ASTM G167-15(2023)

Standard Test Method for Calibration of a Pyranometer Using a Pyrheliometer

SIGNIFICANCE AND USE
4.1 The pyranometer is a radiometer designed to measure the sum of directly solar radiation and sky radiation in such proportions as solar altitude, atmospheric conditions and cloud cover may produce. When tilted to the equator, by an angle β, pyranometers measure only hemispherical radiation falling in the plane of the radiation receptor.  
4.2 This test method represents the only practical means for calibration of a reference pyranometer. While the sun-trackers, the shading disk, the number of instantaneous readings, and the electronic display equipment used will vary from laboratory to laboratory, the method provides for the minimum acceptable conditions, procedures and techniques required.  
4.3 While, in theory, the choice of tilt angle (β) is unlimited, in practice, satisfactory precision is achieved over a range of tilt angles close to the zenith angles used in the field.  
4.4 The at-tilt calibration as performed in the tilted position relates to a specific tilted position and in this position requires no tilt correction. However, a tilt correction may be required to relate the calibration to other orientations, including axis vertical.
Note 1: WMO High Quality pyranometers generally exhibit tilt errors of less than 0.5 %. Tilt error is the percentage deviation from the responsivity at 0° tilt (horizontal) due to change in tilt from 0° to 90° at 1000 W·m23.  
4.5 Traceability of calibrations to the World Radiometric Reference (WRR) is achieved through comparison to a reference absolute pyrheliometer that is itself traceable to the WRR through one of the following:  
4.5.1 One of the International Pyrheliometric Comparisons (IPC) held in Davos, Switzerland since 1980 (IPC IV). See Refs (3-7).  
4.5.2 Any like intercomparison held in the United States, Canada or Mexico and sanctioned by the World Meteorological Organization as a Regional Intercomparison of Absolute Cavity Pyrheliometers.  
4.5.3 Intercomparison with any absolute cavity pyrheliometer t...
SCOPE
1.1 This test method covers an integration of previous Test Method E913 dealing with the calibration of pyranometers with axis vertical and previous Test Method E941 on calibration of pyranometers with axis tilted. This amalgamation of the two methods essentially harmonizes the methodology with ISO 9846.  
1.2 This test method is applicable to all pyranometers regardless of the radiation receptor employed, and is applicable to pyranometers in horizontal as well as tilted positions.  
1.3 This test method is mandatory for the calibration of all secondary standard pyranometers as defined by the World Meteorological Organization (WMO) and ISO 9060, and for any pyranometer used as a reference pyranometer in the transfer of calibration using Test Method E842.  
1.4 Two types of calibrations are covered: Type I calibrations employ a self-calibrating, absolute pyrheliometer, and Type II calibrations employ a secondary reference pyrheliometer as the reference standard (secondary reference pyrheliometers are defined by WMO and ISO 9060).  
1.5 Calibrations of reference pyranometers may be performed by a method that makes use of either an altazimuth or equatorial tracking mount in which the axis of the radiometer's radiation receptor is aligned with the sun during the shading disk test.  
1.6 The determination of the dependence of the calibration factor (calibration function) on variable parameters is called characterization. The characterization of pyranometers is not specifically covered by this method.  
1.7 This test method is applicable only to calibration procedures using the sun as the light source.  
1.8 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.  
1.9 This interna...

General Information

• Status: Published
• Publication Date: 31-Jan-2023
• ICS: 07.060 - Geology. Meteorology. Hydrology
• Technical Committee: G03 - Weathering and Durability
• Drafting Committee: G03.09 - Radiometry

Relations

• Refers: ASTM E824-10(2018)e1 - Standard Test Method for Transfer of Calibration From Reference to Field Radiometers
• Effective Date: 15-Apr-2018
• Refers: ASTM E772-13 - Standard Terminology of Solar Energy Conversion
• Effective Date: 01-Sep-2013
• Refers: ASTM E772-11 - Standard Terminology of Solar Energy Conversion
• Effective Date: 01-Sep-2011
• Refers: ASTM E824-10 - Standard Test Method for Transfer of Calibration From Reference to Field Radiometers
• Effective Date: 01-Dec-2010
• Refers: ASTM E824-05 - Standard Test Method for Transfer of Calibration From Reference to Field Radiometers
• Effective Date: 01-Oct-2005
• Refers: ASTM E772-05 - Standard Terminology Relating to Solar Energy Conversion
• Effective Date: 01-Apr-2005
• Refers: ASTM E824-94(2002) - Standard Test Method for Transfer of Calibration From Reference to Field Radiometers
• Effective Date: 15-May-1994
• Refers: ASTM E824-94 - Standard Test Method for Transfer of Calibration From Reference to Field Radiometers
• Effective Date: 01-Jan-1994
• Refers: ASTM E772-87(2001) - Standard Terminology Relating to Solar Energy Conversion
• Effective Date: 27-Feb-1987
• Refers: ASTM E772-87(1993)e1 - Standard Terminology Relating to Solar Energy Conversion
• Effective Date: 27-Feb-1987

Overview

ASTM G167-15(2023), titled Standard Test Method for Calibration of a Pyranometer Using a Pyrheliometer, is a comprehensive international standard developed by ASTM International. This document defines the procedures and requirements for the accurate calibration of pyranometers-radiometric instruments used to measure global (hemispherical) solar irradiance-by comparison with a reference pyrheliometer. The calibration methods prescribed are crucial for aligning pyranometer measurements with the World Radiometric Reference (WRR), ensuring traceability to internationally recognized irradiance scales.

The scope of ASTM G167-15(2023) harmonizes existing ASTM methods and aligns closely with ISO 9846, making it applicable for all pyranometers regardless of receptor type or orientation. Its adoption supports measurement consistency in solar energy, meteorology, and materials testing worldwide.

Key Topics

• Calibration Procedures: Covers both Type I (using a self-calibrating absolute pyrheliometer) and Type II (using a secondary reference pyrheliometer) calibration methods as defined by the World Meteorological Organization (WMO) and ISO 9060.
• Instrument Orientation: Details calibration for pyranometers in both horizontal and tilted configurations, reflecting practical deployment and field applications.
• Traceability: Ensures calibration results are traceable to the World Radiometric Reference (WRR) through intercomparison programs, satisfying global quality requirements.
• Sources of Uncertainty: Discusses key factors influencing calibration accuracy-such as thermal offset, instrument design (all-black, black-and-white, or solid-state detectors), orientation, and environmental conditions.
• Practical Considerations: Outlines minimum acceptable equipment and procedures, including sun-tracking mounts, shading devices, and data logging systems.
• Applications of Shade Methods: Describes both the alternating and continuous shading techniques for isolating direct and diffuse solar radiation during calibration.

Applications

Calibration in accordance with ASTM G167-15(2023) is foundational for:

• Solar Energy Assessment: Accurate pyranometer calibration is essential for quantifying solar resource availability, optimizing the performance and placement of solar panels, collectors, and photovoltaic systems.
• Materials Durability Testing: Ensures valid irradiance exposures when testing the weathering and degradation of construction materials, coatings, plastics, and textiles.
• Meteorological and Climate Research: Provides precise solar radiation data for climate modeling, atmospheric science, and weather forecasting.
• Reference Transfer: Mandated for secondary standard pyranometers and any device serving as a reference in calibration transfer (following ASTM E842).
• Quality Assurance: Supports compliance with ISO 9060 and alignment with global best practices required by regulatory bodies or contractual agreements.

Laboratories, field researchers, solar energy professionals, and test facilities benefit from adopting this standard, achieving measurement comparability and regulatory acceptance.

Related Standards

• ISO 9846 – Solar Energy: Calibration of a Pyranometer Using a Pyrheliometer
• ISO 9060 – Solar Energy: Specification and Classification of Instruments for Measuring Hemispherical Solar and Direct Solar Radiation
• ASTM E824 – Test Method for Transfer of Calibration from Reference to Field Radiometers
• WMO Guide No. 8 – Measurement of Radiation: Meteorological Instruments and Methods of Observation

These documents, together with ASTM G167-15(2023), form the core standards for solar radiometry and pyranometer calibration, ensuring data quality and harmonization across disciplines and geographic regions.

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Keywords: pyranometer calibration, pyrheliometer, ASTM G167-15, WRR traceability, solar irradiance measurement, reference instrument, ISO 9846, meteorological standards, solar radiation, calibration method.

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: G167 − 15 (Reapproved 2023)
Standard Test Method for
Calibration of a Pyranometer Using a Pyrheliometer
This standard is issued under the fixed designation G167; 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.
INTRODUCTION
Accurate and precise measurements of total global (hemispherical) solar irradiance are required in
the assessment of irradiance and radiant exposure in the testing of exposed materials, determination
of the energy available to solar collection devices, and assessment of global and hemispherical solar
radiation for meteorological purposes.
This test method requires calibrations traceable to the World Radiometric Reference (WRR), which
represents the SI units of irradiance. The WRR is determined by a group of selected absolute
pyrheliometers maintained by the World Meteorological Organization (WMO) in Davos, Switzerland.
Realization of the WRR in the United States, and other countries, is accomplished by the
intercomparison of absolute pyrheliometers with the World Radiometric Group (WRG) through a
series of intercomparisons that include the International Pyrheliometric Conferences held every five
years in Davos. The intercomparison of absolute pyrheliometers is covered by procedures adopted by
WMO and is not covered by this test method.
It should be emphasized that “calibration of a pyranometer” essentially means the transfer of the
WRR scale from a pyrheliometer to a pyranometer under specific experimental procedures.
1. Scope eter as the reference standard (secondary reference pyrheliom-
eters are defined by WMO and ISO 9060).
1.1 This test method covers an integration of previous Test
Method E913 dealing with the calibration of pyranometers
1.5 Calibrations of reference pyranometers may be per-
with axis vertical and previous Test Method E941 on calibra-
formed by a method that makes use of either an altazimuth or
tion of pyranometers with axis tilted. This amalgamation of the
equatorial tracking mount in which the axis of the radiometer’s
two methods essentially harmonizes the methodology with ISO radiation receptor is aligned with the sun during the shading
9846.
disk test.
1.2 This test method is applicable to all pyranometers
1.6 The determination of the dependence of the calibration
regardless of the radiation receptor employed, and is applicable
factor (calibration function) on variable parameters is called
to pyranometers in horizontal as well as tilted positions.
characterization. The characterization of pyranometers is not
specifically covered by this method.
1.3 This test method is mandatory for the calibration of all
secondary standard pyranometers as defined by the World
1.7 This test method is applicable only to calibration pro-
Meteorological Organization (WMO) and ISO 9060, and for
cedures using the sun as the light source.
any pyranometer used as a reference pyranometer in the
1.8 This standard does not purport to address all of the
transfer of calibration using Test Method E842.
safety concerns, if any, associated with its use. It is the
1.4 Two types of calibrations are covered: Type I calibra-
responsibility of the user of this standard to establish appro-
tions employ a self-calibrating, absolute pyrheliometer, and
priate safety, health, and environmental practices and deter-
Type II calibrations employ a secondary reference pyrheliom-
mine the applicability of regulatory limitations prior to use.
1.9 This international standard was developed in accor-
dance with internationally recognized principles on standard-
This test method is under the jurisdiction of ASTM Committee G03 on
Weathering and Durability and is the direct responsibility of Subcommittee G03.09
ization established in the Decision on Principles for the
on Radiometry.
Development of International Standards, Guides and Recom-
Current edition approved Feb. 1, 2023. Published February 2023. Originally
mendations issued by the World Trade Organization Technical
approved in 2000. Last previous edition approved in 2015 as G167 – 15. DOI:
10.1520/G0167-15R23. Barriers to Trade (TBT) Committee.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G167 − 15 (2023)
2. Referenced Documents 3.2.8 hemispherical radiation, n—combined direct and dif-
2 fuse solar radiation incident from a virtual hemisphere, or from
2.1 ASTM Standards:
2π Sr, on any inclined surface.
E772 Terminology of Solar Energy Conversion
3.2.8.1 Discussion—The case of a horizontal surface is
E824 Test Method for Transfer of Calibration From Refer-
denoted global solar radiation (3.2.7).
ence to Field Radiometers
3 3.2.9 pyranometer, n—see Terminology E772.
2.2 WMO Document:
3.2.10 pyranometer, field, n—a pyranometer meeting WMO
World Meteorological Organization (WMO), “Measurement
Good Quality or better (that is, High Quality) appropriate to
of Radiation” Guide to Meteorological Instruments and
field use and typically exposed continuously.
Methods of Observation, seventh ed., WMO-No. 8, Ge-
neva
3.2.11 pyranometer, reference, n—a pyranometer (see also
ISO 9060), used as a reference to calibrate other pyranometers,
2.3 ISO Standards:
which is well-maintained and carefully selected to possess
ISO 9060:1990 Solar Energy—Specification and Classifica-
relatively high stability and has been calibrated using a
tion of Instruments for Measuring Hemispherical Solar
pyrheliometer.
and Direct Solar Radiation
ISO 9846:1993 Solar Energy—Calibration of a Pyranometer
3.2.12 pyrheliometer, n—see Terminology E772 and ISO
Using a Pyrheliometer
9060.
3.2.13 pyrheliometer, absolute (self-calibrating), n—a solar
3. Terminology
radiometer with a limited field of view configuration. The field
3.1 Definitions:
of view should be approximately 5.0° and have a slope angle of
3.1.1 See Terminology E772. from 0.75° to 0.8°, with a blackened conical cavity receiver for
absorption of the incident radiation. The measured electrical
3.2 Definitions of Terms Specific to This Standard:
power to a heater wound around the cavity receiver constitutes
3.2.1 altazimuth mount, n—a tracking mount capable of
the method of self-calibration from first principles and trace-
rotation about orthogonal altitude and azimuth axes; tracking
ability to absolute SI units. The self-calibration principle
may be manual or by a follow-the-sun servomechanism.
relates to the sensing of the temperature rise of the receiving
3.2.2 calibration of a radiometer, v—determination of the
cavity by an associated thermopile when first the sun is
responsivity (or the calibration factor, the reciprocal of the
incident upon the receiver and subsequently when the same
responsivity) of a radiometer under well-defined measurement
thermopile signal is induced by applying precisely measured
conditions.
power to the heater with the pyrheliometer shuttered from the
3.2.3 direct solar radiation, n—that component of solar
sun.
radiation within a specified solid angle (usually 5.0° or 5.7°)
3.2.14 shading-disk device, n—a device which allows
subtended at the observer by the sun’s solar disk, including a
movement of a disk in such a way that the receiver of the
portion of the circumsolar radiation.
pyranometer to which it is affixed, or associated, is shaded
3.2.4 diffuse solar radiation, n—that component of solar
from the sun. The cone formed between the origin of the
radiation scattered by the air molecules, aerosol particles, cloud
receiver and the disk subtends an angle that closely matches the
and other particles in the hemisphere defined by the sky dome.
field of view of the pyrheliometer against which it is compared.
Alternatively, and increasingly preferred, a sphere rather than a
3.2.5 equatorial mount, n—see Terminology E772.
disk eliminates the need to continuously ensure the proper
3.2.6 field of view angle of a pyrheliometer, n—full angle of
alignment of the disk normal to the sun. See Appendix X1.
the cone which is defined by the center of the receiver surface
3.2.15 slope angle, n—the angle defined by the difference in
(see ISO 9060, 5.1) and the border of the limiting aperture, if
radii of the view limiting aperture (radius = R) and the receiver
the latter are circular and concentric to the receiver surface; if
radius (= r) in a pyrheliometer. The slope angle, s, is the
not, effective angles may be calculated (1, 2).
arctangent of R minus r divided by the distance between the
3.2.7 global solar radiation, n—combined direct and diffuse
limiting aperture and the receiver surface, denoted by L:
solar radiation falling on a horizontal surface; solar radiation
-1
s = Tan (R – r)/L. See Ref (1).
incident on a horizontal surface from the hemispherical sky
3.2.16 thermal offset, n—a non-zero signal generated by a
dome, or from 2π Steradian (Sr).
radiometer when blocked from all sources of radiation. Be-
lieved to be the result of infrared (thermal) radiation exchanges
between elements of the radiometer and the environment.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
3.3 Acronyms:
Standards volume information, refer to the standard’s Document Summary page on
3.3.1 ACR—Absolute Cavity Radiometer
the ASTM website.
Available from World Meterological Organization, 7bis, avenue de la Paix,
3.3.2 ANSI—American National Standards Institute
CP2300, CH-1211 Geneva 2, Switzerland, http://www.wmo.int.
3.3.3 ARM—Atmospheric Radiation Measurement Program
Available from International Organization for Standardization (ISO), 1, ch. de
la Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http://www.iso.org.
3.3.4 DOE—Department of Energy
The boldface numbers in parentheses refer to the list of references at the end of
this standard. 3.3.5 GUM—(ISO) Guide to Uncertainty in Measurements
G167 − 15 (2023)
3.3.6 IPC—International Pyrheliometer comparison which was found to be within 60.4 % of the mean of all
absolute pyrheliometers participating therein.
3.3.7 ISO—International Standards Organization
4.6 The calibration method employed in this test method
3.3.8 NCSL—National Council of Standards Laboratories
assumes that the accuracy of the values obtained are indepen-
3.3.9 NIST—National Institute of Standards and Technology
dent of time of year, with the constraints imposed and by the
3.3.10 NREL—National Renewable Energy Laboratory
test instrument’s temperature compensation circuit (neglecting
cosine errors).
3.3.11 PMOD—Physical Meteorological Observatory Da-
vos
5. Selection of Shade Method
3.3.12 SAC—Singapore Accreditation Council
5.1 Alternating Shade Method:
3.3.13 SINGLAS—Singapore Laboratory Accreditation Ser-
5.1.1 The alternating shade method is required for a primary
vice
calibration of the reference pyranometer used in the
3.3.14 UKAS—United Kingdom Accrediation Service
Continuous, Component-Summation Shade Method described
3.3.15 WRC—World Radiation Center in 5.2.
5.1.2 The pyranometer under test is compared with a
3.3.16 WRR—World Radiometric Reference
pyrheliometer measuring direct solar irradiance (or, optionally,
3.3.17 WMO—World Meteorological Organization
a continuously shaded control pyranometer; see Appendix X3
– Appendix X5). The voltage values from the pyranometer that
4. Significance and Use
correspond to direct solar irradiance are derived from the
4.1 The pyranometer is a radiometer designed to measure difference between the response of the pyranometer to hemi-
the sum of directly solar radiation and sky radiation in such
spherical (unshaded) solar irradiance and the diffuse (shaded)
proportions as solar altitude, atmospheric conditions and cloud solar irradiance. These response values (for example, voltages)
cover may produce. When tilted to the equator, by an angle β, are induced periodically by means of a movable sun shade
pyranometers measure only hemispherical radiation falling in disk. For the calculation of the responsivity, the difference
the plane of the radiation receptor. between the unshaded and shaded irradiance signals is divided
by the direct solar irradiance (measured by the pyrheliometer)
4.2 This test method represents the only practical means for
component that is normal to the receiver plane of the pyra-
calibration of a reference pyranometer. While the sun-trackers,
nometer.
the shading disk, the number of instantaneous readings, and the
5.1.3 For meteorological purposes, the solid angle from
electronic display equipment used will vary from laboratory to
which the scattered radiative fluxes that represent diffuse
laboratory, the method provides for the minimum acceptable
radiation are measured shall be the total sky hemisphere,
conditions, procedures and techniques required.
excluding a small solid angle around the sun’s disk.
4.3 While, in theory, the choice of tilt angle (β) is unlimited,
5.1.4 In addition to the basic method, modifications of this
in practice, satisfactory precision is achieved over a range of
method that are considered to improve the accuracy of the
tilt angles close to the zenith angles used in the field.
calibration factors, but which require more operational
4.4 The at-tilt calibration as performed in the tilted position experience, are presented in Appendix X3 – Appendix X5.
relates to a specific tilted position and in this position requires
5.2 Continuous Sun-and-Shade Method (Component Sum-
no tilt correction. However, a tilt correction may be required to
mation):
relate the calibration to other orientations, including axis
5.2.1 The pyranometer is compared with two reference
vertical.
radiometers, one of which is a pyrheliometer and the other a
well-calibrated reference pyranometer equipped with a track-
NOTE 1—WMO High Quality pyranometers generally exhibit tilt errors
of less than 0.5 %. Tilt error is the percentage deviation from the ing shade disk or sphere to measure diffuse solar radiation. The
responsivity at 0° tilt (horizontal) due to change in tilt from 0° to 90° at
reference pyranometer shall be either calibrated using the
1000 W·m .
alternating sun-and shade method described in 5.1, or shall be
4.5 Traceability of calibrations to the World Radiometric
compared against such a pyranometer in accordance with Test
Reference (WRR) is achieved through comparison to a refer- Method E824.
ence absolute pyrheliometer that is itself traceable to the WRR
5.2.2 Global solar irradiance (or hemispherical solar irradi-
through one of the following:
ance for inclined pyranometers) is determined by the sum of
4.5.1 One of the International Pyrheliometric Comparisons
the direct solar irradiance measured with a pyrheliometer
(IPC) held in Davos, Switzerland since 1980 (IPC IV). See multiplied by the cosine of the incidence angle of the beam to
Refs (3-7).
the local horizontal (or inclined plane parallel to the radiometer
4.5.2 Any like intercomparison held in the United States, sensor), plus the diffuse solar irradiance measured with a
Canada or Mexico and sanctioned by the World Meteorological
shaded reference pyranometer mounted in the same configu-
Organization as a Regional Intercomparison of Absolute Cav- ration (tilted or horizontal) as the unit under test.
ity Pyrheliometers.
5.2.3 The smallest uncertainty realized in the calibration of
4.5.3 Intercomparison with any absolute cavity pyrheliom- pyranometers will occur when the pyrheliometer is a self-
eter that has participated in either and IPC or a WMO- calibrating absolute cavity pyrheliometer and when the refer-
sanctioned intercomparison within the past five years and ence pyranometer has itself been calibrated over a range of air
G167 − 15 (2023)
mass (zenith angle) by the component summation (continuous 6.1.5 In consequence of 6.1.1 to 6.1.4, the most accurate
shade) method using a reference diffuse pyranometer with a diffuse irradiance measurement for the component summation
minimal thermal offset (see 6.1). Such a reference pyranometer technique is that made with a black-and-white detector design
must have been calibrated under conditions in which the for the diffuse reference pyranometer.
continuously shaded pyranometer had been itself calibrated by
6.1.6 The calibrations of pyranometers with all-black detec-
the alternating shade method.
tors with an all-black detector reference pyranometer for
diffuse measurement in the component summation technique,
5.3 Comparison of the Alternating and Continuous Shade
will have inherently larger uncertainties, due to the unknown
Methods:
magnitudes of thermal offset voltages in the all-black detectors
5.3.1 A disadvantage of the continuous, or component-
(10, 11).
summation shade method, is that two radiometers must be
6.1.7 Pyranometers utilizing solid-state photoconductive or
employed as reference: a pyrheliometer and a continuously
photovoltaic detectors (for example, silicon photodiodes) have
shaded pyranometer.
limited spectral response ranges (typically only about 75 % of
5.3.2 A disadvantage of the component-summation method
the full solar spectrum), non uniform spectral response, and
is the complexity of the apparatus to effect a continuously
varying temperature and angular response characteristics, de-
moving, that is, tracking, shaded disk/sphere with respect to the
pending on their design. These factors should be considered as
reference pyranometer’s receiver.
additional sources of uncertainty, and included in the uncer-
5.3.3 An advantage of the component-summation method is
tainty analysis of results for calibrations of and measurements
that any number of co-planer pyranometers may be calibrated
from such pyranometers. See Section 15, Measurement Uncer-
at the same time.
tainty.
5.3.4 Calibrations performed using the component-
summation method have the advantage of much lower uncer-
NOTE 3—Because of extreme differences in the spectral power distri-
tainties under conditions of moderately high to high ratios of
bution of total hemispherical and diffuse hemispherical solar radiation, the
use of pyranometers with detectors that have limited spectral response,
direct to diffuse solar radiation.
such as silicon photodiodes, to measure diffuse irradiance can produce
NOTE 2—If an absolute pyrheliometer with a typical uncertainty of
errors of up to several percent in diffuse irradiance (shaded configuration).
0.5 % is used to measure the direct solar radiation when the direct
Thus the alternating shade method of calibration for silicon detector
component is 80 % of the global radiation (as an example), and a
pyranometers, and the use of such radiometers to measure a diffuse
pyranometer with an uncertainty of 4 % is used to measure 20 % of the
reference irradiance is discouraged.
horizontal diffuse solar radiation, resultant uncertainties can be as low as
6.2 Sky Conditions—The measurements made in determin-
1.2 % (as opposed to nearly 4 % for the alternating shade method).
ing the instrument constant shall be performed only under
6. Interferences and Precautions
conditions when the sun is unobstructed by clouds for an
6.1 Pyranometer Design and Thermal Performance—The incremental data taking period. The minimum acceptable direct
solar irradiance on the tilted surface, given by the product of
absolute accuracy of the calibration of thermal detector (ther-
mopile) pyranometers depends on the design of the detector of the pyrheliometer measurement and the cosine of the incident
angle, shall be 80 % of the global solar irradiance. Also, no
the unit under test and the design of the detector of the
pyranometer measuring the diffuse irradiance in the cloud formation shall be within 30° of the sun during the period
that data are taken for record.
component-summation technique.
6.1.1 Pyranometers with thermal sensing elements (thermo-
6.3 Instrument Orientation Corrections—The irradiance
piles) have two basic designs: all black detectors, and black
calibration of a pyranometer is influenced by the tilt angle and
and white detectors. In the former, reference junctions for the
the azimuthal orientation of the instrument about its optical
thermopile are not exposed to solar radiation, and measuring
axis. Orientation effects are minimized by using an altazimuth
junctions are under a black coating exposed to the solar
platform and mounting the tilted pyranometer with the cable
radiation. In the latter, the measuring (under black coatings)
connection mounted pointing downward. When calibrating a
and reference (under white coatings) junctions are exposed to
pyranometer with its axis vertical, the sun angle changes
the same solar and thermal radiation environment.
through a range of azimuths. Hence, the azimuth angle between
6.1.2 Pyranometers with all black detectors have inherent
the sun and the direction of the cable connector or other
thermal imbalance, referred to as thermal offset, which is
reference mark may be significant.
dependent on the exchange of radiation between the detector,
6.3.1 Pyranometers with black-and-white detectors possess
protective domes, and the sky hemisphere (8-12). These offsets
a pattern of alternating reference and measuring thermojunc-
-2
range from equivalent irradiance levels of –5 Wm to -25
-2
tions that significantly affect the azimuthal response of these
Wm , depending on climatic and meteorological conditions.
instruments.
6.1.3 Some all-black detector pyranometers are designed
6.3.2 For maximum accuracy in the alternating shade cali-
with compensating thermopiles to reduce the thermal offset
bration of pyranometers with black-and-white detectors, rota-
-2
signal to the lower limits (–5 Wm ) mentioned in 6.1.2,
tion of the radiometer to at least six different azimuths, in
however the offset is never entirely eliminated in those designs
increments of 60°, is required (12, 13). See Appendix X4.
(10, 11).
6.1.4 Pyranometers with black-and-white detectors have 6.4 Cosine Corrections—This test method permits the pyra-
-2
substantially reduced thermal offsets, in the range of –2 Wm nometer to be tested either with axis vertical (with the
or less (9, 10). pyranometer mounted in an exactly horizontal plane), or with
G167 − 15 (2023)
the axis directed toward the sun by employing an altazimuth 6.7 Physical Environment—Precautions shall be taken to
platform. With the pyranometer’s axis vertical, the zenith and ensure that the horizon is substantially free of natural or
incident angles are the same and never smaller than: manmade objects that obscure more than 5 % of the sky at the
horizon. Special emphasis shall be given to ensure that any
z 5 L 2 δ (1)
objects that do exist above the horizon do not reflect an
where:
additional strong direct beam (specular) component onto the
z = the zenith (or incident angle), test units. It is recommended that the foreground at the
L = the latitude of the site, and
calibration facility be as similar to the foreground where tilted
δ = the solar declination for the day.
instruments are to be deployed as possible.
6.7.1 During calibration, wind conditions are also
6.4.1 The range of minimum incident angles available for
important, since absolute cavity pyrheliometers operating with
test due to the range of latitudes available in the continental
open apertures may be disturbed by strong wind speeds,
U.S. is 2.4° and 24.6° at the summer solstice, and 49.2° and
especially gusts coming from the sun’s azimuthal direction.
71.4° at the winter solstice, for Miami and Seattle, respectively.
Under such conditions, it may be necessary to operate with
The flux calibration is derived from flux measurements made at
wind screens or insulating jackets, or both, around the pyrhe-
incident angles of convenience but referred to the value the
liometer tube if wind-induced instability of the measurements
calibration would have if the measured flux were incident at a
is significant.
specific incidence angle selected by the user (usually 45°).
Therefore, since each calibration involves the cosine and
7. Apparatus
azimuth correction of the pyranometer at each incident angle,
the accuracy of the calibration is limited by the cosine and
7.1 Adjustable Platform—For calibrations performed with
azimuth correction uncertainty. (See Note 8 and Note 12,
the pyranometer’s axis vertical, a level platform is required (all
Sections 10 and 10.3.4.)
field pyranometers to be calibrated are expected to possess
6.4.2 When the pyranometer is calibrated with its axis
spirit levels for final leveling). For calibrations performed with
pointing toward the sun, there are no cosine errors either during
the pyranometer’s axis tilted to the equator, a platform adjust-
calibration or during use as a transfer instrument in the tilted
able in azimuth and tilt from the horizontal with an accuracy of
mode. The incident angles and hence the cosine corrections
greater than 0.5° shall be employed.
should be quantified as “usually less than 1 %.”
7.2 Digital Microvoltmeter—Any digital microvoltmeter
6.4.3 When the pyranometer is calibrated at a fixed tilt from
with a precision of 60.1 % of the average reading, and an
the horizontal (and at a fixed azimuth direction), the calibration
uncertainty of 60.1 % of the radiometers’ calculated outputs at
factor includes the instrument constant and the cosine and
-2
1100 Wm . A data logger having at least three-channel
azimuth correction of the pyranometer at each incident angle.
capacity is required for the alternating shade method, while the
The accuracy of the calibration is therefore limited by the
continuous shade, or component summation, method requires
cosine and azimuth correction uncertainty.
three channels for the reference radiometers and as many
additional channels as there are field pyranometers being
6.5 Environmental Conditions—Under general conditions
of both calibration and use, the pyranometer signal is a calibrated. High temperature stability is required for outdoor
operation. The data sampled from all radiometers should be
function of many parameters, which may affect calibration
factors or data derived from use to a significant degree. Many recorded within about 1 s. A time resolution for calculating the
corresponding solar elevation angle with an uncertainty of less
of these parameters are beyond the scope of this test calibration
method and the control of the practitioner. than 0.1° is required. For documenting the variation of the
measured values during the calibration, the data shall be
6.6 Reference Radiometers—Both the reference pyrheliom-
appropriately recorded.
eter or pyranometer(s) shall not be used as a field instrument
7.3 Field Pyranometer—In principle, this method can be
and its exposure to sunlight shall be limited to calibration or to
applied to any type of pyranometer.
intercomparisons.
7.4 Reference Pyranometer—Pyranometer(s) that are either
NOTE 4—At a laboratory where an absolute cavity pyrheliometer is not
available, it is advisable to maintain a group of two or there pyrheliom- WMO/High Quality, ISO/First Class, ISO/Secondary Standard,
eters which are included in every calibration. These serve as controls to
or possess characteristics that are intermediate between First
detect any instability or irregularity in any of the reference instruments. It
Class and Secondary Standard pyranometers, in terms of the
is also advisable to maintain a set of two or three reference pyranometers
requirements of ISO 9060 and the WMO Guide to Meteoro-
for the same reasons.
logical Instruments and Methods of Observation (1).
6.6.1 Reference radiometers shall be stored in such a
7.5 Primary Standard Pyrheliometer—A self-calibrating ab-
manner as to not degrade their calibration. Exposure to
solute cavity pyrheliometer designated by the WMO Guide to
excessive temperature or humidity can cause instrumental drift.
Meteorological Instruments and Methods of Observation (1)
6.6.2 The distance between the reference radiometer(s) and
and ISO 9060 as a primary standard, and intended for use in
the field pyranometer(s) being calibrated shall be no more than
Type I calibrations.
30 m, otherwise both the reference and field radiometers may
not be similarly affected by the same atmospheric events such
NOTE 5—Self-calibrating absolute cavity pyrheliometers generally have
as, for example, structured turbidity elements. unobstructed apertures, that is, the cavity receiver is open to the
G167 − 15 (2023)
atmosphere. Hence, no question arises concerning the spectral transmis-
7.8.2 A number of types of shading disk devices are
sion of window materials.
described in Appendix X5, several of which are commercially
7.6 Reference Pyrheliometer—A pyrheliometer used to per- available.
form Type II calibrations that meets the WMO Guide to
8. Shaded-Unshaded Timing Sequence
Meteorological Instruments and Methods of Observation (1)
for WHO/High Quality and ISO 9060 specifications for a
8.1 Different methods of timing the shade and unshaded
Secondary Standard, or First Class Pyrheliometer, and selected
portions of the calibration sequence may be used. The most
depending on the accuracy of calibration transfer required.
widely used sequence is to employ equal, or nearly equal,
intervals for the both the shade and unshaded, or illumination,
7.7 Solar Tracker —A solar tracker is required for normal
segments. Typical are 5 min shade and 5 min illumination, and
incident calibrations, that is, with the pyranometers optical axis
6 min shade and 6 min illumination.
pointing to the sun. The tracker may be manually operated
providing it possesses a sun-pointing alignment device that is
8.2 An alternate method consists of using non-equal timing
accurate to 60.3°. When an altazimuth tracking mount is
for the shaded and illuminated segments of the cycle in order
employed, which is the preferable method, it must have a
to lessen the inaccuracies due to an approximately 1 % error
tracking accuracy of 60.5°. An altazimuth tracking mount is
introduced by the inclusion of the pyranometer-body thermal
mandatory for pyrheliometers whose responsivity over the
time constant to the time constant of the instruments thermo-
receiver surface is not circular-symmetrical. Servo-operated
pile. Typically, this consists of shading for approximately 30
bi-directional azimuth and altitude trackers (altazimuth) are
thermopile time constants followed by illumination for a longer
available.
period of time such as 100 to 300 thermopile time constants.
See Refs (14, 15) and Appendix X4 for discussions on time
7.8 Shade Disk Apparatus—Regardless of whether the
constant based timing.
alternating- or the continuous-shade methods are used for
calibration, the geometry of the disk/sphere with respect to the
9. Preparatory Steps
pyranometer’s receiver surface (and transparent glass dome)
are the same.
9.1 Conditioning:
7.8.1 Requirements: 9.1.1 Start the preparatory phase at least 30 min before the
7.8.1.1 The shade disk/sphere shall be positioned perpen-
measurement phase is to begin. Allow for sufficient additional
dicular to the sun’s ray and at a fixed distance d from the center time to determine the pyranometer’s thermopile time constant
of the receiver surface of the pyranometer.
if it is not known.
7.8.1.2 The radius r of the shade disk or sphere should be 9.1.2 Acclimatize the radiometers, electronics and data
larger than the radius of the optical receiver, diffuser, or
acquisition system by exposing the radiometers to the sun.
protective dome of the pyranometer by a minimum of d Absolute cavity pyrheliometers should remain shuttered until
tan(0.5°), where d is the distance from the pyranometer
the measurement sequence begins.
receiver to the shade device, to allow for the divergence of the 9.1.3 Turn on all electronics for a short warm-up period.
sun’s beam and for small tracking errors.
Shade all electronics from direct sunlight.
7.8.1.3 The ratio r/d, where r is the radius of the shade
9.1.4 Adjust all radiometers requiring alignment or leveling,
device, should define an angle at the center of the pyranom-
the solar tracker(s) and the shading disk apparatus.
eter’s receiver surface which corresponds to the field-of-view
9.1.5 Perform electrical continuity and voltage checks, and
angle of the pyrheliometer.
perform any zeroing tests that may be required.
9.1.6 Clean all pyranometer domes and pyrheliometer win-
NOTE 6—All pyrheliometers listed in Refs (1, 13, 14) possess slope
dows.
angles of approximately 1° and field-of-views between 5° and 6°.
NOTE 7—A fixed “shade slope angle” corresponding to the slope angle
9.2 Determination of the Pyranometer’s Thermopile Time
of the pyrheliometer can be stated only for pyranometers which are
Constant:
operated in a position normal to the sun. For pyranometers calibrated at
9.2.1 Illuminate the field (test) pyranometer to be calibrated
fixed position, regardless of tilt, the shade slope angle varies according to
the angle of incidence of the ray on the receiver plane. for 10 min (unshaded) and record the signal V . Then shade the
u
pyranometer dome only for 60 s and record the signal V . Again
s
7.8.1.4 Those parts of the disk mounting rod that obscure
illuminate (unshaded) the pyranometer and, taking continuous
the field-of-view angle of the pyranometer should be as small
(not less than every 5 s if not analog) voltage readings,
as possible in order to restrict the disturbance of the signal to
determine the time required for the response signal to reach
less than a total of 0.5 % when taking into consideration both
95 % of the final steady state value V . Record the time, t , as
u c
the mount and any restrictions from neighboring instruments.
the instrument’s thermopile time constant.
7.8.1.5 The shade disk must be easily removed and replaced
in terms of shading and unshading of the pyranometers
10. Procedure for the Alternating Shade Method
hemispherical glass dome such that the time spent in shading
NOTE 8—Equations 2 and 3 in this method include interpolation of the
and unshading requires less than 5 % of the phase duration.
shaded (diffuse) measurement voltages over two cycles of shading. This
requires the assumption that the diffuse and direct beam irradiance are
both smoothly and linearly changing over the period between the two
A source of supply for the solar tracker is Kipp and Zonen, Delft, Holland,
shadings. A more direct, instantaneous, quantitative value for the shaded
(Model 2AP). If you are aware of alternate suppliers, please provide this information
to ASTM Headquarters. Your comments will receive careful consideration at a voltage can be obtained by using the ratio of the voltage signal of the unit
meeting at the responsible technical committee, which you may attend. under test to the signal of a continuously shaded pyranometer. See
G167 − 15 (2023)
Appendix X3 and Appendix X4.
10.2.2 Take each series of measurements in accordance with
the timing sequence presented in Fig. 1, consisting of n + 1
10.1 Mounting:
shade intervals, during which the sensor is exposed to diffuse
10.1.1 Mount the self-calibrating absolute cavity pyrheli-
radiation only, alternating with n intervals during which the
ometer (hereinafter designated the primary reference
pyranometer is unshaded and exposed to hemispherical solar
radiometer), or a secondary reference pyrheliometer (if a Type
radiation.
II calibration is desired) on either an altazimuth or equatorial
sun tracker. If an equatorial tracker is used, set the latitude
10.2.3 The value of the time interval t should be from 20 to
o
angle adjustment of the tracker to the exact local latitude. Align
60 response time constants determined in 9.2.1 and should be,
the reference pyrheliometer with the sight mechanism pro-
typically, 2 min to 5 min for WMO High Quality or ISO First
vided.
Class pyranometers. The setting of the same time interval for
10.1.2 For calibration of the field pyranometer with axis
the shading and illuminated sequences is based on the assump-
vertical, mount the field (test) and any monitoring pyranom-
tion that the response times of the pyranometer’s thermopile
eters used on a horizontal plate. Rotate each until the instru-
during increasing and decreasing signals, that is, during shad-
ment cable connector faces the equator and level all instru-
ing and illumination, are approximately the same.
ments with the leveling screws and bubble levels provided.
10.2.4 Record the following values in accordance with Fig.
NOTE 9—Mounting the body of a radiometer flush with a mounting
1: diffuse solar radiation signal V measured with the shaded
D,β
plate will induce unwanted thermal transients that will affect the calibra-
pyranometer for n + 1 intervals, including reflected solar
tion of radiometers with thermal sensors and is not permitted.
irradiance if β ≠ 1 (read and record at the end of each odd
10.1.3 For calibrations of field pyranometers either at nor-
numbered shading interval nt ); hemispherical solar radiation
o
mal incidence (that is, on a sun-tracking platform) or at a fixed,
signal V measured at the end of each even numbered
G,β
equator-facing tilt β from the horizontal, first precisely level
exposed (illuminated) interval nt for n intervals; direct solar
o
the instruments on an exactly horizontal platform using the
radiation signal V measured at each nt interval for 2n + 1
I o
same technique as in 10.1.2. After leveling, mount the pyra-
measurements; and a measurement of the ambient air
nometers either on a tilt table that is precisely adjusted to the
temperature, or pyranometer and pyrheliometer case
required tilt from the horizontal, or on an altazimuth follow-
temperatures, T, measured at least at the beginning and end of
the-sun mount for normal incidence calibrations.
each series.
10.1.4 While the instruments leveling procedure can com-
10.2.5 Record the time of each measurement required in
pensate for somewhat non-level platforms when calibrating
10.2.4 precisely in order to accurately calculate the solar
pyranometers with axis vertical, it is essential that the horizon-
incidence angles (see 12.1 – 12.4).
tal platform used to perform the initial instrument leveling on
10.2.6 Restrict the number of intervals n such that the total
an exactly level, horizontal platform for instruments being
duration of the series s is no more than 36 min (in order to
calibrated at tilts from the horizontal.
ensure that the mean value of each series is associated with a
10.2 Equal Shade/Unshaded Time Intervals:
small range of solar elevation and temperature.
10.2.1 Take 2n + 1 voltage readings for each series of a set
10.2.7 For a pyranometer with a black and white detector, or
of s series of measurements performed over not less than two
any source of azimuthal asymmetry, steps 10.2.1 to 10.2.6
days, depending on sky conditions and the degree of scatter in
should be repeated after the radiometer has been rotated 60
the measurements observed within each series. The value s
degrees in azimuth. Record the azimuthal rotation angle with
should not be less than six for clear sky conditions with little
the signal data. Repeat the sequence until the radiometer
cirrus formation, to ten for haze and cirrus conditions. The
returns to the original azimuth (6 rotations). See Appendix X4.
essential requirement is that a sufficient number of series be
obtained during which the mean solar incidence angles deviate
10.3 Determination of the Calibration Factor:
less and 65° from the mean angle representing the normal
10.3.1 Determine the responsivity R (i) and the mean re-
S
operating conditions of the pyranometer being calibrated.
¯
sponsivity R , expressed as microvolts per watt per square
S
–2 –2
meter (μV.watt .m ) for each measurement and for the series,
respectively, in accordance with:
$V ~2i! 2 0.5@V ~2i 2 1!1V ~2i11!#%
G,β D,β D,β
R i 5 (2)
~ !
s
V 2i F cos η 2i
$ ~ ! @ ~ !#%
I P
and:
n
R
S i
( ~ !
i51
¯
R S 5 (3)
~ !
S
n
where:
or V (2i + 1)
D,β
i = indicates the measurement within the series,
FIG. 1 Measurement Sequence for the Alternating Sun-and-Shade
S = indicates the series,
Method Using Equal Timing Intervals
G167 − 15 (2023)
If a reduction formula f(T,T ) is available and there are some
V (2i) = the hemispherical solar irradiance signal mea-
n
G,β
series in which the temperature deviates significantly from the
sured at position 2i within the series, in
desired value T , then apply the correction factor to each R
millivolts, for example;
n S
according to:
V (2i – = the diffuse solar irradiance signal for the shaded
D,β
1) interval measured at position (2i – 1) or (2i + 1) ρ
R 5 f T S ,T R S (6)
@ ~ ! # ~ !
or V (2i within the series, in millivolts, for example; n s
(
D,β
ρ
S51
+ 1)
NOTE 12—For some types of pyranometers, temperature coefficients α
V (2i)F = the direct solar irradiance calculated from the
are specified such that the correction factor is simply f(T,
I P
T ) = [1 – α(T – T )].
product of the pyrheliometer signal and its
n n
calibration factor F ;
P
10.3.6 Present the final result also in the form of a calibra-
η(2i) = the angle between the direction of the solar
tion factor F, expressed in watts per square metres per
beam and the perpendicular to the plane of the
microvolt:
pyranometer’s receiver at the time correspond-
ing to position 2i. The angle of incidence η is
F 5 (7)
R
calculated from the equations given in 12.1 and
12.2 taking into account the inclined position of
and the responsivity R.
the pyranometer β and the solar position. The
NOTE 13—Note 11 in 10.3.5 applies to the derived calibration factor as
expression cos[η(2i)] in Eq 2 and 3 is unity for
a function of η, the incidence/zenith angle, as well as to the responsivity.
normal incident calibrations using a sun-
following tracker to maintain the pyranometer’s
11. Procedure for the Continuous Sun-and-Shade
axis pointing to the sun.
(Component Summation) Method
n = the number of readings of E and E to be used
G,β I
11.1 Mounting:
from the total number of reading intervals (2n +
11.1.1 Mount the reference pyrheliometer as prescribed in
1).
10.1.1.
10.3.2 For a pyranometer with a black-and-white detector,
11.1.2 Mount the reference pyranometer, which has been
perform the computations in 10.3.1 for each of 6 incremental
previously calibrated by the alternating shade method (10.2 or
60° azimuthal rotation positions.
10.3) on the appropriate platform depending on the tilt from the
10.3.3 Identify and reject those R (i) which deviate by more horizontal chose (0° to β) selected for the calibrations, or on the
S
¯
than 1 % from R . If more than n /2 are rejected, eliminate the appropriate tracker for normal incidence calibrations.
S
series from further calculations.
NOTE 14—For the best absolute accuracy, the reference pyranometer
should have the lowest thermal offset possible. Presently, only pyranom-
NOTE 10—The 61 % deviation limit specified in 10.3.2 will result in R
s
eters with black and white detectors, or all-black pyranometers with
values for restricted ranges of zenith/incidence angles, and not all
compensating thermopiles connected in opposition to the active detectors
zenith/incidence angles, since all pyranometers eventually deviate by
are known to meet this requirement. The method of Appendix X3 for
more than 61 % from a mean value at some zenith/incident angles as
calibrating the black-and-white reference pyranometer will produce the
zenith/incidence angles increase.
lowest uncertainty in the reference irradiance (14).
10.3.4 If there are sufficient R (i), calculate a corrected
11.1.3 Affix the shade disk over the reference pyranometer,
S
value R :
and ensure that it will remain rigid and optically aligned
S
throughout the entire calibration procedure. Use of an
¯
R 5 R ~i,i 5 1,n for ifij! (4)
S S
automatic, sun-tracking shade disk is recommended, although
a manually adjusting disk can be used albeit with considerable
where: j are those measurements i which were identified as
¯ difficulty.
deviating by 1 % from R .
s
11.1.4 Mount the test (field) pyranometer(s) being cali-
10.3.5 If ρ calibration series are carried out at the desired
brated on the appropriate platform(s) or on an altazimuth sun
parameter range
...