMO-No. 8, but not having self-calibrating capability.
mutually perpendicular polarizing elements, and generally
equipped with one or more retardation plates for qualitative
pyrradiometer, spherical, n—instrument for measuring total
observations of relative optical retardation by color flux incident from a 4π sr solid angle onto a spherical
differentiation. C162
surface.
primary standard pyrheliometers, n—pyrheliometers, se-
radiometer, n—a general class of instruments designed to
lected from the group of absolute pyrheliometers (see detect and measure radiant energy. G113
self-calibrating absolute cavity pyrheliometer). E816
radiometer, broad-band, n—a relative term generally applied
pyranometer, n—a radiometer with a hemispherical field-of- toradiometerswithinterferencefiltersorcut-on/cut-offfilter
view (i.e. a 2π sr solid angle) used to measure the total solar pairs having a FWHM between 20 and 70 nm and with
radiant energy incident upon a surface per unit time per unit tolerances in center (peak) wavelength and FWHM no
area. This energy includes the direct radiant energy, diffuse greater than 62 nm. G130
radiant energy, and reflected radiant energy from the back-
radiometer, narrow-band, n—a relative term generally ap-
ground.
plied to radiometers with interference filters with FWHM ≤
pyranometer, field, n—a pyranometer meeting World Meteo- 20 nm and with tolerances in center (peak) wavelength and
rological Organization (WMO) Second Class “moderate FWHM no greater than 62 nm. G130
E772 − 15 (2021)
radiometer, wide-band, n—a relative term generally applied World Radiometric Reference, WRR, n—the mean of a
toradiometerswithcombinationsofcut-offandcut-onfilters selected group of at least four World Metrological Organi-
with FWHM greater than 70 nm. G130 zation (WMO) self calibrating absolute cavity pyrheliom-
eters maintained at the World Radiation Center, Physical
radiometry, n—measurement of the quantities associated with
Meteorological Organization, Davos (WRC/PMOD) at
radiation. E349
Davos, Switzerland. The WRR is accepted as representing
thephysicalunitsoftotalsolarirradiancewithanuncertainty
reference pyrheliometer, n—pyrheliometers of any category
of 0.3 % and a confidence of 99 %.
serving as a reference in calibration transfer procedures.
DISCUSSION—As of 1970 the WRR replaced the “International
Theyareselectedandwell-testedinstruments(seeTable2of
PyrheliometricScaleof1956,IPS56”,the“SmithsonianPyrheliometric
ISO 9060), that have a low rate of yearly change in
Scale of 1913, SI13” and the “Angstrom Scale of 1905, A05”, as
responsivity. The reference pyrheliometer may be of the
follows: WRR/IPS56 = 1.026, WRR/SI13 = 0.977, and WRR/A05 =
same type, class, and manufacturer as the field radiometers
1.026. See WMO-No. 8, Section 7.1.2.2.
in which case it is specially chosen for calibration transfer
SOLAR ENERGY – GENERAL
purposes and is termed a secondary standard pyrheliometer
(see ISO 9060), or it may be of the self-calibrating cavity
absorber, n—that part of a solar collector whose primary
type (see self-calibrating absolute cavity pyrheliometer).
function is to absorb radiant energy and transform it into
E816
another form of energy.
DISCUSSION—A thermal absorber usually possesses a solid surface
reflectometer, n—an instrument for the measurement of quan-
through which energy is transmitted by thermal conduction to the
tities pertaining to reflection. E349
transfer fluid; however, the transfer fluid itself can be the absorber in
the case of an optically transparent container and a “black liquid”. A
refractometer, n—an optical instrument used to measure the
photovoltaic absorber converts part of the incident solar flux into
index of refraction of an unknown sample.
electrical energy, and part to thermal energy.
roll-wave gauge, n—instrument used to monitor and quantify
albedo—discouraged in favor of the preferred term, reflec-
roller wave surface distortion, typically present in thermally
tance.
tempered flat glass processed in a horizontal roller-hearth
tempering furnace. C1651 altazimuthal mount, n—a supporting device that facilitates
tracking of the sun and allows rotation about horizontal and
secondary standard pyrheliometer, n—pyrheliometers of
vertical axes. It can be used to aim equipment such as
high precision and stability whose calibration factors are
heliostats, concentrating collectors, exposure specimens, or
derived from primary standard pyrheliometers. This group
radiometers.
comprises absolute cavity pyrheliometers that do not fulfill
the requirements of a primary standard pyrheliometer. E816 apparent solar time, apt[h], n—thehoursoftheday(i.e.time)
as computed from the position of the sun (see solar noon).
self-calibrating absolute cavity pyrheliometer, n—a radiom-
eter consisting of either a single- or dual-conical heated auxiliary energy subsystem, n—in solar energy applications,
cavity that, during the self-calibration mode, displays the equipment using nonsolar energy sources to supplement or
backup the output provided by a solar energy system.
powerrequiredtoproduceathermopilereferencesignalthat
isidenticaltothesamplingsignalobtainedwhenviewingthe
cloud cover, n—that portion of the sky which is covered by
sun with an open aperture. The reference signal is produced
clouds, usually expressed in tenths of sky covered.
by the thermopile in response to the cavity irradiance
resulting from heat supplied by a cavity heater with the
collector cover (glazings)—see cover plate, collector.
aperture closed. E816
collector, concentrating, n—a solar collector that uses
spectrophotometer, n—instrument for measuring the ratio of
reflectors, lenses, or other optical elements to redirect and
two spectral radiometric qualities. E349
concentrate the solar irradiance on the collector aperture
ontoanabsorberofwhichthesurfaceareaissmallerthanthe
spectroradiometer, n—an instrument for measuring the radi-
collector aperture area.
ant energy of a light source at each wavelength throughout
the spectrum. G138
collector, flat plate, n—a non-concentrating solar collector in
which the absorbing surface is essentially planar.
strain viewer—see polariscope.
collector, line-focus, n—a concentrating collector that focuses
sunphotometer, n—a narrow-band radiometer (see
the solar flux in one dimension only.
radiometer, narrow-band) that measures relative direct
solar intensity at a number of discrete wavelengths that are collector, point focus, n—a concentrating collector that fo-
selectedfordeterminationofatmosphericopticaldepthsdue
cuses the solar flux to a point, i.e. in two dimensions.
to constituents, especially aerosol scattering and molecular
collector, tracking, n—a solar collector that moves so as to
absorption by water vapor and ozone.
follow the apparent motion of the sun during the day,
sun radiometer—see sunphotometer. rotating about one axis or two orthogonal axes.
E772 − 15 (2021)
concentration ratio—see concentration ratio, geometric and Fresnel-reflector system, n—flat mirrors arranged in an array
concentration ratio, photovoltaic. such that they reflect onto a target, the illuminated area of
which simulates the shape and size of the flat mirror. (Such
concentration ratio, geometric, n—of a concentrating solar
anarraysimulatestheray-tracingofaparabolictroughofthe
thermal collector, the ratio of the collector aperture area to
same aperture angle.)
the absorber area.
heliostat, n—a reflector that is mechanically positioned so that
concentration ratio, photovoltaic, n—the total irradiance at
solar flux is reflected onto a stationary receiver or target.
the front surface of a photovoltaic cell intended for use
–2
inside a concentrating collector, divided by 1000 W·m . in-service conditions, n—the normal conditions to which a
−2
system and its components will be exposed during their
DISCUSSION—The 1000 W·m value of the denominator in this ratio
is equal to the total irradiance from the standard reporting conditons
operational lifetimes. This does not include stagnation con-
definedforperformancemeasurementsmadewithrespecttotheTables
ditions; see stagnation conditions.
G173 reference spectral irradiance distribution (see Test Methods
E948 and E1036). Because this value is defined as “one-sun”, the
insolation—discouraged in favor of the preferred term, solar
normalization changes the total irradiance into a multiplicative factor
irradiance.
corresponding to the magnitude of the concentration, which is some-
DISCUSSION—Insolation is sometimes used as a synonym for radiant
times referred to as the “number of suns”.
–2 –2
exposure, with units of J·m or the non-SI equivlaent kWh·m . This
usage is also discouraged.
concentrating collector—see collector, concentrating.
–2 –1
isohel, MJ·m ·year , n—a line on a map connecting points
concentrator, n—an optical device (lenses or mirrors) that, as
thatreceiveequalamountsofsolarradiationoveraperiodof
part of a solar collector, receives the unconcentrated solar
one year.
irradiance over a large area aperture and redirects and
focuses (concentrates) it to a smaller area (the receiver).
isopleth, n—a line on a chart or graph connecting points
having a specified constant value of a single variable as a
cover plate, collector, n—a sheet of transparent (or translu-
function of two other specified variables.
cent) glazing placed above the absorber in a solar collector,
to provide thermal and environmental protection.
line-focus collector—see collector, line-focus.
design life, n—the period of time during which a system or
natural-type environment, n—in solar energy applications,
component is expected to perform its intended function,
the natural aspects of the outdoor exposure elements (or
without significant degradation of performance and without
simulation),includingchangeswithtime,thatmayaffectthe
requiring major maintenance or replacement.
performance of a collector through degradation of collector
materials or physical damage to the collector configuration.
direct radiation, n—radiation received from a small solid
Typical aspects include radiant exposure, ambient
angle centered on the sun’s disk, on a given plane (see ISO
temperature, and rain impingement.
9060). That component of sunlight is the beam between an
observer, or instrument, and the sun within a solid conical
natural weathering, n—outdoor exposure of materials to
angle centered on the sun’s disk and having a total included
unconcentrated sunlight, the purpose of which is to assess
planar field angle of 5 to 6° (see also Test Method E816).
the effects of environmental factors on various functional
and decorative parameters of interest. G113
direct beam radiation—see direct radiation.
operating conditions, extreme, n—unusual physical condi-
direct solar radiation—see direct radiation.
tions to which a component or system may be exposed and
equatorial mount, n—a sun-tracking mount, usually clock-
for which it is not designed or intended to withstand, nor is
driven,whoseaxisofrotationisparalleltothatoftheearth’s it required to withstand by a local regulatory agency.
axis of rotation.
operating conditions, normal, n—the usual range of physical
exposure racks, at-latitude, n—racks that hold specimens at conditions (for example, temperature, pressure, wear and
aninclinationangleequaltothelatitudeoftheracklocation,
tear, weather) for which the component or system was
facing the equator. designed.
flat plate collector—see collector, flat plate. plant, solar, n—a generic term for any solar energy collection
system, either photovoltaic or thermal; its usage is discour-
Fresnel lens, circular, n—a sheet of transparent material into
aged in favor of the specific terms system, photovoltaic or
whichconcentricgrooveshavebeenformedinsuchapattern
system, solar thermal energy.
thatlightwillbefocusedaswithalens.(Focusingmirrorsof
similar design are also available.) point focus collector—see collector, point focus
Fresnel lens, linear, n—a sheet of transparent material into receiver, n—in solar energy systems, that part of the solar
which parallel grooves have been formed in such a pattern collector to which the solar irradiance is finally directed or
that light will be focused as by a cylindrical lens. (Focusing redirected, and includes the absorber and any associated
mirrors of similar design are also available.) glazings through which the redirected energy must pass.
E772 − 15 (2021)
shading—see shadowing. solar irradiance, global tilted—discouraged in favor of the
preferred term solar irradiance, hemispherical tilted.
shadowing, v—the act of casting a shadow across any surface.
–2
solar irradiance, hemispherical, E [W·m ], n—on a given
H
solar, adj—referring to radiometric quantities, indicates that
plane, the solar radiant flux received from within the 2π
the radiant flux involved has the sun as its source, or has the
steradian field of view of a tilted plane from the portion of
relative spectral distribution of the sun’s radiant flux.
theskydomeandtheforegroundincludedintheplane’sfield
of view, including both diffuse and direct solar radiation.
solar, adj—referring to optical properties,indicatesaweighted
G173
average of the spectral property, with a standard solar
DISCUSSION—For the special condition of a horizontal plane the
spectral irradiance distribution as the weighting function.
hemispherical solar irradiance is properly termed global solar
solar degradation, n—the process by which exposure to solar irradiance, E . Incorrectly, global tilted, or total global irradiance is
G
oftenusedtoindicatehemisphericalirradianceforatiltedplane.Incase
energy deteriorates the properties of materials and compo-
of a sun-tracking receiver, this hemispherical irradiance is commonly
nents; or, the deterioration of materials and components
called global normal irradiance. The adjective global should refer only
produced by exposure to solar energy.
to hemispherical solar radiation on a horizontal, not a tilted, surface.
solar energy, n—electromagnetic energy emitted by the sun.
G173
The solar radiation incident on the top of the terrestrial
–2
solar irradiance hemispherical tracking, W·m , n—on a
atmosphere is called extraterrestrial solar radiation; 97
plane that always pointed normal to the sun with a tracking
percent of which is confined to the spectral range 290 to
device, the solar radiant flux received from within the 2π sr
3000 nm. WMO-No. 8
field-of-view of a tilted plane from the portion of the sky
solar flux, Φ [J/s], n—radiant flux received from the sun.
dome and the foreground included in the plane’s field of
view, including both diffuse and direct solar radiation.
–2
solar irradiance, E [W·m ], n—irradiancereceivedfromthe
s
–2
sun.
solar irradiance, hemispherical tilted, W·m ,
DISCUSSION—Solar irradiance is a function of distance between the
n—hemispherical solar irradiance incident on a non-
sun and the place of measurement, falling off as the inverse of the
horizontal stationary surface; see discussion for solar
square of the separation. Typically, the place of measurement is the
irradiance, hemispherical.
surface of the earth, thus sometimes the term “terrestrial solar irradi-
ance” is used. Note that the distance between the sun and the Earth
solar irradiance, total global—discouraged in favor of the
changes because the Earth’s orbit is elliptical; the resulting variation in
preferred term solar irradiance, hemispherical tilted.
solar irradiance at the top of the Earth’s atmosphere is approximately
–2
63.0 %.
solar irradiance, instantaneous, W·m —see solar irradi-
–2
ance.
solar irradiance, diffuse, W·m , n—
...