IEC 62788-1-4:2016

IEC 62788-1-4 ®
Edition 1.0 2016-09
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Measurement procedures for materials used in photovoltaic modules –
Part 1-4: Encapsulants – Measurement of optical transmittance and calculation
of the solar-weighted photon transmittance, yellowness index, and UV cut-off
wavelength
Procédures de mesure des matériaux utilisés dans les modules
photovoltaïques –
Partie 1-4: Encapsulants – Mesurage du facteur de transmission optique
et calcul du facteur de transmission photonique à pondération solaire,
de l'indice de jaunissement et de la fréquence de coupure des UV

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IEC 62788-1-4 ®
Edition 1.0 2016-09
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Measurement procedures for materials used in photovoltaic modules –

Part 1-4: Encapsulants – Measurement of optical transmittance and calculation

of the solar-weighted photon transmittance, yellowness index, and UV cut-off

wavelength
Procédures de mesure des matériaux utilisés dans les modules

photovoltaïques –
Partie 1-4: Encapsulants – Mesurage du facteur de transmission optique

et calcul du facteur de transmission photonique à pondération solaire,

de l'indice de jaunissement et de la fréquence de coupure des UV

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.160 ISBN 978-2-8322-3634-5

– 2 – IEC 62788-1-4:2016  IEC 2016
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references. 5
3 Terms and definitions . 6
4 Principle . 7
5 Apparatus . 7
6 Test specimens . 7
6.1 Nominal (and unweathered) transmittance to the cell . 7
6.2 Weathering studies . 8
6.3 Glass for superstrates/substrates . 9
6.4 Number of specimens . 9
6.5 Preconditioning of specimens . 9
7 Measurement procedure . 9
7.1 General . 9
7.2 Specimen preparation . 9
7.3 Instrument calibration (baseline measurements) . 9
7.4 Specimen measurements . 10
7.5 Witness measurements . 10
8 Calculation and expression of results . 10
8.1 Post-processing of data . 10
8.2 Calculation of weighted transmittance . 10
8.3 Calculation of the Yellowness Index (YI) . 11
8.4 Calculation of the UV cut-off wavelength . 11
9 Uncertainty of measurements . 11
10 Test report. 12
Annex A (informative) Advanced analysis of transmittance (absorption coefficients) . 14
Annex B (informative) Applying the quantum efficiency of a specific cell technology . 16
Bibliography . 18

Table 1 – Details of the solar weight transmittance parameters . 11

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MEASUREMENT PROCEDURES FOR MATERIALS
USED IN PHOTOVOLTAIC MODULES –

Part 1-4: Encapsulants – Measurement of optical transmittance and
calculation of the solar-weighted photon transmittance,
yellowness index, and UV cut-off wavelength

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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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 62788-1-4 has been prepared by IEC technical committee 82:
Solar photovoltaic energy systems.
The text of this standard is based on the following documents:
FDIS Report on voting
82/1148/FDIS 82/1165/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.

– 4 – IEC 62788-1-4:2016  IEC 2016
A list of all parts in the IEC 62788 series, published under the general title Measurement
procedures for materials used in photovoltaic modules, can be found on the IEC website.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
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.
MEASUREMENT PROCEDURES FOR MATERIALS
USED IN PHOTOVOLTAIC MODULES –

Part 1-4: Encapsulants – Measurement of optical transmittance and
calculation of the solar-weighted photon transmittance,
yellowness index, and UV cut-off wavelength

1 Scope
This part of IEC 62788 provides a method for measurement of the optical transmittance of
encapsulation materials used in photovoltaic (PV) modules. The standardized measurements
in this procedure quantify the expected transmittance of the encapsulation to the PV cell.
Subsequent calculation of solar-weighted transmittance allows for comparison between
different materials. The results for unweathered material may be used in an encapsulation
manufacturer’s datasheets, in manufacturer’s material or process development, in
manufacturing quality control (material acceptance), or applied in the analysis of module
performance.
This measurement method can also be used to monitor the performance of encapsulation
materials after weathering, to help assess their durability. The standardized measurements
are intended to examine an interior region within a PV module, e.g., without the effects of
oxygen diffusion around the edges of the cells. Subsequent calculation of yellowness index
allows for quantification of durability and consideration of appearance. The change in
transmittance, yellowness index, and ultraviolet (UV) cut-off wavelength may be used by
encapsulation or module manufacturers to compare the durability of different materials.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements 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 60904-3, Photovoltaic devices – Part 3: Measurement principles for terrestrial
photovoltaic (PV) solar devices with reference spectral irradiance data
ISO 291:2008, Plastics – Standard atmospheres for conditioning and testing
ISO 11664-1:2007, Colorimetry – Part 1: CIE standard colorimetric observers
ISO 11664-2:2007, Colorimetry – Part 2: CIE standard illuminants
ISO 13468-2:1999, Plastics – Determination of the total luminous transmittance of transparent
materials – Part 2: Double-beam instrument
ISO 17223:2014, Plastics – Determination of yellowness index and change in yellowness
index
ASTM E424-71:2007, Standard test methods for solar energy transmittance and reflectance
(Terrestrial) of sheet material

– 6 – IEC 62788-1-4:2016  IEC 2016
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:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
NOTE In cases where definitions already exist, refer to IEC TS 61836. Calculations related to these definitions
are given in Clause 8.
3.1
solar-weighted transmittance of photon irradiance
–2 –1 –1
proportion of the solar spectral photon irradiance (E , m ·s ·nm ) optically transmitted
pλ
through the specimen, throughout the range of the terrestrial solar spectrum (280 nm to
2 500 nm) (see Table 1)
–2 –1 –1
Note 1 to entry: The photon irradiance (E , m ·s ·nm ) accounts for the wavelength-specific energy of the
pλ
–2 –1
optical flux and should not be confused with spectral irradiance (E , W·m ·nm ).
λ
3.2
representative solar-weighted transmittance of photon irradiance
–2 –1 –1
proportion of the solar spectral photon irradiance (E , m ·s ·nm ) optically transmitted
pλ
through the specimen, throughout the range of the terrestrial solar spectrum (300 nm to
1 250 nm) (see Table 1)
Note 1 to entry: In the case of a PV device, the representative solar-weighted transmittance of photon irradiance
is defined throughout the range of the spectrum utilized by a representative PV device (which may not include
wavelengths as low as 280 nm or as great as 2 500 nm).
3.3
UV cut-off wavelength
λ
cUV
wavelength of light below which the encapsulation is considered optically absorbing and
above which the encapsulation is considered transmitting
Note 1 to entry: In this procedure, the absolute transmittance of 10 % (corresponding to the optical absorbance of
1) is considered as the threshold of the UV cut-off wavelength. As described further in [9] , the UV cut-off
wavelength may also be used to quantify the effects of weathering.
3.4
weathering
process of subjecting specimens to environmental conditions that could include ultra-violet
radiation, temperature, humidity, and ozone
Note 1 to entry: Weathering may occur in artificial or natural environments. Weathering could occur at the nominal
(field) or an accelerated rate.
3.5
yellowness index
YI
calculated value identifying the yellowness of the test specimen perceived by a human
observer (see ASTM E313-10)
Note 1 to entry: YI may be used to quantify the effects of weathering.
_____________
Numbers in square brackets refer to the Bibliography.

4 Principle
The total spectral transmittance of laminated specimens, containing encapsulation material,
shall be measured using a spectrophotometer equipped with an integrating sphere. Solar-
weighted transmittance, yellowness index, and UV cut-off wavelength will be subsequently
calculated from the transmittance measurements.
The transmittance measured using this procedure may be used in a more advanced optical
analysis to improve the accuracy of PV performance analysis or distinguish between different
encapsulation materials, as described in Annex A. The transmittance measured using this
procedure may be used to estimate module performance (current yield) if the quantum
efficiency of the PV cell is known, as described in Annex B. The method does not attempt to
account for variations in transmittance with the angle of incidence, which may vary with time
of day, sky conditions, and geometry of the module, especially if concentration is used.
5 Apparatus
The test instrument shall consist of a double beam spectrophotometer equipped with an
integrating sphere. A single beam spectrophotometer may be used if the port reflectance can
be properly accounted for, as in Annex A and [2]. Details regarding the construction and
configurations of the test instrument may be found in ISO 13468-2 or ASTM E424-71. A
measurement range of at least 280 nm to 2 500 nm is required for calculation of the solar-
weighted transmittance using the AM1.5 global spectrum as in IEC 60904-3. A wavelength
increment no larger than 1 nm is preferred for the measurement, however increments up to
5 nm are permitted with linear interpolation to 1 nm.
An integrating sphere of at least 100 mm in diameter with a port area of < 5 %, as in [11], is
recommended to reduce the error in the measurement. The port area as in [9] should not
exceed 13 %.
6 Test specimens
6.1 Nominal (and unweathered) transmittance to the cell
Specimens shall be constructed using a laminate structure of glass/encapsulation, as
described in Annex A and [12].
The solar-weighted transmittance and representative solar-weighted transmittance, as
calculated in Clause 8, may be used for the purpose of reporting on an encapsulation
manufacturer’s datasheet.
The specimens shall contain an examination region free from visible flaws including:
scratches, pits, sink marks, bubbles, or other imperfections. The examination region shall be
at least 50 % larger in diameter than the measurement area of the test instrument.
NOTE 1 A spot size of 1 cm × 1 cm is common in many commercial spectrophotometer instruments. Use of
specimens at least 2 to 3 times this size will improve uniformity (resulting from fabrication) and handling (during
measurement).
The size (length and width) should be adequate to allow the specimen to fit inside the test
instrument.
The nominal thickness of the encapsulation specimens shall be equal to the thickness
intended for use in PV modules.
Specimens should be cured (if applicable) according to the manufacturer’s specification and
using a process as similar as possible to the method used in the intended manufacturing
process.
– 8 – IEC 62788-1-4:2016  IEC 2016
The thickness of the encapsulation portion of the test specimen shall be measured after its
preparation. The thickness shall be taken as the average of three measurements obtained at
different locations on the test region of the specimen.
Glass plates shall be parallel with minimal edge pinch or edge flare. I.e. the encapsulation
thickness at any of the corners shall not be more than 10 % different than that in the centre of
the sample. In a typical bag laminator this will require the use of a frame around the samples
during lamination, but any other means of accomplishing this specification is acceptable.
The back surface of the specimens (the exposed encapsulation intended to face to the
integrating sphere) shall not be intentionally textured.
For additional resolution to more accurately distinguish between materials, subsequent
studies may utilize a thicker encapsulation layer that may be analysed to determine the
optical attenuation coefficient as described in Annex A.
If the encapsulation material is intended to be used with superstrates other than glass, the
same procedure may be used in a subsequent study. Specimens using polymeric superstrates
may be prone to optical polarization occurring within the instrument. As in [9] and [12], a
depolarizer should be used with the instrument to minimize the effects of polarization.
NOTE 2 The effect of haze in specimens prone to optical haze can be mitigated through the use of a diffusing
film, as described in [13].
6.2 Weathering studies
A glass/encapsulation/glass laminate specimen geometry is recommended.
The size (length and width) should be adequate to allow the specimen to fit inside the test
instrument.
As described in [14], the minimum size of 5 cm × 5 cm is recommended for weathering
specimens based on previous examinations of poly (ethylene-co-vinyl acetate).
Large specimens are preferred in weathering studies, because a test region may be
distinguished, where the diffusion of oxygen or moisture is limited.
Other geometries may be used with this procedure to evaluate the effects of weathering. For
example, a permeable polymeric backsheet facilitates the examination of moisture ingress.
Some PV modules make use of an edge seal to reduce moisture permeation.
Separate “blank” pieces of superstrate or substrate may be weathered with the test
specimens to quantify the degradation of those components.
The specimens shall contain an examination region free from visible flaws including:
scratches, pits, sink marks, bubbles or other imperfections. The examination region shall be
at least 50 % larger in diameter than the measurement area of the test instrument.
The nominal thickness of the encapsulation specimens shall be as intended for use in the PV
module. Specimens should be cured (if applicable) according to the manufacturer’s
specification and as similar as possible to the method used in the intended manufacturing
procedure.
The thickness of the encapsulation in the laminate may be controlled by inserting a removable
material around the specimen perimeter.

6.3 Glass for superstrates/substrates
Measurements of the nominal (unweathered) transmittance to the cell for the purpose of
encapsulation manufacturer’s datasheets shall be performed using (3 ± 0,2) mm thick silica
glass. The glass shall have smooth, defect-free surfaces that are sufficiently flat and parallel
such that the diffuse component of transmitted light is less than 1 % between 280 nm and
2 500 nm.
The solar-weighted transmittance of photon irradiance of silica glass, which may be used to
verify that the composition of the glass is appropriate, is approximately 93 %, because the
reduction in transmittance comes from reflections at the surfaces. As in [1] and [12], the
transmittance of the glass should be greater than 90 % at 280 nm.
The glass shall not be coated or contain antireflective layers. The glass shall not be
intentionally textured.
Subsequent examination beyond that intended for the encapsulation material datasheet,
including weathering, may be performed according to this procedure using other superstrate
and/or substrate materials that can incorporate other optical features, e.g., antireflective
coatings, surface texture, and untempered PV glass.
6.4 Number of specimens
A minimum of 3 replicates shall be used for the determination of the transmittance to the cell
or in weathering studies. Optical characteristics, including transmittance, YI, and the UV cut-
off wavelength shall be subsequently calculated using the average of the three separate
specimens, with the range of the measurements indicated to identify their variability.
6.5 Preconditioning of specimens
Specimens used for the purpose of datasheet reporting shall be maintained at (23 ± 2) °C,
(50 ± 5) % RH for at least 24 h, as recommended in ISO 291, prior to optical measurement.
The use of elevated temperature and humidity conditions in a weathering study may result in
a supersaturated moisture condition within specimens, affecting their optical transmittance
when they are returned to the laboratory ambient condition. In such cases, specimen
conditioning, including a controlled environmental chamber, may be applied to prevent
spurious effects, e.g., optical haze or moisture related absorptance. Specimen-condition
effects may be verified using periodic measurements. Refer to the weathering test procedure
for any specific details related to specimen conditioning and storage intermittent to
weathering.
7 Measurement procedure
7.1 General
Transmittance measurements shall be performed in accordance with the procedure in
ISO 13468-2.
7.2 Specimen preparation
Prior to measurement, specimens should be free of dust, grease or other contaminants. The
specimens and instrument should be in thermal equilibrium prior to measurement.
7.3 Instrument calibration (baseline measurements)
Allow the instrument lamp to adequately equilibrate after it has been lighted, observing the
typical warm-up period, e.g., 15 min or as recommended by the instrument manufacturer.
Perform the correction scan(s) to compensate for the instrument baseline signal.

– 10 – IEC 62788-1-4:2016  IEC 2016
The 100 % transmittance baseline measurement should be performed in air, with no
superstrate, specimen, or substrate material present. The 0 % transmittance baseline
measurement should also be performed, if possible.
Periodic measurement of the baseline is recommended to minimize instrument drift and
ensure the measured values are accurate. The instrument drift occurring over an extended
measurement session may be instrument specific.
It is recommended to maintain the instrument drift below 0,05 % to minimize the uncertainty of
measurement (Clause 9). The instrument drift should be considered in the instrument bias
when the uncertainty of measurements is determined.
7.4 Specimen measurements
Perform the transmittance measurements for the test specimens over the wavelength range of
at least 280 nm to 2 500 nm using a 1 nm increment.
Linear interpolation to a 1 nm increment may be used when only a coarser measurement
increment (maximum of 5 nm) is available. The error associated with a coarser increment may
be more influential at shorter wavelengths (where YI and the UV cut-off wavelength are
determined) than at longer wavelengths (where only the solar-weighted transmittance is
affected). When applied, the use of linear interpolation should be noted in the test report.
Discontinuities associated with changes in the optical components (including detector, light
source, and/or monochromator) may occur during the measurement. Such discontinuities can
be minimized via the instrument settings.
It may be useful to extend the range of measurement in durability studies to provide insight
into the results. For example, by measuring UV wavelengths as low as 200 nm, the integrity of
UV absorbers and stabilizers can be confirmed from the UV cut-off wavelength.
The spectral bandwidth of the measurement should be less than or equal to the increment of
the measurement, i.e., 1 nm or 5 nm.
7.5 Witness measurements
Perform the transmittance measurements on a witness specimen at the beginning of each
measurement session to ensure proper operation of the instrument and minimize the
measurement error. Perform the transmittance measurements of any witness specimens using
the same procedure applied to the test specimen(s). The witness specimens may include a
traceable standard specimen, laboratory working witness specimen, or the silica
superstrate/substrate material.
8 Calculation and expression of results
8.1 Post-processing of data
The measurements obtained from three separate specimens shall be averaged at each
wavelength increment. The range (difference of the maximum and minimum) shall also be
determined at each wavelength increment.
8.2 Calculation of weighted transmittance
The solar-weighted transmittance may be calculated from formula (1):

[ ] [ ]
τλ E λ dλ
pλ
∫
τ =
sw
(1)
[ ]
E λ dλ
pλ
∫
Where
τ refers to the solar-weighted transmittance (%);
sw
τ  is the measured transmittance of the specimen (%);
λ  is the wavelength of light (nm); and
E is the reference global spectral photon irradiance (as given in IEC 60904-3).
pλ
The details for the calculation of the various solar-weighted transmittance parameters are
identified in Table 1. The parameters include: the “solar-weighted” transmittance, which may
be applied generally, based on a broad solar spectrum; and the “representative solar-
weighted” transmittance, which may be applied for most commercially available single junction
terrestrial PV cells (including silicon, cadmium telluride, copper indium gallium selenide, and
gallium arsenide) based on their useable wavelength range. The solar-weighted transmittance
(as well as YI) obtained from the spectrophotometer measurements (1 nm interval) shall be
calculated using a discretized sum, by means of the modified trapezoidal integration
technique, as in IEC 60904-3. The denominator for formula (1) is specified in Table 1 for the
purpose of verification, based on IEC 60904-3.
Table 1 – Details of the solar weight transmittance parameters
Parameter Symbol Lower bound Upper bound
[ ]
E λ dλ
pλ
wavelength wavelength
∫
nm nm –2 –1
m ·s
“solar-weighted” transmittance τ 280 2 500
4,15502132187479 × 10
sw
“representative solar-weighted” τ 300 1 250
3,03000915425 × 10
rsw
transmittance
NOTE The solar-weighted spectral transmittance may be more specifically applied to estimate module
performance, i.e., current yield, if the quantum efficiency of the PV cell is known, as described in Annex B.

8.3 Calculation of the Yellowness Index (YI)
YI shall be calculated according to the procedure in ISO 17223. The three tristimulus
coefficients shall be determined using the CIE Standard D65 Illuminant spectrum (as in
ISO 11664-2), and the CIE 1964 XYZ colour space (for a human observer with a 10° field of
view, as in ISO 11664-1). YI shall be calculated for a wavelength increment, e.g., 1 nm,
consistent with the measured transmittance data.
NOTE Additional details related to the YI may be found in ASTM E313-10 and ASTM E308-08.
8.4 Calculation of the UV cut-off wavelength
The UV cut-off wavelength, λ , shall be determined as the longest wavelength (to the
cUV
nearest nm) in the UV range (where λ≤400 nm) where the transmittance equals 10 % or less.
In cases where the UV cut-off wavelength is known to be less than 280 nm, the range of
measurement should be extended below 280 nm to quantify its specific value.
9 Uncertainty of measurements
The uncertainty associated with the randomness of measurement may be estimated from:

– 12 – IEC 62788-1-4:2016  IEC 2016
n
∆τ[λ]E [λ]dλ
pλ
∫
σ = = S (σ [λ]) (2)
ran n τ
∑
E [λ]dλ
pλ i=1
∫
where
σ represents the random variation of measurement;
ran
∆τ is the uncertainty of the transmittance measurement;
λ is the wavelength of light;
E is the reference spectral photon irradiance;
pλ
S is the coefficient for Student’s t-distribution (3,18245 for 3 specimens at the 95 %
n
confidence interval), and
σ is the standard deviation of the transmittance measurements (at each wavelength
τ
increment).
The final (reported) uncertainty may be estimated from:
2 2
σ = (σ ) +(σ ) (3)
tot inst ran
where
σ represents the total uncertainty;
tot
σ is the instrument bias (accounting for the particular make of the instrument, the
inst
instrument settings (including integration time), as well as instrument drift); and
σ is the random variation of measurement (the accuracy of the instrument as well as the
ran
specimen variability).
σ may be evaluated for the test instrument from the average of 10 measurements
ran
(multiplied by the corresponding S value of 2,22814) with no specimen present (nominal
n
transmittance of 100 %). Similarly, σ may be evaluated for the test instrument from a series
inst
of successive periodic measurements with no specimen present, with the rate of drift being
determined from a trendline fit. While σ may vary with the lamp age, a single representative
tot
evaluation is expected to adequately represent the test instrument used within a particular
test laboratory.
Calculate the uncertainty for other test results (such as YI) using the same method as for
transmittance, i.e., the method of propagation of error. These estimates should also account
for the uncertainty associated with instrument bias in addition to random error.
The repeatability and reproducibility of the method, including solar-weighted transmittance,
yellowness index, and the UV cut-off wavelength, are described in Annex A and [12].
10 Test report
A report of the tests shall be prepared by the test agency. The report shall contain the detail
specification for the specimens. Each certificate or test report shall include at least the
following information:
a) a title;
b) name and address of the test laboratory and location where the tests were carried out;
c) unique identification of the certification or report and of each page;
d) name and address of client, where appropriate;

e) description and identification of the item tested, including specimen type, specimen
thickness (and its range of variation), the specimen size (length and width), the
superstrate/substrate material(s) and their characteristics;
f) characterization and condition of the test item, including the method and details of
specimen preparation (including curing, lamination, or similar processing if applicable) and
preconditioning;
g) date of receipt of test item and date(s) of test, where appropriate;
h) identification of test method used; including the make and model of the spectrophotometer
and the integrating sphere;
i) reference to sampling procedure, where relevant;
j) any deviations from, additions to, or exclusions from, the test method and any other
information relevant to a specific test, such as environmental conditions; and the
procedure(s) and condition(s) used for weathering and any preconditioning conducted
prior to measurements;
k) measurements, examinations and derived results supported by tables, graphs, sketches
and photographs as appropriate including the complete set of the tabulated average
transmittance values and the corresponding range of the averaged values; the estimated
uncertainty of the transmittance measurement (instrument); the averaged solar-weighted
transmittance of photon irradiance and the corresponding range of the averaged values;
the averaged representative solar-weighted transmittance of photon irradiance and the
corresponding range of the averaged values (as well as the wavelength range
considered); the UV cut-off wavelength and its uncertainty; and any failures observed;
l) the yellowness index and its uncertainty (which should be determined after each
weathering interval in addition to its original value in the case of weathering experiments);
m) a statement of the estimated uncertainty of the test results (where relevant); the
measurement of the witness specimen (if utilized) and its deviation from its witness
values. When applicable, the details of the witness specimen (such as its preparation,
composition, and thickness) shall be specified;
n) a signature and title, or equivalent identification of the person(s) accepting responsibility
for the content of the certificate or report, and the date of issue;
o) where relevant, a statement to the effect that the results relate only to the items tested;
p) a statement that the certificate or report shall not be reproduced except in full, without the
written approval of the laboratory.

– 14 – IEC 62788-1-4:2016  IEC 2016
Annex A
(informative)
Advanced analysis of transmittance (absorption coefficients)
The measurements described in this procedure may be supplemented to allow for a more
rigorous optical analysis. The determination of the optical attenuation coefficient allows the
optical transmittance to be generalized and extended to any possible configuration. Such
analysis may be used to improve the accuracy of PV performance analysis, account for
changes in the module design (such as the thickness of the encapsulation), or to distinguish
between different encapsulation materials. Characteristics including transmittance, YI, and the
UV cut-off wavelength strongly vary with the specimen thickness; the methods described in
this annex may therefore be used to standardize measurements to a representative thickness.
Optical transmittance also depends upon the material layers present within the specimen. A
more rigorous analysis, summarized in this annex, may be used to consider loss of
transmitted light, including the reflectance at material interfaces.
Hemispherical reflectance and transmittance data may be used to calculate the attenuation
coefficient for a minimally absorbing material. The terminology “low absorbing” implies that
light reflected from the rear interface is not fully absorbed prior to reaching the front interface.
For this method, the specimen is ideally flat, free standing, of uniform known thickness, and
minimally scattering, i.e., minimal optical haze. The method is based on work presented in [1].
As described in [2] and [3], the absorptance is first determined using the formula:
α [λ]= 1−τ [λ]−ρ [λ]
(A.1)
c m m
Nomenclature in the formula includes: α , the calculated optical absorptance (normalized
c
intensity); τ , the measured optical transmittance (normalized intensity); ρ , the measured
m m
optical reflectance (normalized intensity); and λ, the wavelength (m). The measured
reflectance should be corrected to an absolute scale (using a certified calibrated reflectance
specimen) for spectrophotometers that report relative reflectance measurements. The
absolute reflectance of the specimen may be determined if its measured relative reflectance is
multiplied by the reflectance of a calibrated specimen. The measured transmittance and
measured reflectance include the effects of multiple passes of light, occurring as the result of
internal reflections, as described in [4].
A subsequent analysis of the measured reflectance and transmittance data may be used to
account for the internal reflections, in order to represent a single optical pass through the
material. The reflections occurring at the air/sample interface as well as the absorptance for a
single pass (occurring normal to the specimen), can be determined from the measured values
using the formulas:
 2 2
2 2
 
 
− 1+ τ λ + 2ρ λ − ρ λ + 1+(τ [λ]) + 2ρ [λ]−(ρ [λ]) + 4(ρ [λ]− 2)ρ [λ]
( []) [] ( [])  
m m m m m
m m m
 
 
 
(A.2)
ρ[λ]
=
i
2(ρ [λ]− 2)
m
ρ[λ]α [λ]−α [λ]
i c c
α [λ]= (A.3)
sp
ρ[λ]α [λ]+ρ[λ]−1
i c i
(A.4)
ψ[λ]=− ln(1−α [λ])
sp
z
New nomenclature in the formulas includes: ρ , the reflectance at the
...