IEC 60904-4:2009

Photovoltaic devices - Part 4: Reference solar devices - Procedures for establishing calibration traceability

IEC 60904-4:2009 sets the requirements for calibration procedures intended to establish the traceability of photovoltaic reference solar devices to SI units as required by IEC 60904-2. Applies to photovoltaic (PV) reference solar devices that are used to measure the irradiance of natural or simulated sunlight for the purpose of quantifying the performance of PV devices. The use of a PV reference solar device is required in the application of IEC 60904-1 and IEC 60904-3.

Dispositifs photovoltaïques - Partie 4: Dispositifs solaires de référence - Procédures pour établir la traçabilité de l'étalonnage

La CEI 60904-4:2009 fixe les exigences relatives aux procédures d'étalonnage, dans le but d'établir la traçabilité des dispositifs solaires photovoltaïques de référence en unités SI, comme cela est stipulé par la CEI 60904-2. La présente norme s'applique aux dispositifs solaires photovoltaïques (PV) de référence utilisés pour mesurer l'éclairement énergétique de la lumière solaire naturelle ou simulée, afin de quantifier les performances des dispositifs PV. L'utilisation d'un dispositif solaire PV de référence est requise lors de l'application des CEI 60904-1 et CEI 60904-3.

General Information

• Status: Published
• Publication Date: 08-Jun-2009
• ICS: 27.160 - Solar energy engineering
• Technical Committee: TC 82 - Solar photovoltaic energy systems
• Drafting Committee: WG 2 - TC 82/WG 2
• Current Stage: DELPUB - Deleted Publication
• Start Date: 12-Nov-2019
• Completion Date: 13-Feb-2026

Relations

• Revised: IEC 60904-4:2019 - Photovoltaic devices - Part 4: Photovoltaic reference devices - Procedures for establishing calibration traceability
• Effective Date: 05-Sep-2023

Overview

IEC 60904-4:2009 is an international standard published by the International Electrotechnical Commission (IEC) focused on photovoltaic (PV) devices, specifically addressing reference solar devices and the procedures for establishing calibration traceability. This standard ensures that PV reference solar devices, which measure the irradiance of natural or simulated sunlight, are calibrated in a manner traceable to the International System of Units (SI). Traceability is essential for reliable performance quantification of PV devices and for maintaining global consistency in solar irradiance measurements.

IEC 60904-4:2009 complements related standards such as IEC 60904-1, IEC 60904-2, and IEC 60904-3 by establishing rigorous calibration practices and uncertainty analysis methods for reference solar devices, often used with single-junction photovoltaic technologies, predominantly crystalline silicon.

Key Topics

• Calibration Traceability: Comprehensive procedures are defined to ensure an unbroken and documented chain of calibration that links PV reference solar devices directly to SI units. The standard emphasizes traceability through internationally recognized primary and secondary standards.

• Reference Standards and Instruments:

  • Primary Standards: These include devices like the World Radiometric Reference (WRR), a set of cavity radiometers maintained by national metrology institutes and the World Standard Group.
  • Secondary Standards: Instruments periodically calibrated against primary standards, such as commercial cavity radiometers verified every five years via the International Pyrheliometer Comparison (IPC).
  • Primary and Secondary References: Practical reference solar devices (such as solar cells or pyrheliometers) used in laboratories for transferring calibration to working devices.

• Uncertainty Analysis: Detailed guidance on statistical evaluation and reporting of measurement uncertainties ensures confidence in calibration results. The standard references ISO/IEC Guide 98-3 for expressing measurement uncertainty.

• Calibration Procedures: The document specifies validated methods for conducting solar irradiance calibration using natural sunlight or solar simulators. Annex A illustrates examples such as differential spectral responsivity calibration and solar simulator methods, addressing the complexities of spectra and intensity measurement.

• Marking and Reporting: Requirements for consistent marking on reference solar devices and comprehensive calibration reports are defined to preserve traceability information and support quality assurance in measurement practices.

Applications

IEC 60904-4:2009 is indispensable for professionals and organizations involved in:

• Photovoltaic Module and System Testing: Ensuring accurate solar irradiance measurement is fundamental when measuring PV module efficiencies and performance under standard test conditions.

• Calibration Laboratories: Laboratories that provide calibration services for solar measurement instruments rely on this standard to maintain international traceability and measurement accuracy.

• Solar Research and Development: Researchers developing new PV technologies or solar simulators use the standard’s recommendations to verify device performance with reliable irradiance data.

• Quality Assurance Programs: Manufacturers and certification bodies apply this traceability framework to meet international benchmarking standards and regulatory requirements.

By adhering to IEC 60904-4:2009, stakeholders in the solar industry ensure high precision in irradiance measurements that underpin PV energy conversion assessments, system design, and comparative performance analysis globally.

Related Standards

• IEC 60904-1: Photovoltaic devices – Part 1: Measurement of photovoltaic current-voltage characteristics.
• IEC 60904-2: Photovoltaic devices – Part 2: Requirements for reference solar devices.
• IEC 60904-3: Photovoltaic devices – Part 3: Measurement principles for terrestrial photovoltaic (PV) solar devices with reference spectral irradiance data.
• ISO/IEC 17025: General requirements for the competence of testing and calibration laboratories.
• ISO 9059: Solar energy – Calibration of field pyrheliometers by comparison to a reference pyrheliometer.
• ISO 9846: Solar energy – Calibration of a pyranometer using a pyrheliometer.
• ISO/IEC Guide 98-3:2008: Uncertainty of measurement – Guide to the expression of uncertainty in measurement (GUM).

These related standards collectively support a robust ecosystem of best practices and technical requirements for photovoltaic device measurement and calibration traceability.

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Keywords: IEC 60904-4, photovoltaic devices, reference solar devices, calibration traceability, PV measurement, solar irradiance calibration, photovoltaic standards, World Radiometric Reference, uncertainty analysis, calibration procedures, solar simulator, pyrheliometer calibration, solar energy standards.

Standards Content (Sample)

IEC 60904-4 ®
Edition 1.0 2009-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Photovoltaic devices –
Part 4: Reference solar devices – Procedures for establishing calibration
traceability
Dispositifs photovoltaïques –
Partie 4: Dispositifs solaires de référence – Procédures pour établir la traçabilité
de l'étalonnage
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IEC 60904-4 ®
Edition 1.0 2009-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Photovoltaic devices –
Part 4: Reference solar devices – Procedures for establishing calibration
traceability
Dispositifs photovoltaïques –
Partie 4: Dispositifs solaires de référence – Procédures pour établir la traçabilité
de l'étalonnage
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
T
CODE PRIX
ICS 27.160 ISBN 978-2-88910-323-2
– 2 – 60904-4 © IEC:2009
CONTENTS
FOREWORD.3

1 Scope and object.5

2 Normative references .5

3 Terms and definitions .5

4 Requirements for traceable calibration procedures of PV reference solar devices .7

5 Uncertainty analysis .8

6 Calibration report.8

7 Marking .8
Annex A (informative) Examples of validated calibration procedures.10
Bibliography.24

Figure 1 – Schematic of most common reference instruments and transfer methods
used in the traceability chains for solar irradiance detectors. .7
Figure A.1 – Block diagram of differential spectral responsivity calibration
superimposing chopped monochromatic radiation DE(l) and DC bias radiation E .18
b
Figure A.2 – Optical arrangement of differential spectral responsivity calibration. .19
Figure A.3 – Schematic apparatus of the solar simulator method. .21

Table 1 – Examples of reference instruments, used in a traceability chain of time and
solar irradiance.7
Table A.1 – Typical uncertainty components (k = 2) of global sunlight method .15
Table A.2 – Typical uncertainty components (k = 2) of a differential spectral
responsivity calibration .18
Table A.3 – Example of uncertainty components (k = 2) of a solar simulator method
calibration.21
Table A.4 – Typical uncertainty components (k = 2) of a solar simulator method
calibration when WRR traceable cavity radiometer is used .21
Table A.5 – Typical uncertainty components (k = 2) of a direct sunlight method .23

60904-4 © IEC:2009 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
PHOTOVOLTAIC DEVICES –
Part 4: Reference solar devices –

Procedures for establishing calibration traceability

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60904-4 has been prepared by IEC technical committee 82: Solar
photovoltaic energy systems.
The text of this standard is based on the following documents:
CDV Report on voting
82/533/CDV 82/561/RVC
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of IEC 60904 series, under the general title Photovoltaic devices, can be
found on the IEC website.
– 4 – 60904-4 © IEC:2009
The committee has decided that the contents of this publication will remain unchanged until

the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in

the data related to the specific publication. At this date, the publication will be

• reconfirmed,
• withdrawn,
• replaced by a revised edition, or

• amended.
60904-4 © IEC:2009 – 5 –
PHOTOVOLTAIC DEVICES –
Part 4: Reference solar devices –

Procedures for establishing calibration traceability

1 Scope and object
This part of IEC 60904 sets the requirements for calibration procedures intended to establish
the traceability of photovoltaic reference solar devices to SI units as required by IEC 60904-2.
This standard applies to photovoltaic (PV) reference solar devices that are used to measure
the irradiance of natural or simulated sunlight for the purpose of quantifying the performance
of PV devices. The use of a PV reference solar device is required in the application of
IEC 60904-1 and IEC 60904-3.
This standard has been written with single junction PV reference solar devices in mind, in
particular crystalline Silicon. However, the main part of the standard is sufficiently general to
include other technologies. The methods described in Annex A, however, are limited to single
junction technologies.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60904-2, Photovoltaic devices – Part 2: Requirements for reference solar devices
ISO/IEC 17025, General requirements for the competence of testing and calibration
laboratories
ISO 9059, Solar energy – Calibration of field pyrheliometers by comparison to a reference
pyrheliometer
ISO 9846, Solar energy – Calibration of a pyranometer using a pyrheliometer

ISO/IEC Guide 98-3: 2008, Uncertainty of measurement – Part 3: Guide to the expression of
uncertainty in measurement (GUM: 1995)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
NOTE The different reference instruments for the traceability chain of solar irradiance are defined in this Clause.
Table 1 lists and compares them with those in use for time. Figure 1 shows schematically the most common
traceability chains, based on the methods described in Annex A.
3.1
primary standard
a device, which implements physically one of the SI units or directly related quantities. They
are usually maintained by national metrology institutes (NMIs) or similar organisations
entrusted with maintenance of standards for physical quantities. Often referred to also just as
the «primary», the physical implementation is selected such that long-term stability, precision

– 6 – 60904-4 © IEC:2009
and repeatability of measurement of the quantity it represents are guaranteed to the maximum

extent possible by current technology.

NOTE The World Radiometric Reference (WRR) as realized by the World Standard Group (WSG) of cavity

radiometers is the accepted primary standard for the measurement of solar irradiance.

3.2
secondary standard
a device, which by periodical comparison with a primary standard, serves to maintain

conformity to SI units at other places than that of the primary standard. It does not necessarily

use the same technical principles as the primary standard, but strives to achieve similar long-

term stability, precision and repeatability.

NOTE Typical secondary standards for solar irradiance are cavity radiometers which participate periodically
(normally every 5 years) in the International Pyrheliometer Comparison (IPC) with the WSG.
3.3
primary reference
the reference instrument which a laboratory uses to calibrate secondary references. It is
compared at periodic intervals to a secondary standard. Often primary references can be
realised at much lower costs than secondary standards.
NOTE Typically a solar cell is used as a reference solar device for the measurement of natural or simulated solar
irradiance.
3.4
secondary reference
the measurement device in use for daily routine measurements or to calibrate working
references, calibrated at periodic intervals to a primary reference.
NOTE The most common secondary references for the measurement of natural or simulated solar irradiance are
solar cells and solar modules.
3.5
traceability
the requirement for any PV reference solar device, to tie its calibration value to SI units in an
unbroken and documented chain of calibration transfers including stated uncertainties.
NOTE The WRR has been compared twice to the SI radiometric scale and shown to be within their mutual
uncertainty levels. Therefore traceability to WRR automatically provides traceability to SI units. However, the
uncertainty of the ratio WRR/SI units needs to be taken into account. The World Radiation Center (WRC)
recommends a rectangular uncertainty distribution with 0,3 % half-width. A third comparison is currently underway
and should be published in the future.
J. Romero, N.P. Fox, C. Fröhlich metrologia 28 (1991) 125-8

J. Romero, N.P. Fox, C. Fröhlich metrologia 32 (1995/1996) 523-4

60904-4 © IEC:2009 – 7 –
Table 1 – Examples of reference instruments, used in a traceability chain
of time and solar irradiance
Reference instrument Time Solar irradiance

Primary standard Cesium atomic clock at Group of cavity radiometers constituting the World Standard
National Metrology Institute Group (WSG) of the World Radiometric Reference (WRR)

(NMI)
Cryogenic trap detector
Standard lamp
Secondary standard Cesium atomic clock on GPS Commercially available cavity radiometers compared every 5

(Global Positioning System) years at the International Pyrheliometer Comparison (IPC)
satellites
Standard detector calibrated against a trap detector
Spectroradiometer calibrated against a standard lamp
Primary reference GPS receiver, set to show Normal incidence pyrheliometer (NIP) (ISO 9059)
time
Reference solar device (IEC 60904-2 and IEC 60904-4)
Secondary reference Quartz watch Pyranometer (ISO 9846)
Reference solar device (IEC 60904-2)

WSG Trap detector Standard lamp
Primary
standard
IPC
Secondary
Absolute radiometer Spectroradiometer
Standard detector
standard
ISO 9059 IEC 60904-4
Primary
Reference solar device
NIP
reference
IEC 60904-2
ISO 9846
Secondary Reference solar device
Pyranometer
reference
IEC  858/09
NOTE Direct traceability of absolute radiometers to SI radiometric scale may also be available.
Figure 1 – Schematic of most common reference instruments and transfer methods
used in the traceability chains for solar irradiance detectors
4 Requirements for traceable calibration procedures of PV reference solar
devices
A traceable calibration procedure is necessary to transfer calibration from a standard or
reference measuring solar irradiance (such as cavity radiometer, pyrheliometer and
pyranometer) to a PV reference solar device. The requirements for such procedures are as
follows:
– 8 – 60904-4 © IEC:2009
a) Any measurement instrument required and used in the transfer procedure shall be an

instrument with an unbroken traceability chain.

b) A documented uncertainty analysis.

c) Documented repeatability, such as measurement results of laboratory intercomparison, or

documents of laboratory quality control.

d) Inherent absolute precision, given by a limited number of intermediate transfers.

NOTE 1 Normally the transfer would be from a secondary standard to a PV reference solar cell constituting a

primary reference.
NOTE 2 The transfer from one reference solar device to another is covered by IEC 60904-2.

5 Uncertainty analysis
An uncertainty estimate according to MISC UNCERT – ED. 1.0 (1995-01) shall be provided for
each traceable calibration procedure. This estimate shall provide information on the
uncertainty of the calibration procedure and quantitative data on the following uncertainty
factors for each instrument used in performing the calibration procedure. In particular:
a) Component of uncertainty arising from random effects (Type A).
b) Component of uncertainty arising from systematic effects (Type B).
Nevertheless a full uncertainty analysis has to be performed for the implementation of the
calibration method by a particular laboratory.
6 Calibration report
The calibration report shall conform to the requirements of ISO/IEC 17025 and shall normally
include at least the following information:
a) title (e.g. ”Calibration Certificate”);
b) name and address of laboratory, and location where the tests and/or calibrations were
carried out, if different from the address of the laboratory;
c) unique identification of the report (such as serial number) and of each page, the total
number of pages and the date of issue;
d) name and address of the client placing the order;
e) description and unambiguous identification of the item(s) tested or calibrated;
f) date of receipt of calibration item(s) and date(s) of performance of test or calibration, as
appropriate;
g) calibration results including the temperature of the device at which the calibration was
performed;
h) reference to sampling procedures used by the laboratory where these are relevant to the
validity or application of the results;
i) the name(s), title(s) and signature(s) or equivalent identification of person(s) authorising
the report;
j) where relevant, a statement to the effect that the results relate only to the items tested or
calibrated.
7 Marking
The calibrated reference solar device shall be marked with a serial number or reference
number and the following information attached or provided on an accompanying certificate:
a) date of (actual or present) calibration;

60904-4 © IEC:2009 – 9 –
b) calibration value and its temperature coefficient (if applicable).

– 10 – 60904-4 © IEC:2009
Annex A
(informative)
Examples of validated calibration procedures

A.1 General
This annex describes examples of calibration procedures for PV reference solar cells as

primary reference devices, together with their stated uncertainties. These procedures serve to

establish the traceability of reference solar devices to SI units as required by IEC 60904-2.
Primary reference devices calibrated in accordance with these procedures serve to establish
the traceability of further PV reference solar devices.
As already mentioned in Clause 1, the methods in this annex are limited to PV single junction
technology. Moreover, they have currently only been validated for crystalline Silicon
technology, although they should be applicable to other technologies.
The methods have been implemented in various laboratories around the world and validated
in international intercomparisons, most notably the World Photovoltaic Scale (WPVS).
However, the description in this standard is more generalised. For details of the various
implementations, the references in peer-reviewed publications are given at the end of each
procedure.
The uncertainty estimates are based on U (coverage factor k = 2) for all single components.
The combined expanded uncertainty is calculated as the square root of the sum of squares of
all components. The uncertainties provided are simplified versions (restricted to the main
components) as provided by the laboratories having implemented the procedure. These
uncertainty calculations serve as guidelines and will have to be adapted to the particular
implementation of each procedure in a given laboratory. The uncertainties achieved by any
implementation of these methods might be considerably different. Uncertainties quoted have
to be based on an explicit analysis and cannot be taken by reference to the uncertainty
estimates in this standard.
A.1.1 Examples of validated methods

A. 2 Global sunlight method
A. 3 Differential spectral responsivity calibration
A. 4 Solar simulator method
A. 5 Direct sunlight method
A.1.2 List of common symbols
I short circuit current of reference cell
SC
T temperature of reference cell
j
M irradiance correction factor (see below)
G
M temperature correction factor (see below)
T
T temperature coefficient α of the short-circuit current (IEC 60891) normalized to
coef
the short-circuit current at 25 °C and expressed in 1/ °C
MMF mismatch factor (see below)
λ wavelength
S(λ) spectral response of reference cell
s(λ) differential spectral responsivity of reference cell
E (λ) spectral irradiance distribution of natural or simulated sunlight
m
(λ) standard or reference spectral irradiance distribution according to IEC 60904-3
E
s
G direct irradiance
dir
G diffuse in-plane irradiance
dif
G total in-plane irradiance
T
60904-4 © IEC:2009 – 11 –
–2
E irradiance at STC (= 1 000 Wm )
STC
CV calibration value, i.e. I at STC
SC
AM air mass
STC standard test conditions (1 000 W/m , 25 °C and E (λ))
s
P local air pressure
P 101 300 Pa
θ solar elevation angle
A.1.3 Common equations
The methods described in Clauses A.2, A.4 and A.5 have some common calculations, which

are detailed in this subclause. Details of the various implementations are then described in

each subclause.
–2
The I is normally not measured at exactly 1 000 Wm , but at an irradiance level close to it.
SC
Under the assumption that the I of the reference cell varies linearly with irradiance, the
SC
following correction is made:
W
−2 m
I (1 000 Wm ) = I M = I (A.1)
SC SC G SC
G
T
STC mandate a device temperature of 25 °C, but measurements will not always be taken at
this temperature. The deviations in temperature should be accounted for in the uncertainty
budget. It is also possible to correct I from the measurement temperature T to 25 °C by
SC j
multiplying with the temperature correction factor M defined by
T
I (T )
SC j
I (25 °C) = I (T )M = (A.2)
SC SC j T
1− T()25 °C − T
coef j
The correction for the difference in spectral sensitivity of the reference cell to be calibrated
and the device used to measure the irradiance can be described as a MMF
4000 nm 4000 nm
S(λ) ⋅ E (λ) ⋅ dλ E (λ) ⋅ dλ
s m
∫ ∫
300 nm 300 nm
MMF = (A.3)
4000 nm 4000 nm
S(λ) ⋅ E (λ) ⋅ dλ E (λ) ⋅ dλ
m s
∫ ∫
300 nm 300 nm
NOTE The integration range is taken based on the definition of E (λ). If the measurement range, in particular that
s
of E (λ), does not cover this entire range, suitable approximation, extrapolation or modelling can be used, but
m
needs to be accounted for in the uncertainty calculation.
The calibration value CV of the reference cell is then calculated as
CV = I M M MMF (A.4)
SC G T
A.1.4 References documents
– C. R. Osterwald et al. “The results of the PEP’93 intercomparison of reference cell
calibrations and newer technology performance measurements: Final Report”, NREL/TP-
520-23477 (1998) 209 pages.
– C. R. Osterwald et al. “The world photovoltaic scale: an international reference cell
calibration program”, Progress in Photovoltaics 7 (1999) 287-297.
– K. Emery “The results of the First World Photovoltaic Scale Recalibration”, NREL/TP-520-
27942 (2000) 14 pages.
– 12 – 60904-4 © IEC:2009
– Winter el al.: “The results of the Second World Photovoltaic Scale Recalibration”, Proc. of

st
the 31 IEEE PVSC 3-7 January 2005, Orlando, Florida, USA, pp. 1011-1014.

A.2 Global sunlight method
The establishment of traceability is based on the calibration using the Continuous Sun-and-

Shade Method as described in ISO 9846. The reference solar cell to be calibrated is

compared under natural sunlight with two reference radiometers, namely a pyrheliometer

measuring direct solar irradiance and a pyranometer measuring diffuse solar irradiance by

application of a continuous shade device under normal incidence conditions. The total solar

irradiance is determined by the sum of direct irradiance and diffuse in-plane irradiance. As a

pyrheliometer, a secondary standard is used in the form of an absolute cavity radiometer
compared at 5-year intervals with the World Standard Group (WSG) establishing the World
Radiometric Reference (WRR). The calibration factor for the photovoltaic reference cell is
determined from the measured short circuit current, scaled to 1 000 W/m and corrected for
spectral mismatch (IEC 60904-7) based on the measured spectral irradiance of the global
sunlight and the relative spectral response of the reference solar cell to be calibrated.
Under certain conditions the simplified global sunlight method is applicable. The short-circuit
current of the reference cell is scaled to 1 000 W/m and then plotted versus pressure
corrected geometric air mass. The calibration value is determined from a linear least square
fit at air mass 1,5. A spectral mismatch correction is not required and hence the
measurements of the spectral irradiance of the sunlight and the spectral response are not
necessary. In the simplified version of the global sunlight method no explicit spectral
mismatch correction is performed and it is replaced by conditions which should ensure that
the spectral irradiance of the natural sunlight is sufficiently close to the defined standard
spectral irradiance (IEC 60904-3) that the uncertainty component is smaller than quoted in
Table A.1. Although this should be ensured by the conditions listed in the description of the
method below, it should be explicitly verified (preferentially by using the global sunlight
method). After this validation the simplified version can be applied as long as the boundary
conditions are the same as during the validation.
NOTE 1 The verification and validation will produce numerical values for both methods. If the agreement between
these numerical values is within the uncertainty budget of the methods, the simplified method shall be deemed
validated.
NOTE 2 The simplified procedure gives accurate results for devices with a spectral response over a broad range
of the solar spectrum e.g. crystalline silicon devices. Significant errors may be introduced for narrow spectral
response devices.
A.2.1 Equipment
a) A mounting platform, which can be oriented normal to the sun within an accuracy of ±0,5°
throughout the calibration run.

b) A cavity radiometer, traceable to WRR.
c) A pyranometer, traceable to WRR.
d) A shading device to provide shade to item c). The field angle, viewing angle and aperture
angle provided by the shade shall compensate the respective descriptive angles of the
cavity radiometer of item b).
e) A temperature controlled mounting block for the reference device under test capable of
maintaining the cell temperature at (25 ± 2)°C throughout all calibration runs.
f) Traceable means to measure the short circuit current of the solar cell to an accuracy of
±0,1 % or better.
g) Traceable means to measure the signal of the pyranometer to an accuracy of ±0,5 % or
better.
h) A spectroradiometer capable of measuring the spectral irradiance of the total in-plane
natural sunlight in the wavelength range of 350 – 2 500 nm (or larger).
NOTE 1 Not required in simplified version.

60904-4 © IEC:2009 – 13 –
i) Apparatus to determine the relative spectral response of the reference solar cell.

NOTE 2 Not required in simplified version.

j) Means to measure the sun’s elevation to a precision of ±2°. Alternatively, the elevation of

the sun during the data sampling can be taken from almanacs or computed, as long as the

precision requirement is met for the instant of data sampling. The latter normally requires

traceable means to measure time for the computation of air mass.

NOTE 3 Only required in simplified version.

k) A manometer to measure the local air pressure P to an accuracy of ±250 Pa or better.

NOTE 4 Only required in simplified version.

A.2.2 Measurements
A calibration according to this standard shall be performed only on clear, sunny days with no
visible cloud cover within 30 degrees of the sun.
a) Determine the relative spectral response of the reference cell to be calibrated.
NOTE 1 Not required in simplified version.
b) Select the site and/or the season of the year to ensure that the sun’s elevation reaches an
angle during the course of the day which corresponds to AM 1,5 (41,8 degrees at P ).
c) Mount the cavity radiometer on the sun-pointing device (item A.2.1.a). Available
radiometers have their own electronic unit which shall be connected to the instrument
following the manufacturer’s recommendations. Allow sufficient time to stabilise the
electronic unit.
d) Mount the reference solar cell to be calibrated coplanar on the mounting platform,
attaching it to the mounting block and maintain the cell temperature at (25 ± 2) °C.
e) Mount the pyranometer intended to measure diffuse solar irradiance coplanar on the
mounting platform. Ensure that within the field of view of the pyranometer no reflective
surfaces may influence the measurement result. Mount the shading device and ensure
that the sensitive area of the pyranometer is pointed to the centre of the shade.
f) Mount the spectroradiometer coplanar on the mounting platform.
NOTE 2 Not required in simplified version.
g) Take simultaneous readings according to the following steps:
1) Ensure the alignment of all instruments with respect to the sun and the proper
alignment of the shading device.
2) Ensure that the temperature of the reference solar cell is within the limits given in d).
3) Record G , the direct normal irradiance as indicated by the cavity radiometer.
dir
4) Record G , the diffuse in-plane irradiance as indicated by the pyranometer
dif
5) Record I , the short circuit current of the reference solar cell to be calibrated
SC
6) Record E(λ), the spectral irradiance of the global natural sunlight.
NOTE 3 Not required in simplified version.
7) Measure θ, the solar elevation angle, or alternatively, record the hour, minute and
second of the data sampling and calculate the sun’s elevation.
NOTE 4 Only required in simplified version.
8) Record P, the local air pressure.
NOTE 5 Only required in simplified version.
9) Repeat Steps 1 to 6 several times.
NOTE 6 Not required in simplified version.

– 14 – 60904-4 © IEC:2009
10) Repeat steps 1 to 5, 7 and 8 at least every 5 min for several hours before and after

solar noon, spanning the range of air mass from below AM 1,5 to above AM 3,0 in both

time periods.
NOTE 7 Only required in simplified version.

h) Repeat the whole measurement procedure on at least two other days.

A.2.3 Data analysis
For all data points taken, apply in sequence the following steps:

a) Reject data points where G , G or I deviate by more than ±3 % when compared to the
dir dif sc
previous data point.
G = G + G .
b) Calculate the total irradiance
T dir dif
c) Scale the measured short circuit current I of the reference solar cell to be calibrated to
sc
1000 W/m according to Equation A.1.
d) Correct for temperature according to Equation A.2.
NOTE 1 This is normally not required as the temperature is maintained as described in A.2.2.d) and the allowed
temperature deviation is accounted for in the uncertainty budget.
e) Correct for spectral mismatch according to Equation A.3, where E (λ) is the measured
m
spectral irradiance of the global natural sunlight.
f) Calculate the calibration value according to Equation A.4.
g) Average all calibration values for one day to obtain CV .
h) Repeat steps a) to g) for the other days of measurement runs to obtain CV , CV , . CV
2 3 n
accordingly.
i) Determine the arithmetic average of all n CV values analysed according to the above
i
steps which yields the final calibration value for the reference device:
CV = (CV + CV + . + CV ) / n. (A.5)

1 2 n
j) In the simplified version the steps e) to g) are replaced as follows:
1) Reject data points for which the ratio G /G is either smaller than 0,1 or larger than
dif T
0,3. Also reject data points where G is outside the range 800 – 1 200 W/m .
T
NOTE 2 This to ensure that data used for the analysis are taken during atmospheric conditions close to the
standard reference spectrum.
2) Using the sun’s elevation angle and the atmospheric pressure, calculate the air mass
(AM) at the moment of measurement according to:
AM = P / (P0 × sin(θ)) (A.6)
3) Reject all data samples where AM is larger than 3.
4) Plot the value of I obtained after step d) versus the air mass value AM of each
sc i
corresponding measurement sample.
5) By using a linear least-square technique, calculate the slope (m) and offset (b) of the
best fit straight line of the data set. In order to balance the fit, all short circuit current
readings should be averaged for AM bins of 0,01 before performing the fit. Both
morning and afternoon have to contribute at least 33 % of the total number of
measurement samples used for the Least-Squares fit.
NOTE 3 For a good straight line fit, 10 data points shall be considered as minimum. The smaller the uncertainty of
the procedure, the more data points in the least-squares fit are close to AM 1,5.
NOTE 4 It is permissible to use only data from half a day. However, in the final average, at least data from three
different days with at least two mornings and two afternoons have to be included.
6) Calculate the calibration value of the reference device by the formula:
CV = m × AM + b   with AM = 1,5 (A.7)
60904-4 © IEC:2009 – 15 –
7) Perform steps h) and i).
A.2.4 Uncertainty estimates
In Table A.1, typical values of the uncertainty components for the global sunlight method (left
column) and its simplified version (right column) are listed, resulting in combined expanded

uncertainties U (with coverage factor k = 2) of 0,8 % and 1,1 % respectively.

Table A.1 – Typical uncertainty components (k = 2) of global sunlight method

Uncertainty in measurement of short circuit current 0,1 %

0,1 %
Uncertainty due to unstable cell temperature (± 2 K)
Uncertainty of direct irradiance 0,4 %
Uncertainty of diffuse irradiance 1,6 %
Uncertainty of total irradiance (80 % direct and 20 % diffuse) 0,6 %
Uncertainties due to spectral mismatch correction (IEC 60904-7) or spectral
irradiance deviations between test conditions and the reference spectral
0,3 % 0,4 %
irradiance of AM 1,5 (IEC 60904-3)
Variations of data on different days 0,3 % 0,8 %
Combined expanded uncertainty 0,8 % 1,1 %

A.2.5 References documents
– K.A. Emery, C.R. Osterwald, L.L. Kazmerski, and R.E. Hart, (1988c), Calibration of
Primary Terrestrial Reference Cells When Compared With Primary AM0 Reference Cells,
Proceedings of the 8th PV Solar Energy Conference, Florence, pp. 64-68.
– K. A. Emery, C.R. Osterwald, S. Rummel, D.R. Myers, T.L. Stoffel, and D. Waddington, “A
Comparison of Photovoltaic Calibration Methods,” Proc. 9th European Photovoltaic Solar
Energy Conf., Freiburg, W. Germany, September 25-29, 1989, pp. 648-651.
– K.A. Emery, D. Waddington, S. Rummel, D.R. Myers, T.L. Stoffel, and C.R. Osterwald,
“SERI Results from the PEP 1987 Summit Round Robin and a Comparison of Photovoltaic
Calibration Methods,” SERI tech. rep. TR-213-3472, March 1989.
– Gomez, T, Garcia L, Martinez G, "Ground level sunlight calibration of space solar cells.
Tenerife 99 campaign," Proc. 28th IEEE PVSC, 1332-1335, (2000).
– J. Metzdorf, T. Wittchen, K. Heidler, K. Dehne, R. Shimokawa, F. Nagamine, H.
Ossenbrink, L. Fornarini, C. Goodbody, M. Davies, K. Emery, and R. Deblasio, “The
Results of the PEP '87 Round-Robin Calibration of Reference Cells and Modules,- Final
Report” PTB technical report PTB-Opt-31, Braunschweig, Germany, November 1990,
ISBN 3-89429-067-6.
– H. Müllejans, A. Ioannides, R. Kenny, W. Zaaiman, H. A. Ossenbrink, E. D. Dunlop
“Spectral mismatch in calibration of photovoltaic reference devices by global sunlight
method” Measurement Science and Technology 16 (2005) 1250-1254.
– H. Müllejans, W. Zaaiman, E. D. Dunlop, H. A. Ossenbrink “Calibration of photovoltaic
reference cells by global sunlight method”, Metrologia 42 (2005) 360-367.
– H. Müllejans, W. Zaaiman, F. Merli, E. D. Dunlop, H. A. Ossenbrink “Comparison of
traceable calibration methods for primary photovoltaic reference cells” Progress in
Photovoltaics 13 (2005) 661-671.
– F.C. Treble and K.H. Krebs, “Comparison of European Reference Solar Cell Calibrations”,
Proc. 15th IEEE PV Spec. Conf., 1981, pp. 205-210.
– R. Whitaker, G. Zerlaut, and A. Purnell, “Experimental demonstration of the efficacy of
global versus direct beam use in photovoltaic performance prediction of flat plate
photovoltaic modules”, Proc 16th IEEE PVSC, pp. 469-474, 1982.

– 16 – 60904-4 © IEC:2009
A.3 Differential spectral responsivity calibration (DSR calibration)

Traceability is based on a calibration of spectral responsivity based on standard detectors

directly traceable to SI units. The calibration value is computed from the measured absolute

spectral responsivity of the reference cell and the reference solar spectral irradiance

distribution. The spectral responsivity calibration is transferred from the standard detector

irradiance level to the solar irradiance level over many orders of magnitude with no

restrictions to the solar cell concerning linearity or spectral match.

A.3.1 Equipment
The following apparatus is required (see Figures A.1 and A.2)
–2 –1
a) a monochromator producing chopped spectral irradiance of at least 1 mWm nm within
the wavelength range covering the spectral responsivity of the reference solar cell to be
calibrated, with a traceable wavelength setting;
b) lamp(s) with lens or mirror entrance optics (recommended are quartz-halogen lamp to
cover wavelengths above 400 nm; and Xenon-arc lamps for wavelengths below 400 nm);
c) a bias light source, meeting in spectral irradiance, uniformity and temporal stability the
requirements of Class CBA as defined in IEC 60904-9;
d) a chopped monochromatic beam, traceable in its wavelength calibration, for the absolute
calibration at one or more discrete wavelengths. The non-uniformity shall be smaller than
± 3 % within the active area of the device to be calibrated;
e) a monitor photodiode large enough to monitor the radiation power of the monochromatic
beam of a) and d);
f) standard radiation detector(s) with temperature control directly traceable to SI units.
These detectors shall be of photodiodes with the best available linearity, uniformity and
stability;
g) adjustable aperture (imaged onto the reference cell);
h) means for maintaining the temperature of the reference cell at (25 ± 2)°C;
i) means for measuring the AC short-circuit currents of the reference cell, the standard
detector(s) and the monitoring detector, e.g. with a lock-in amplifier. The variation of the
amplification factor of such amplifiers shall be less than 0,1 % over the signal ranges
used. Preferably the same amplifier is used for the reference cell and the standard
detector;
j) means for measuring the DC component of the reference cell I as defined in step A.3.2.f.
...