IEC 60904-2:2015

IEC 60904-2 ®
Edition 3.0 2015-01
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
Photovoltaic devices –
Part 2: Requirements for photovoltaic reference devices

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IEC 60904-2 ®
Edition 3.0 2015-01
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
Photovoltaic devices –
Part 2: Requirements for photovoltaic reference devices

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-2231-7

– 2 – IEC 60904-2:2015 RLV  IEC 2015

CONTENTS
FOREWORD . 3

1 Scope and object . 5

2 Normative references . 5

3 Terms and definitions . 6

4 Selection of reference device . 8

4.1 General requirements . 8

4.2 Additional requirements for single reference cell in a multi-cell package . 9
4.3 Additional requirements for reference modules . 9
4.4 Requirements for built-in shunt resistors . 9
5 Temperature measurement . 10
6 Electrical connections . 10
7 Calibration . 10
8 Data sheet Report . 11
9 Marking . 12
10 Packaging . 12
10.1 Recommended packaging for use in natural sunlight . 12
10.2 Recommended packaging for use under solar simulators . 12
10.3 Single cell package . 12
11 Care of reference devices . 12
12 Calibration of secondary reference devices against a primary reference cell . 13
12.1 General . 13
12.2 Natural sunlight . 13
12.3 Simulated sunlight . 14
12.4 Test procedure . 14
13 Calibration of working solar reference device against a secondary reference solar
device. 15
Bibliography . 16

Figure 1 – Single-cell package . 7
Figure 2 – Single Reference cell in a multi-cell package . 9

INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
PHOTOVOLTAIC DEVICES –
Part 2: Requirements for photovoltaic reference devices

FOREWORD
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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This redline version of the official IEC Standard allows the user to identify the changes
made to the previous edition. A vertical bar appears in the margin wherever a change
has been made. Additions are in green text, deletions are in strikethrough red text.

– 4 – IEC 60904-2:2015 RLV  IEC 2015

International Standard IEC 60904-2 has been prepared by IEC Technical Committee 82: Solar
photovoltaic energy systems.
This third edition cancels and replaces the second edition, published in 2007. It constitutes a
technical revision.
The main technical changes with regard to the previous edition are as follows:

– addition of a test procedure in simulated sunlight of subsequent measurement of primary

and secondary reference device;

– definition of standard test conditions;

– reduction of allowed diffuse component for secondary reference cell calibration to 20 %.
The text of this standard is based on the following documents:
FDIS Report on voting
82/893/FDIS 82/918/RVD
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 in the IEC 60904 series, published under the general title Photovoltaic
devices, can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC website 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.
IMPORTANT – The “colour inside” logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this publication using a colour printer.

PHOTOVOLTAIC DEVICES –
Part 2: Requirements for photovoltaic reference solar devices

1 Scope and object
This part of IEC 60904 gives requirements for the classification, selection, packaging, marking,

calibration and care of photovoltaic reference solar devices.
This standard covers solar photovoltaic reference devices used to determine the electrical
performance of solar photovoltaic cells, modules and arrays under natural and simulated
sunlight. It does not cover solar photovoltaic reference devices for use under concentrated
sunlight.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60891, Photovoltaic devices – Procedures for temperature and irradiance corrections to
measured I-V characteristics
IEC 60904-1, Photovoltaic devices – Part 1: Measurements of photovoltaic current-voltage
characteristics
IEC 60904-3, Photovoltaic devices – Part 3: Measurement principles for terrestrial
photovoltaic (PV) solar devices with reference spectral irradiance data
IEC 60904-4, Photovoltaic devices – Part 4: Reference solar devices – Procedures for
establishing calibration traceability
IEC 60904-5, Photovoltaic devices – Part 5: Determination of the equivalent cell temperature
(ECT) of photovoltaic (PV) devices by the open-circuit voltage method

IEC 60904-7, Photovoltaic devices – Part 7: Computation of the spectral mismatch correction
for measurements of photovoltaic devices
IEC 60904-8, Photovoltaic devices – Part 8: Measurement of spectral responsivity of a
photovoltaic (PV) device
IEC 60904-9, Photovoltaic devices – Part 9: Solar simulator performance requirements
IEC 60904-10, Photovoltaic devices – Part 10: Methods of linearity measurement
IEC 61215, Crystalline silicon terrestrial photovoltaic (PV) modules – Design qualification and
type approval
IEC 61646, Thin-film terrestrial photovoltaic (PV) modules – Design qualification and type
approval
– 6 – IEC 60904-2:2015 RLV  IEC 2015

IEC TS 61836, Solar photovoltaic energy systems – Terms, definitions and symbols

3 Description Terms and definitions

Reference solar devices are specially calibrated devices which are used to measure natural or

simulated irradiance or to set simulator irradiance levels for measuring the performance of

other solar devices having similar spectral response, optical characteristics, dimensions and

electrical circuitry.
For the purposes of this document, the terms and definitions given in IEC TS 61836 and the

following apply.
3.1
calibration traceability
traceability as defined in IEC 60904-4
Note 1 to entry: Photovoltaic reference devices are distinguished by their position in a chain of calibration
traceability as follows.
3.2
primary reference device
photovoltaic reference device whose calibration is based on a radiometer or standard detector
or standard light source traceable to SI units as defined in IEC 60904-4.
3.3
secondary reference device
photovoltaic reference device calibrated in natural or simulated sunlight against a primary
reference device.
3.4
working reference device
photovoltaic reference device calibrated in natural or simulated sunlight against a secondary
reference device.
3.5
Construction of reference devices
Depending on their intended use, reference devices need to meet different requirements in
terms of their mechanical construction, optical properties and electrical circuit. The following
types are in use
reference devices
specially calibrated photovoltaic devices which are used to measure natural or simulated

irradiance or to set simulator irradiance levels for measuring the performance of other
photovoltaic devices
3.6
reference cell
A reference single photovoltaic cell used primarily for transfer of calibration values
Note 1 to entry: For practical reasons, such cells are small in surface area, and are usually mounted on a fixture
which ensures reproducibility in mounting, thermal control and electrical connections. A typical sample is sketched
in Figure 1.
Recommended use: laboratory primary and secondary reference.

Wiring space
Window receptacle
Solar cell cavity
Voltage and
Cable
current leads
clamp
Field of view not
less than 160°
IEC
Figure 1 – Single-cell package
3.6.1
reference cell with protective cover but without encapsulant
photovoltaic reference cell similar to 3.6, but provided with a protective cover
Note 1 to entry: Recommended use: as a laboratory primary, secondary and working reference, in particular when
calibrating measuring the performance of other photovoltaic devices using solar simulators or when calibrating
outdoors natural sunlight with direct beam only.
3.6.2
encapsulated reference cell
photovoltaic reference cell similar to 3.6, but encapsulated in a protective assembly so as to
withstand short-term outdoor exposure
Note 1 to entry: Recommended use: as a laboratory primary, secondary and working reference, in particular when
calibration in measurements of the performance of other photovoltaic devices under natural sunlight are performed.
Note 2 to entry: If the encapsulation system has been demonstrated to withstand long-term outdoor exposure,
applying test levels according to IEC 61215, such reference cells may also be suitable to be used as a monitoring
device for long-term assessment of operational photovoltaic arrays.
3.7
multi-cell reference devices
A reference device matching the mechanical and optical features of a test specimen, and
responds to variations in the geometrical distribution of the incident radiation in the same way
as the test specimens.
photovoltaic device consisting of several photovoltaic cells
Note 1 to entry: Recommended use: as the diffuse component of natural sunlight and non-normal incidence of
simulated sunlight interact with encapsulants and back sheets of a module and influence the amount of irradiation
irradiance which a particular cell receives, it is recommended that reference devices used for measuring sub-
assemblies of modules and arrays be encapsulated in a multi-cell package, simulating the neighbouring optical
parameters matching the mechanical and optical features of a the test specimen (module, sub-assemblies of
modules, arrays) and reflectivity of the front cover so as to respond to variations in the geometrical distribution of
the incident radiation in the same way as the test specimen.
3.8
single reference cell in a multi-cell package
single photovoltaic cell mounted in such a package that frame, encapsulation system, shape,
size and spacing of the cells surrounding the reference cell shall be it are the same as in the
module to be tested.
– 8 – IEC 60904-2:2015 RLV  IEC 2015

Note 1 to entry: The surrounding cells may be real or dummies that have the same optical properties. The dotted

line in Figure 1 indicates the minimum acceptable size of the multi-cell package for outdoor testing.

3.9
reference module
photovoltaic module consisting of the encapsulation of a series and/or parallel connection of

solar photovoltaic cells
Note 1 to entry: The use of a full-size reference module is recommended in Recommended use: for measuring

other modules in order to achieve correspondence of dimensions, mechanical construction, optical properties and
electrical circuitry of the reference module and test specimen module, so as to minimize discrepancies due to
simulator non-uniformity, internal reflections or temperature distribution.

3.10
built-in shunt resistors
For practical reasons, reference devices may be shunted by a precision resistor, in order to
avoid user-provided means of establishing short circuit condition. In such cases, the precision
resistor should be chosen such as to ensure that the reference device operates sufficiently
near to short-circuit condition, meeting the requirement:

I ×R < 0,03× V
SC CAL OC
where
R is the precision shunt resistor;
CAL
I is the short circuit current of the reference device at reference conditions;
SC
V is the open circuit voltage at reference conditions.
OC
The long-term stability of such resistors shall also meet the stability requirements of the
reference device. Calibration values of such reference devices shall be stated with the
dimension [V∙W–1∙m2], if the calibration has been performed on the voltage drop across the
shunt resistor. The temperature coefficient of the built-in shunt resistor is part of the
temperature coefficient of the calibration value of the reference device.
NOTE It is recommended that the shunt resistor be a removable 4-wire resistor, to allow for periodic checking of
the reference device stability by taking an I-V curve per IEC 60904-1.

resistor connected across the output terminals of photovoltaic devices
Note 1 to entry: The resistor shunts the output of the photovoltaic device providing an output voltage to be
measured and avoiding user-provided means of establishing short circuit condition.
4 Selection of reference device

4.1 General requirements
Depending on their intended use, reference devices need to meet different requirements in
terms of their spectral responsivity, mechanical construction, optical properties, dimensions
and electrical circuit. The spectral responsivity of the reference device, for example, is
determined by the transmission of any protective cover in front of the device and the spectral
responsivity of the device itself. Therefore the overall spectral responsivity can be adapted by
using suitable filters as or in addition to the protective cover.
A reference device shall meet the following requirements:
a) photovoltaic characteristics shall be stable according to the requirements in Clause 11;
b) the output signal of the reference device shall vary linearly with irradiance, as defined in
IEC 60904-10, over the range of interest.

4.2 Additional requirements for single reference cell in a multi-cell package

The dotted line in Figure 2 indicates the minimum acceptable size of a multi-cell package.

Reference cell Minimum size of multi-cell package

IEC
Figure 2 – Single Reference cell in a multi-cell package
4.3 Additional requirements for reference modules
Additional requirements apply to reference modules:
a) it should not contain Bypass diodes:
– general reference modules, that are used to measure a range of module types and
geometries, should not contain bypass diodes. The presence or absence of bypass
diodes shall be noted and considered in conjunction with the measurement conditions,
in particular spatial non-uniformity of the irradiance on the module during measurement;
– for reference modules, that are intended to be matched to the module under test, the
number, type and connection of bypass diodes (if present) shall match those in the
module under test.
b) If they are made from discrete cells, these shall be matched in short -circuit current
and fill-factor within ±2 % as follows depending on the intended use of the reference
module:
– if only the short circuit current of the reference module will be used the short circuit
current of the individual cells shall be matched to within ± 1 %;
– if other parameters (such as maximum power) are used additionally or exclusively,
both the short circuit current and the fill factor of the individual cells shall be matched
to within ± 1 %.
The matching of the individual cells is the responsibility of the manufacturer of the
reference module, bearing in mind that matching may also be influenced by encapsulation
or lamination. The cell matching needs not be checked by the calibration laboratory.
However, if I-V curves of the reference module indicate inconsistent response (i.e. steps
are noted in the I-V curve), the I-V curve should be measured under light that is known to
be uniform (e.g. natural sunlight) to determine whether there is evidence that the cells
within the module are matched within 1 %. If the module exhibits evidence of > 1 %
mismatch between cells, the module shall not be used as reference module.
4.4 Requirements for built-in shunt resistors
The resistor shall be chosen such as to ensure that the reference device operates sufficiently
near to short-circuit condition, meeting the requirement:
I × R < 0,03 × V (1)
SC CAL OC
– 10 – IEC 60904-2:2015 RLV  IEC 2015

where
R is the shunt resistor;
CAL
I is the short circuit current of the reference device at reference conditions;
SC
V is the open circuit voltage at reference conditions.
OC
If a shunted reference cell does not meet the requirement of formula (1), it shall only be used

at irradiances (± 5 %) and temperatures (± 2 ºC) at which it was calibrated.

The long-term stability of such resistors shall also meet the stability requirements of the

reference device. Calibration values of such reference devices shall be measured as the

voltage drop across the shunt resistor and stated with the dimension [V] at standard test
conditions (see Clause 7). The temperature coefficient of the built-in shunt resistor is part of
the temperature coefficient of the calibration value of the reference device. As the uncertainty
in the calibration may be strongly dependent on the shunt resistor stability and temperature
coefficient, respective values should be provided with the reference cell data sheet.
If a shunted reference cell is to be used for low irradiance measurements, either a dedicated
cell can be constructed with the restriction of formula (1), where the short circuit current is
considered at the desired low irradiance rather than at STC. Alternatively a shunted cell can
have a larger shunt resistor, but requires a separate calibration for each irradiance and
temperature it is to be used at.
It is recommended that the shunt resistor be a removable 4-wire resistor, to allow for periodic
checking of the reference device stability by taking an I-V curve per IEC 60904-1.
Formula (1) means that the measured output voltage of a shunted reference cell shall be less
than 3 % of its open circuit voltage. For typical crystalline Silicon this equates to about 20 mV
output.
5 Temperature measurement
Means shall be provided for determining the reference cell temperature or, for reference
modules, the equivalent cell temperature (ECT), according to IEC 60904-5. The required
uncertainty for temperature measurements shall be less than ± 2,0 °C for all reference
devices. A minimum accuracy of ±1,0 °C for the temperature sensor is suggested to achieve
this uncertainty in the temperature measurement.
6 Electrical connections
The electrical connections to reference cells shall consist of a four-wire contact system
(Kelvin probe). Care shall be taken to avoid measurement errors due to voltage drops along
the cell’s contact bars and the package wiring.
The electrical connections to the reference module shall be designed to meet the
requirements of IEC 60904-1 (Clause 4).
7 Calibration
Each reference device shall be calibrated in terms of its calibration value at the desired
–2
reference conditions, normally standard test conditions (STC) (1 000 W·m , 25 ºC device
temperature with the reference spectral irradiance distribution as defined in IEC 60904-3).
Methods for calibrating primary reference devices are included in IEC 60904-4. A method of
calibrating secondary reference devices is described in Clause 12. The calibration of working
reference devices is treated in Clause 13.

The relative spectral response of responsivity at short-circuit current conditions of each

reference device shall be measured in accordance with IEC 60904-8. If for reference modules

this cannot be measured directly, it shall be deduced from measurements made on

representative encapsulated solar photovoltaic cells.

The temperature coefficient of each reference device shall be measured in accordance with

IEC 60891.
8 Data sheet Report
Each time a reference device is calibrated, the following information shall be recorded on a
data sheet:
– Identification number
– Type (primary reference cell; secondary reference device, working reference device)
– Cell manufacturer
– Material type
– Type of package
– Type and dimension of cell(s)
– Circuit diagram, in particular of any connectors
– Calibration organization
– Site and date of calibration
– Method of calibration (refer to standard)
– Radiometer or standard lamp characteristics (where applicable)
– Primary reference cell identification (where applicable)
– Simulator characteristics (where applicable)
– Type of temperature sensor (where applicable)
– Relative spectral response Spectral responsivity
– Temperature coefficient of calibration value
– Calibration value at reference conditions
– Reference conditions
– Estimated uncertainty
– Shunt resistor nominal resistance and temperature coefficient (where applicable)
– Either the mismatch correction value used in the measurement or an estimate of the

uncertainty introduced by using the mismatched reference device.
For reference cells without fixed electrical connection to the cell, the following information
shall be recorded on the data sheet:
– Illustration of type, shape and location of electrical contacts during calibration.
For reference modules, the following information shall be recorded in addition on the data
sheet:
– manufacturer
– model designation
– serial number
– cell technology
– construction and dimensions of module
– electrical circuit layout
– 12 – IEC 60904-2:2015 RLV  IEC 2015

– presence or absence of bypass diodes and if present their number and type.

9 Marking
The reference device shall carry a clear, indelible serial or identification number for cross-

reference to its data sheet.
10 Packaging
10.1 Recommended packaging for use in natural sunlight

The reference device used for measurement in natural sunlight shall should respond to
variations in the geometrical distribution of the incident radiation in the same way as the test
specimens (cells, sub-assemblies of cells, modules). As encapsulants and back sheets
respond to the diffuse component of natural sunlight, it is recommended that reference cells
used for measuring modules be enclosed in a multi-cell package (see Figure 2), simulating
the neighbouring optical parameters of a module.
In this case, the frame, the encapsulation system, the shape, and the size and spacing of the
cells surrounding the reference cell shall be the same as in the module to be tested. The
surrounding cells may be real or dummies that have the same optical properties. The dotted
line in Figure 2 indicates the minimum acceptable size of the multi-cell package for outdoor
testing.
10.2 Recommended packaging for use under solar simulators
In some simulators which allow multiple reflections of light to and from the test specimen, the
irradiance in the test plane may change depending on whether or not the test specimen is
present.
Therefore, in order to measure accurately the irradiance that will be present when the test
specimen is in place, the reference cells devices used in such simulators shall be packaged in
the same way as the test specimen, so that the change in irradiance due to multiple
reflections is the same for both the reference cell device and the test specimen.
Reference cells used for measurements in simulators designed to minimize any error from
multi-reflected light may be packaged singly or, if not intended for day-to-day use, mounted in
the unpackaged state on a temperature controlled block.
Alternatively, the requirements given for reference cells for use in natural sunlight may be
followed.
10.3 Single cell package
If a single cell package is used, the following recommendations are made:
a) The field of view should be at least 160°.
b) All surfaces in the package within the cell’s field of view should be non-reflective, with
an absorption of at least 0,95 in the cell’s wavelength response responsivity band.
c) The material used for bonding the cell to the holder should be resistant to degradation,
either electrically or optically. Its physical characteristics should remain stable over the
entire period of intended use.
d) The use of a protective window is recommended. If encapsulated, the space between
the window and the cell should be filled with a stable transparent encapsulant. Both
the protective window and the encapsulant should be transparent over the wavelength
range in which the PV reference device has a non-zero spectral responsivity. The
refractive index of the encapsulant should be similar (within 10 %) to that of the

window to minimize errors due to the internal reflection of light. The transparency,

homogeneity and adhesion of the encapsulant should not be adversely affected by

ultra-violet light and operational temperatures.

e) The protective window may embody a filter to match the spectral response

responsivity of the reference cell to that of the test specimen, provided that the other

requirements of d) are met.
Figure 1 shows an example of a suitable single cell package. Other suitable single cell

packages can be found in JIS C8910 and C8911 or the World Photovoltaic Scale found in the

reference C.R. Osterwald, S. Anevsky, K. Bücher, A.K. Barua, P. Chaudhuri, J. Dubard, K.
Emery, B. Hansen, D. King, J. Metzdorf, F. Nagamine, R. Shimokawa, Y.X. Wang, T. Witchen,

W. Zaaiman, A. Zastrow, and J. Zhang, “The World Photovoltaic Scale: An International

Reference Cell Calibration Program,” Progress in Photovoltaics Research and Applications,
vol. 7, pp. 287-297, 1999 (see Bibliography).
11 Care of reference devices
It is recommended that reference devices be recalibrated on an annual basis.
The window of a packaged reference device shall be kept clean and scratch-free.
Uncovered reference cells shall be preserved against damage, contamination and degradation.
A reference device exhibiting any defect which might impair its function shall not be used.
If The calibration value of a reference device has changed might change systematically as a
function of time for successive calibrations. If the calibration value of a reference device has
changed by more than 1 % with respect to its previous calibration or by more than 5 % of its
initial calibration, it shall not be used as a reference device.
12 Calibration of secondary reference devices against a primary reference cell
12.1 General
This Clause describes a procedure for calibrating a secondary reference device in natural or
simulated sunlight against a primary reference cell whose calibration is traceable to SI units
according to IEC 60904-4. The spectral response match responsivity mismatch between the
primary reference cell and that of the secondary reference device under the illumination used
for the calibration shall be determined according to IEC 60904-7. If the spectral mismatch
correction is less than 1 %, the mismatch correction may be omitted.

The procedure can be applied using both natural and simulated sunlight according to the
requirements in IEC 60904-1 with the following restrictions.
12.2 Natural sunlight
Calibration in natural sunlight shall be carried out under the following conditions:
a) Clear, sunny weather, with the diffuse irradiance not greater than 30 20 % of the
global irradiance.
b) No observable cloud formations.
–2
c) Total irradiance (sun + sky + ground reflection) not less than 800 W·m , as
measured by the primary reference cell.
d) Air mass between AM1 and AM2.
e) Radiation sufficiently stable so that the variation in reference cell output signal is less
than ± 0,5 % over the time taken for a measurement.

– 14 – IEC 60904-2:2015 RLV  IEC 2015

12.3 Simulated sunlight
If simulated sunlight is used The simulated sunlight for calibration can either be continuous or

pulsed. Normally the primary reference cell and the secondary reference device are

positioned side by side and measured simultaneously. In this case, the simulator shall be of

Class AAA in accordance with IEC 60904-9 with the additional requirement that the non-
uniformity of irradiance is less than ±1 % within the surface that includes the device to be

calibrated and the primary reference device. The requirement of class A with respect to

temporal instability as defined in IEC 60904-9 needs only be met for the short term instability

(STI) as the long term instability (LTI) is not relevant in this case. In case that the primary and

secondary reference device are of the same or similar size (ratio of active areas between 0,5

and 2,0), an additional measurement shall be taken exchanging their positions. A valid result

is only obtained if both measurements agree within the measurement uncertainty.
If the requirement of non-uniformity of less than ± 1 % cannot be met for calibrating reference
modules made from a series connection of cells, the simulator shall be class A for spatial non-
uniformity and a detailed uncertainty analysis shall be provided taking into account mismatch
in short-circuit current of the individual cells.
In the special case that the primary reference cell and the secondary reference device are of
the same or similar size (ratio of active areas between 0,5 and 2,0) and that the continuous
simulated sunlight is stable, the two devices can be placed in the same position one after the
other and measured consecutively. In this case the simulator shall be of Class AAA in
accordance with IEC 60904-9 with the additional requirement that the LTI of irradiance is less
than ± 1 %, where the LTI is with respect to the total time period needed for the
measurements. A pulsed solar simulator may also be suitable if the repeatability of each pulse
is better than 1 %. This shall be recorded using a suitable monitor cell.
In any case a detailed measurement uncertainty analysis considering both, spatial and
temporal non-uniformity of the simulated sunlight needs to be provided, considering the
specifics of the chosen measurement strategy.
12.4 Test procedure
12.4.1 Before the initial calibration, measure the relative spectral response responsivity and
temperature coefficient of short circuit current of the secondary reference device, using the
procedures specified in IEC 60904-8, IEC 61215 or IEC 61646 and IEC 60891, respectively.
Whenever a change in the calibration value at the reference conditions of more than 2 % with
respect to this initial calibration is detected, the measurements of spectral responsivity and
temperature coefficient shall be repeated.
12.4.2 Adjust the mount so that the devices are normal to the light source within ± 5°.

– Mount the primary reference cell and the secondary reference device co-planar within ± 1°
and in close proximity on the same mount (for simultaneous measurement).
– Mount the primary reference cell in the designated position (for consecutive measurement).
12.4.3 Control the cell temperature of both the primary reference cell and the secondary
reference device at (25 ± 2) °C. Where this is not practical, readings of output signal must
shall be subsequently corrected to 25 °C in accordance with IEC 60891.
12.4.4 The spectral mismatch shall be corrected according to IEC 60904-7. Appropriate
measurements of the specral irradiance shall be recorded.
12.4.5 Record simultaneous readings of the output signal and temperature:
– of both primary reference cell and secondary reference device (for simultaneous
measurement) The spectral irradiance shall be determined. For outdoor measurements the
spectral irradiance shall be measured while the output signals are measured.,

– of the primary reference cell, (for consecutive measurement).

12.4.6 Repeat step 12.4.5 until five successive sets of readings are obtained in which:

– the ratio of the output signals (corrected to 25 °C and for spectral mismatch as required)
does not vary by more than ± 0,5 % (for simultaneous measurement),

– the output signals for the primary reference cell (corrected to 25 °C and for spectral

mismatch as required) do not vary by more than ± 0,5 %; then remove primary reference

cell and mount the secondary reference device in the same position and repeat step

12.4.5 with the same constraints as for the primary reference cell (for consecutive
measurement).
12.4.7 If the primary reference cell and the secondary reference device are of the same or
similar size (ratio of active areas between 0,5 and 2,0) and are measured simultaneously,
exchange the position between the primary reference cell and the secondary reference device
and repeat steps 12.4.5 and 12.4.6.
12.4.8 When calibrating in natural sunlight, steps 12.4.2 to 12.4.6, inclusive, shall be
performed at least twice a day on at least three separate days.
12.4.9 From the acceptable data, calculate the ratio:
Output signal of secondary reference device at 25 °C

Output signal of primary reference cell at 25 °C
12.4.10 Multiply the calibration value of the primary reference cell by the calculated:
– mean of the ratios (for simultaneous measurement);
– ratio of the means of the output signals (for consecutive measurement),
to obtain the calibration value of the secondary reference device. If measurements have been
taken with the positions of the primary reference cell and the secondary reference device
exchanged, calculate the calibration value for both cases. The results are only valid if both
agree within the measurement uncertainty. Both values are to be reported and their geometric
average is to be used as the calibration value.
13 Calibration of working solar reference device against a secondary solar
reference device
For calibration of a working solar reference device against secondary solar reference device

the above procedure may be applied, omitting the spectral mismatch correction if the
secondary and working standard reference are of the same material and construction,
otherwise the same procedure as described in Clause 12 should be applied.

– 16 – IEC 60904-2:2015 RLV  IEC 2015

Bibliography
JIS C8910, Primary reference solar cells

C.R. Osterwald, S. Anevsky, K. Bücher, A.K. Barua, P. Chaudhuri, J. Dubard, K. Emery, B.
Hansen, D. King, J. Metzdorf, F. Nagamine, R. Shimokawa, Y.X. Wang, T. Witchen, W.

Zaaiman, A. Zastrow, and J. Zhang, “The World Photovoltaic Scale: An International

Reference Cell Calibration
...