ISO 9060:2018

INTERNATIONAL ISO
STANDARD 9060
Second edition
2018-11
Solar energy — Specification and
classification of instruments for
measuring hemispherical solar and
direct solar radiation
Énergie solaire — Spécification et classification des instruments de
mesurage du rayonnement solaire hémisphérique et direct
Reference number
©
ISO 2018
© ISO 2018
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ii © ISO 2018 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Instruments to measure hemispherical solar radiation — Pyranometers .3
4.1 General physical design . 3
4.2 Types. 4
4.3 Classification . 4
4.3.1 General. 4
4.3.2 Pyranometer specifications . 5
4.3.3 Classification criteria . 7
4.3.4 Identification of classification . 8
5 Instruments to measure direct solar radiation—Pyrheliometers .8
5.1 General physical design . 8
5.2 Types. 9
5.2.1 Absolute pyrheliometer . 9
5.2.2 Compensation pyrheliometer . 9
5.2.3 Pyrheliometers without self-calibration capability . 9
5.3 Classification . 9
5.3.1 General. 9
5.3.2 Pyrheliometer specifications .10
5.3.3 Classification criteria .10
5.3.4 Identification of classification .11
6 Final remarks .12
Annex A (informative) Comments on the specifications given in Tables 1 to 2.14
Bibliography .18
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).
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.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 180, Solar energy, Subcommittee SC 1,
Climate — Measurement and data.
This second edition cancels and replaces the first edition (ISO 9060:1990), which has been technically
revised. The main changes compared to the previous edition are as follows:
— in addition to thermopile radiometers, other technology options have been included such as
photoelectric sensors as long as they fulfil the requirements specified in this document;
— the spectral error is used to characterize the spectral responsivity;
— to further characterize the radiometers, the additional properties “spectrally flat” and “fast
response” can be added to the classification if the radiometers fulfil specific criteria;
— more intuitive names have been introduced for the classes: “A”, “B”, “C”.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
iv © ISO 2018 – All rights reserved

Introduction
This document is one of a series of standards that specify methods and instruments for the measurement
of solar radiation in support to solar energy utilization.
Accurate solar radiation data are used in meteorology and are needed for developing solar energy
appliances, in particular for performance testing, solar radiation simulation and resource assessment.
The measurement of radiation is needed for determination of the conversion efficiencies of solar
appliances. The specification and classification of these instruments are needed in order to enable the
comparison of solar radiation data on a worldwide basis. In addition, this classification is intended
to assist end users/consumers and entities requiring and tendering radiometers with the choice
or comparison of instruments, to protect end users/consumers and to offer a level playing field for
manufacturers.
The specification and classification of solar radiometers specified in this document provides an accuracy
ranking and focuses on application specific requirements and qualities. However, solar radiometers are
used in a wide range of applications with often conflicting requirements. The best radiometer for one
application may be inadequate for a different application. In order to address this issue at least partly, a
sensor of a given class can be assigned the additional properties “fast response” and/or “spectrally flat”
to further characterize the radiometers.
INTERNATIONAL STANDARD ISO 9060:2018(E)
Solar energy — Specification and classification of
instruments for measuring hemispherical solar and direct
solar radiation
1 Scope
This document establishes a classification and specification of instruments for the measurement of
hemispherical solar and direct solar radiation integrated over the spectral range from approximately
0,3 μm to about 3 μm to 4 μm.
Instruments for the measurement of hemispherical solar radiation and direct solar radiation are
classified according to the results obtained from indoor or outdoor performance tests. This document
does not specify the test procedures.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
hemispherical solar radiation
solar radiation received by a plane surface from a solid angle of 2π sr
Note 1 to entry: Approximately 97 % to 99 % of the hemispherical solar radiation incident at the Earth’s surface
[1]
is contained within the wavelength range from 0,3 μm to 3 μm . Generally, hemispherical solar radiation is
composed of direct solar radiation and diffuse solar radiation (solar radiation scattered in the atmosphere) as
well as solar radiation reflected by the ground.
3.2
global horizontal irradiance
hemispherical solar radiation received by a horizontal plane surface
Note 1 to entry: The tilt angle and the azimuth of the receiver surface should be specified, e.g. horizontal.
3.3
direct solar radiation
radiation received from a small solid angle centred on the sun’s disc, on a given plane
Note 1 to entry: In general, direct solar radiation is measured by instruments with field-of-view angles of up to
6°. Therefore a part of the scattered radiation around the sun’s disc (circumsolar radiation or aureole) is also
included (see 5.1). Historic pyrheliometers of the Angström type (compensation pyrheliometer) have a larger
field of view of up to 15°. A more detailed definition of circumsolar radiation and related parameters can be
found in Reference [2].
Note 2 to entry: Approximately 97 % to 99 % of the direct solar radiation received at the ground is contained
[1]
within the wavelength range from 0 μm to 3 μm .
Note 3 to entry: The tilt angle of the receiver surface should be specified, e.g. horizontal or normal to the direct
solar radiation.
3.4
diffuse solar radiation
diffuse radiation
hemispherical solar radiation minus coplanar direct solar radiation
Note 1 to entry: For the purposes of solar energy technology, diffuse radiation includes solar radiation scattered
in the atmosphere as well as solar radiation reflected by the ground, depending on the inclination of the receiver
surface.
Note 2 to entry: The tilt angle and the azimuth of the receiver surface should be specified, e.g. horizontal.
3.5
pyranometer
radiometer designed for measuring the irradiance on a plane receiver surface which results from the
radiant fluxes incident from the hemisphere above within the wavelength range from approximately
0,3 µm to about 3 µm to 4 µm
Note 1 to entry: The spectral range (50 % transmittance points) given is only nominal. Depending on the
radiometer design, the spectral limits of its responsivity can be different from the limits mentioned above.
3.6
pyrheliometer
radiometer designed for measuring the irradiance which results from the solar radiant flux from a well-
defined solid angle the axis of which is perpendicular to the plane receiver surface
Note 1 to entry: It follows from this definition that pyrheliometers are used to measure direct solar radiation at
normal incidence. Typical opening half angles of common and historical pyrheliometers range from 2,5° to 7,5°.
−3
Reference [3] recommends that the opening half-angle is 2,5° (6 10 sr) and the slope angle 1° for all new designs
of direct solar radiation instruments. The opening half-angle is measured from the centre of the (circular)
receiver aperture to the edge of the view-limiting aperture. The slope angle is the opening half-angle of the cone
defined by both apertures. For mathematical definitions of the angles, see 5.1 b). A more detailed description of
the influence of circumsolar radiation on the pyrheliometers can be found in Reference [2].
Note 2 to entry: The spectral responsivity of field pyrheliometers is often limited to the range of approximately
0,3 µm to 3 µm, depending on the radiometer properties. The spectral range (50 % points) given is only nominal.
Depending on the radiometer design, the spectral limits of its responsivity can be different from the limits
mentioned above.
3.7
diffusometer
radiometer designed for measuring the diffuse solar radiation, consisting of a pyranometer and a
shading structure which can be a shading ball, a shading disk, a shading ring, a rotating shadowband or
a shading mask
Note 1 to entry: Shading balls and disks shall be tracked to the sun, so that the pyranometer is shaded. Shading
[4]
disks and their tracking are defined in ISO 9846 . The centre of a shading ball is tracked to the same point as the
centre of a shading disk. The diameter of the ball corresponds to the diameter of the disk. The shaded opening
angle and slope angle of shading balls and -disks for the sun in the zenith shall be 2,5° and 1°.
Note 2 to entry: Shading rings are positioned such that the pyranometer is shaded for all solar positions occurring
throughout approximately two days. Shading rings shall be adjusted approximately every two days. Shading
rings therefore prevent not only the direct radiation, but also a part of the diffuse radiation from reaching the
pyranometer and only an approximation of the diffuse radiation can be measured.
Note 3 to entry: A rotating shadowband is rotated around the pyranometer so that this pyranometer is shaded for
some time during the rotation. The pyranometer measures an approximation of the diffuse radiation when the
shadowband shades the sensor. The pyranometer measures the hemispherical radiation when the shadowband
is below the pyranometer’s field-of-view. When the shadowband’s shadow is close to the sensor, but not on
the sensor the hemispherical radiation except of the blocked diffuse radiation is measured. With these three
measurements so-called rotating shadowband irradiometers determine the diffuse radiation.
2 © ISO 2018 – All rights reserved

Note 4 to entry: Shading masks throw a shadow on one or various pyranometers depending on the solar position.
3.8
offset correction
value added algebraically to the uncorrected result of a measurement to compensate for systematic error
Note 1 to entry: The offset correction is equal to the negative of the estimated systematic error.
Note 2 to entry: Since the systematic error cannot be known perfectly, the compensation cannot be complete.
3.9
correction factor
numerical factor by which the uncorrected result of a measurement is multiplied to compensate for
systematic error
Note 1 to entry: Since the systematic error cannot be known perfectly, the compensation cannot be complete.
[SOURCE: ISO/IEC Guide 98-3:2008, B.2.24]
3.10
acceptance interval
interval of permissible measured quantity values
[6]
[SOURCE: BIPM, 2012 , 3.3.9]
3.11
tolerance interval
interval of permissible values of a property
[6]
[SOURCE: BIPM, 2012 , 3.3.5]
3.12
guard band
interval between a tolerance limit and a corresponding acceptance limit
[6]
[SOURCE: BIPM, 2012 , 3.3.11]
3.13
accuracy class
class of measuring instruments or measuring systems that meet stated metrological requirements that
are intended to keep measurement errors or instrumental uncertainties within specified limits under
specified operating conditions
[SOURCE: ISO/IEC Guide 99:2007, 4.25, modified — Notes have been deleted.]
4 Instruments to measure hemispherical solar radiation — Pyranometers
4.1 General physical design
Pyranometers are radiometers used to measure hemispherical solar radiation (see 3.1, 3.2, 3.4, 3.5
and 3.7).
Thermal sensors transform radiant energy into thermal energy with a consequent rise in the
temperature of the receiving surface. This rise in temperature is balanced by various kinds of heat
losses to thermal sinks (e.g. the body of the pyranometer and ambient air).
The thermal sensor of a pyranometer is protected from wind, rain and dust as well as the exchange of
thermal radiation by one or two transparent domes and/or a diffusor whose spectral transmittance
confines the spectral range of responsivity to the interval between approximately 0,3 µm and 3 µm
(50 % transmittance points).
Photodiode pyranometers use photodiodes as sensors that convert the incoming radiation in electrical
energy. The photodiodes are often placed below a diffusor.
Because of the spectral limits of the measurement, thermal sensors have an advantage compared to
photodiode sensors as they can achieve a nearly uniform spectral responsivity required for low spectral
errors. The spectral irradiance error is the error introduced by the change in the spectral distribution
of the incident solar radiation and the difference between the spectral responsivity of the radiometer
with respect to a radiometer with completely homogeneous spectral responsivity in the wavelength
range of interest.
Other technologies exist that are not mentioned in this document.
The main parts of common pyranometers are:
a) the sensor;
b) the transparent dome(s) or diffusor, which cover(s) concentrically the receiving surface; and
c) the body, which is often shielded by a sun-screen, and used as a thermal reference.
4.2 Types
One type of pyranometer is the “thermoelectric” pyranometer which is equipped with a thermopile
(sometimes called a thermobattery) measuring the difference in temperature between the receiving
surface (active junctions) and the body (passive junctions). The position and number of the active and
passive junctions vary depending on the different pyranometer models. Generally, these sensors are
covered by one or two concentric glass dome(s) or a diffusor.
Another type of pyranometer is the “photoelectric” pyranometer which is equipped with a photoelectric
receiver (using e.g. silicon photodiode or photovoltaic cell) measuring photovoltaic power. This type of
pyranometer is usually called “Si-pyranometer”. Often these sensors are placed below a diffusor. The
diffusor can have the shape of a cylinder or other shapes.
4.3 Classification
4.3.1 General
The classification of pyranometers is based exclusively on the measuring specifications of the
instruments. The classification is not based upon manufacturing technologies but rather on criteria
deduced from the various applications of pyranometers. Following this principle, any technical device
which produces a signal when irradiated (e.g. a photovoltaic cell) could be classified as a pyranometer
according to this document.
Most of the classification criteria (see Table 1) are of general relevance, whereas others may be
important only for specific applications.
Therefore statements about the overall measurement uncertainty can only be made on an individual
basis, taking all relevant factors into account.
The classification scheme is based on various specifications, as given in 4.3.2 and various classification
criteria, as given in 4.3.3.
The classification can be understood as an accuracy ranking. The letters indicate the typically reached
accuracy for well-maintained measurements when compared under the same measurement conditions.
The accuracy decreases in alphabetic order (A reaches a better accuracy than B or C). However, the
accuracy ranking does not mean that a radiometer of higher accuracy class is more accurate than
another radiometer of lower class under all conditions. First of all, as different radiometers can have
different maintenance requirements and e.g. susceptibility to soiling, the term “well-maintained” is
important in the previous statement. Furthermore, depending on the application and the measurement
conditions, a sensor of a lower class can be more appropriate in some cases. For example, radiometers
4 © ISO 2018 – All rights reserved

have different response times. In order to be able to identify radiometers that are adequate for the
measurement of highly variable data (e.g. overirradiance events), additional classes are defined by
adding the term “fast response” before the name of the class (e.g. fast response pyranometer of class A;
see also 4.3.3). Furthermore, comparing fast response sensors to slower sensors is more complex. A
fast response sensor of the same class has a higher accuracy for high temporal resolution than a slower
sensor of the same class if the response time is the only difference between the sensors and if the
sampling rate of the datalogger is adequate to the response time. For a high variability of the irradiance,
a fast response radiometer of a given class might even be more appropriate than a slower sensor of a
higher class.
Spectral errors can be an issue depending on the site’s meteorological conditions if the radiometer has
a significant spectral selectivity. The spectral selectivity is the percentage deviation of the spectral
[12]
responsivity from the corresponding mean within the range 0,35 µm and 1,5 µm . A low spectral
selectivity is also desirable for the measurement of reflected irradiance and albedo. Therefore, further
additional classes are defined by adding the term “spectrally flat radiometer” before the name of the class.
NOTE 1 The accuracy of measured solar radiation data depends not only on the instrument characteristics
used for the classification of the instrument but also on:
a) the calibration procedure;
b) the measurement conditions and maintenance including cleaning;
c) the environmental conditions; and
d) data logger uncertainty and setting (e.g. sampling rate) if the instrument provides an analogue signal.
NOTE 2 The most accurate determination of global irradiance under stable conditions is believed to be that
derived from the direct irradiance as measured by a highest-class pyrheliometer and the diffuse solar irradiance
as measured by a highest-class pyranometer shaded from the sun by a disc or a ball.
4.3.2 Pyranometer specifications
Pyranometer specifications are given as the acceptance intervals and guard bands for certain
parameters. The specifications can be grouped as follows.
a) The response time (a measure of the stabilization period for an accurate reading under realistic
irradiance changes).
b) The zero off-set including zero offsets of electronics (a measure of the stability of the zero-point
specified for the effect of thermal radiation, for a temperature transient and other influences).
c) The dependence of responsivity on:
1) ageing effects (a measure of the long-term stability, assuming regular and proper maintenance
including cleaning of the pyranometer);
2) the level of irradiance (a measure of the nonlinearity);
3) the direction of the irradiance (a measure of the deviations from the ideal “cosine behaviour”
and its azimuthal variation);
4) the clear sky spectral error for the most relevant irradiance component (a measure of the
deviation of the spectral responsivity of the radiometer from a completely flat spectral
responsivity);
5) the temperature of the radiometer body;
6) the tilt angle of the receiving surface; and
7) additional signal processing errors (The additional signal processing errors contain data
acquisition and analogue to digital conversion that might be carried out in the instrument and
all other processing steps carried out within the instrument that are not covered by the criteria
a, b and c1 to c6.).
1)
NOTE 1 The spectral selectivity used in ISO 9060:1990 is not the spectral error. The spectral selectivity was
defined as the maximum percentage deviation of the spectral responsivity within 0,35 µm and 1,5 µm from the
mean spectral responsivity within 0,35 µm and 1,5 µm. For some sensors such as photodiode sensors the spectral
responsivity can be 0 for some wavelengths in the defined wavelength range. Hence, the spectral selectivity
can reach 100 % and more. Also some sensors with specific diffusors might have higher spectral selectivities
or errors. The knowledge of the spectral range alone is not sufficient to determine the spectral selectivity or
the spectral error. The specification of the spectral range also requires the specification of a percentage of the
maximum spectral responsivity at which the wavelength limits are given (e.g. 50 %).
NOTE 2 Diffusometers are also included in this document. Diffusometers are partially classified by Table 1, as
the used pyranometer can be classified according to Table 1. The remaining part of diffusometers is only described
in this document by its type (shading disk, shading ball, shading ring, rotating shadowband or shading mask).
Table 1 — Pyranometer classification list
Specifica- Name of the classes, acceptance intervals and width
Parameter
tion param- of the guard bands (in brackets)
eter No.
Name of the class A B C
(see 4.3.2)
Roughly corresponding class from Secondary First class Second class
1)
ISO 9060:1990 standard
a Response time (see also 4.3.3 on fast < 10 s (1 s) < 20 s (1 s) < 30 s (1 s)
response pyranometers):
time for 95 % response
b Zero off-set:
−2 −2 −2 −2
a) response to −200 W·m net ±7 W·m ±15 W·m ±30 W·m
−2 −2 −2
thermal radiation (2 Wm ) (2 Wm ) (3 Wm )
−1 −2 −2 −2
b) response to 5 K·h change in ±2 W·m ±4 W·m ±8 W·m
−2 −2 −2
ambient temperature (0,5 Wm ) (0,5 Wm ) (1 Wm )
−2 −2 −2
c) total zero off-set including the ±10 W·m ±21 W·m ±41 W·m
−2 −2 −2
effects a), b) and other sources (2 W·m ) (2 W·m ) (3 W·m )
c1 Non-stability: ±0,8 % ±1,5 % ±3 %
(0,25 %) (0,25 %) (0,5 %)
percentage change in responsivity
per year
c2 Nonlinearity: ±0,5 % ±1 % ±3 %
(0,2 %) (0,2 %) (0,5 %)
percentage deviation from the respon-
−2
sivity at 500 W·m due to the change
−2
in irradiance within 100 W·m to
−2
1 000 W·m
−2 −2 −2
c3 Directional response (for beam radia- ±10 W·m ±20 W·m ±30 W·m
−2 −2 −2
tion): (4 W·m ) (5 W·m ) (7 W·m )
the range of errors caused by assuming
that the normal incidence responsivity
is valid for all directions when measur-
ing from any direction (with an inci-
dence angle of up to 90° or even from
below the sensor) a beam radiation
whose normal incidence irradiance is
−2
1 000 W·m
NOTE  The acceptance intervals should not be used for uncertainty estimations for conditions different from the ones
stated for each criterion. In particular the spectral error can be different under different conditions. The spectral error for
diffuse horizontal irradiance measurements is also different from that for global horizontal irradiance.
1) Now withdrawn.
6 © ISO 2018 – All rights reserved

Table 1 (continued)
Specifica- Name of the classes, acceptance intervals and width
Parameter
tion param- of the guard bands (in brackets)
eter No.
Name of the class A B C
(see 4.3.2)
Roughly corresponding class from Secondary First class Second class
1)
ISO 9060:1990 standard
c4 Clear sky global horizontal irradiance ±0,5 % ±1 % ±5 %
spectral error: (0,1 %) (0,5 %) (1 %)
maximum spectral error observed for
a set of global horizontal irradiance
clear sky spectra defined in this docu-
ment (see A.7 related to the calcula-
tion of the spectral error; see 4.3.3 on
spectrally flat pyranometers)
c5 Temperature response: ±1 % ±2 % ±4 %
(0,2 %) (0,2 %) (0,5 %)
percentage deviation due to change in
ambient temperature within the inter-
val from −10 °C to 40 °C relative to the
signal at 20 °C
c6 Tilt response: ±0,5 % ±2 % ±5 %
(0,2 %) (0,5 %) (0,5 %)
percentage deviation from the re-
sponsivity at 0° tilt (horizontal) due
to change in tilt from 0° to 180° at
−2
1 000 W·m irradiance
−2 −2 −2
c7 Additional signal processing errors ±2 W·m ±5 W·m ±10 W·m
−2 −2 −2
(2 W·m ) (2 W·m ) (2 W·m )
NOTE  The acceptance intervals should not be used for uncertainty estimations for conditions different from the ones
stated for each criterion. In particular the spectral error can be different under different conditions. The spectral error for
diffuse horizontal irradiance measurements is also different from that for global horizontal irradiance.
4.3.3 Classification criteria
For the classification, the specifications given in Table 1 shall be ver
...