IEC TS 60904-13:2018
(Main)Photovoltaic devices - Part 13: Electroluminescence of photovoltaic modules
Photovoltaic devices - Part 13: Electroluminescence of photovoltaic modules
IEC TS 60904-13:2018(E) specifies methods to:
capture electroluminescence images of photovoltaic modules,
process images to obtain metrics about the images taken in quantitative terms, and
provide guidance to qualitatively interpret the images for features in the image that are observed. This document is applicable to PV modules measured with a power supply that places the cells in the modules in forward bias.
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Standards Content (Sample)
IEC TS 60904-13 ®
Edition 1.0 2018-08
TECHNICAL
SPECIFICATION
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inside
Photovoltaic devices –
Part 13: Electroluminescence of photovoltaic modules
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IEC TS 60904-13 ®
Edition 1.0 2018-08
TECHNICAL
SPECIFICATION
colour
inside
Photovoltaic devices –
Part 13: Electroluminescence of photovoltaic modules
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-5991-7
– 2 – IEC TS 60904-13:2018 © IEC 2018
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Imaging . 7
4.1 Apparatus . 7
4.1.1 Electroluminescence imaging camera . 7
4.1.2 Dark room imaging studio or environment . 9
4.1.3 Power supply . 10
4.1.4 Computer interface with camera and power supply for image capture . 11
4.1.5 Image processing and displaying software . 11
4.1.6 Safety and handling . 12
4.2 Procedure . 12
4.2.1 Camera settings and positioning . 12
4.2.2 Camera settings . 13
4.2.3 Sharpness determination and classification . 14
4.2.4 Imaging . 16
4.2.5 Image correction . 17
4.3 Image signal-to-noise ratio . 18
4.3.1 General . 18
4.3.2 Imaging procedure . 18
4.3.3 Analysis . 18
4.3.4 SNR criteria . 19
5 Evaluation of EL images . 20
5.1 Principles of electroluminescence . 20
5.2 Image interpretation . 21
5.2.1 Series resistance . 21
5.2.2 Minority carrier lifetime and diffusion length . 21
5.2.3 Shunt resistance . 21
5.2.4 Assignment of root cause . 21
5.3 Histogram-based analysis of the electroluminescence signal . 21
5.3.1 General . 21
5.3.2 Image information . 22
5.3.3 Bias current effects . 22
5.3.4 Analysis of intensity distributions . 22
5.3.5 Variance . 22
5.3.6 Kurtosis . 22
5.3.7 Skewness . 22
5.3.8 Pixel (or area)-weighted electroluminescence relative to an ideal module . 22
6 Reporting. 22
Annex A (normative) Procedures for image correction. 24
A.1 Dark current and stray light removal. 24
A.2 Vignetting . 24
A.2.1 Vignetting correction . 24
A.2.2 Vignetting as a function of angle from the optical axis . 24
A.2.3 Correction for vignetting . 24
Annex B (informative) Focus . 26
B.1 General . 26
B.2 Application of the Tenengrad function and Sobel operator . 26
Annex C (normative) Quantifying solar cell cracks in photovoltaic modules . 27
C.1 General . 27
C.2 Cell crack modes . 27
C.3 Basis of cell damage quantification . 28
C.4 Procedure . 30
Annex D (informative) . 32
D.1 Qualitative interpretation of electroluminescence images crystalline Si PV
modules . 32
D.2 Qualitative interpretation of electroluminescence images in thin-film PV modules . 36
Bibliography . 39
Figure 1 – Various semiconductor detector materials and their absolute spectral
response [1] . 8
Figure 2 – Electroluminescence emission spectra for (a) Si, (b)
ZnO/CdS/Cu(In,Ga)Se (CIGS) [2], and (c) CdS/CdTe [3] . 8
Figure 3 – Example of frame subtraction given in Figure 3a) to Figure 3c), with images
taken in ideal dark room conditions given in Figure 3d) . 12
Figure 4 – EL image with introduced two edges using aluminum tape . 15
Figure 5 – Edge gradient image 𝑮𝑮𝑮𝑮𝑮𝑮𝑮𝑮𝑮𝑮 from the Figure 4 EL image’s first derivative in
orthogonal direction 𝑮𝑮𝑮𝑮,𝒚𝒚 . 15
Figure 6 – Excerpt of the EL image of Figure 4 and plot of image intensity values
along line L . 15
Figure 7 – Images of regions of multicrystalline silicon solar cells with three SNR
values as labeled . 20
Figure 8 – Emission of light (hν) associated with the electroluminescence process in
solar cells of PV modules . 20
Figure 9 – Scheme for labeling position of cells in a module viewed from the light-
facing side according to coordinates (i,j) in portrait orientation (a) or rotated into
landscape orientation (b), which shall be indicated if applicable . 23
Figure B.1 – EL image of a solar cell (left) and a silicon module (right) . 26
Figure C.1 – Single cell region of a module with 0,1 × I applied showing crack types,
sc
as labeled . 27
Figure C.2 – Example of normalized EL intensity histograms calculated from the EL
images of modules with various levels of cell cracking and resulting power degradation,
indicated by P . 29
max
Figure C.3 – Example of quantifying solar cell cracks in photovoltaic modules: (a) EL
image produced with 0,1· I forward bias current, and (b) image of regions
sc
considered damaged . 31
Table 1 – Detectors and their applicable wavelengths . 7
Table 2 – Sharpness classes, examples of images meeting the criteria of the classes,
and examples of distinguishable features . 16
Table D.1 – Descriptions of observables, features, and known causes, along with
electroluminescence images for crystalline Si modules . 32
Table D.2 – Descriptions of observables, features, and known causes, along with
electroluminescence images for thin-film modules . 36
– 4 – IEC TS 60904-13:2018 © IEC 2018
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PHOTOVOLTAIC DEVICES –
Part 13: Electroluminescence of photovoltaic modules
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
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The main task of IEC technical committees is to prepare International Standards. In
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• the required support cannot be obtained for the publication of an International Standard,
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• the subject is still under technical development or where, for any other reason, there is the
future but no immediate possibility of an agreement on an International Standard.
Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC TS 60904-13, which is a technical specification, has been prepared by IEC technical
committee 82: Solar photovoltaic energy systems.
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
82/1292/DTS 82/1424/RVDTS
Full information on the voting for the approval of this technical specification can be found in
the report on voting indicated in the above table.
This document 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 document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.
– 6 – IEC TS 60904-13:2018 © IEC 2018
PHOTOVOLTAIC DEVICES –
Part 13: Electroluminescence of photovoltaic modules
1 Scope
This part of IEC 60904 specifies methods to:
a) capture electroluminescence images of photovoltaic modules,
b) process images to obtain metrics about the images taken in quantitative terms, and
c) provide guidance to qualitatively interpret the images for features in the image that are
observed.
This document is applicable to PV modules measured with a power supply that places the
cells in the modules in forward bias.
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 TS 61836:2016, Solar photovoltaic energy systems – Terms, definitions and symbols
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TS 61836 as well as
the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electaropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
electroluminescence
EL
emission of optical radiation resulting from the application of electrical energy
3.2
open circuit
electrical circuit that has a break, or “open”, somewhere in the conductive path
Note 1 to entry: A module or laminate exhibits an “open circuit” if defective or damaged so that no current can
flow through it when attached to an external circuit at the module electrical connection points.
Note 2 to entry: A PV module itself is in open circuit condition if one or both of the module electrical connection
points are not connected to anything or current is not flowing as defined in IEC TS 61836:2016, 3.4.57.
3.3
forward bias
forcing current flow with a power supply where the leads are connected to those of the same
polarity (+ and -) on the sample
3.4
barrel distortion
distortion in the image whereby rectangular features in an image appear expanded, as in a
curved barrel wall
3.5
vignetting
reduction of an image's brightness at the periphery compared to the image center
4 Imaging
4.1 Apparatus
4.1.1 Electroluminescence imaging camera
4.1.1.1 Camera detector
Detectors are typically light-sensing pixels consisting of charge coupled devices (CCD) or
complementary metal oxide semiconductor (CMOS) devices arranged in a focal-plane array.
They may be cooled, usually with thermoelectric cooling, to achieve better signal-to-noise
ratio by means of reducing device dark current originating from thermally generated charges.
Semiconductor light absorber materials in the detector shall be sensitive to the EL emission of
the device under test. Example semiconductor light absorber materials and their useful
wavelengths of detection for PV module characterization are given in Table 1. The spectral
response for some semiconductor detectors is given in Figure 1. The typical emission
spectrum for Si, ZnO/CdS/Cu(In,Ga)Se (CIGS) and CdS/CdTe heterostructure solar cells are
given in Figure 2. The signal strength obtained during EL measurements will be proportional
to the product of the spectral response and the emission. For a given EL signal from a sample,
a greater spectral response at the wavelength of interest will typically permit a shorter
exposure time. Spectral response of Si detectors to Si cell PV module EL emission is
relatively low. Commercial Si detectors frequently offer the best resolution, but spectral
response of silicon detectors is typically compromised as a result.
Table 1 – Detectors and their applicable wavelengths
Detector Sensitive wavelengths
µm
Ge 0,8 to 1,7
InGaAs 0,7 to 2,6
Si 0,3 to 1,1
InAs 1,0 to 3,8
– 8 – IEC TS 60904-13:2018 © IEC 2018
1,2
InGaAs
0,8
0,6
Ge
Si
0,4
UV-Si
0,2
200 600 1 000 1 400 1 800
Wavelength (nm)
IEC
Figure 1 – Various semiconductor detector materials
and their absolute spectral response [1]
Relevant parameters in choosing detectors include number of pixels, noise, quantum
efficiency at the wavelength of interest, and dynamic range. This document contains
provisions for various image resolutions. Choice of camera to obtain images that meet the
sharpness classes given subsequently in 4.2.3 is made with respect to its imaging sensor and
lens, with consideration to working distance (WD) and field of view (FOV). Theoretically,
camera sensor resolution translates one-to-one with the image resolution. The resolution of
an image in one of the dimensions (length or width) of an orthogonal array of pixels is the
number of pixels in the image in that dimension.
IEC
IEC
IEC
(a) (b) (c)
Figure 2 – Electroluminescence emission spectra for
(a) Si, (b) ZnO/CdS/Cu(In,Ga)Se (CIGS) [2], and (c) CdS/CdTe [3]
Sensor resolution = Image resolution = FOV/smallest image feature
For example, if it is desired to image features of 2,5 mm onto a pixel and the length of the
module to be imaged (corresponding to the FOV) is 1 600 mm, the sensor resolution (in pixels)
required in this dimension is 640. This example implies the camera being in focus and
neglects image sharpness considerations (see 4.2.3).
___________
Numbers in square brackets refer to the Bibliography.
Response (A/W)
The camera response function (CRF) relates the actual quantity of light impinging on each
element of the sensor array to the pixel values that the camera outputs. When the same
object is captured at different exposure times but with an otherwise identical camera setup, a
non-linear CRF causes the resulting image intensity distribution to exhibit nonlinearity, even
after application of a correction for exposure time. Therefore, when analyzing image
intensities, either the linearity of the CRF needs to be assured (basic methods are commonly
found in the camera literature), or a correction for non-linearity needs to be performed using
image processing. Scientific grade Si or InGaAs-based sensors are often linear or have a
correction for non-linearity embedded. Neglecting non-linearity will cause erroneous results
when applying procedures for image correction that are given in Annex A or any quantitative
analysis.
To obtain maximum image resolution and electroluminescence signal, the optical axis of the
camera is placed perpendicularly and as close as possible to the module face to image the
solar cell or module area. Images captured at the highest resolution may require a longer
exposure time and time to transfer from the camera and process. Binning features may exist
to combine pixels for lower resolution and shorter image processing times. Gain feature may
exist to amplify the signal of the EL image.
4.1.1.2 Lens
Lenses shall be free of absorption filters or coatings that remove the infrared near the band-
gap of the semiconductor material to be examined. Optical glass is generally suitable,
however Ge lenses will be necessary for measuring EL from the very low band gap materials
(under 0,6 eV). Lenses vary from telephoto to wide-angle in focal length. Choice will depend
on the specific application and geometric considerations when capturing the image. Wide-
angle lenses that have short focal lengths used in conjunction with the higher resolution
cameras capture a larger FOV. The camera may be placed much closer to the subject, which
is useful when space is constrained. Some wide-angle lens optics however cause undesirable
barrel distortion in the images that will require correction by post-processing. Lenses with
longer focal lengths generally have less barrel distortion and can therefore more accurately
image a module, whereby the resulting images may require little or no correction by post
processing. Lenses may feature components that correct for the difference between visible
and infrared wavelengths, which can aid in focusing.
Lenses typically have an aperture with the size referred to by a f-number. Ignoring differences
in light transmission efficiency, a lens set to a greater f-number has less light gathering area
and projects less electroluminescence signal to the image sensor. Depth of field increases
with increasing f-number. Image sharpness is related to f-number through two different optical
effects; aberration, due to imperfect lens design, and diffraction, which is due to the wave
nature of light. Many wide-angle lenses will result in significant vignetting at the edges of the
image when using a smaller f-number.
4.1.1.3 Filters
Filters on the camera lens may be used to help cut light of extraneous wavelengths from
being detected. 850 nm to 950 nm long-pass filters may be used when imaging near band-
edge EL from modules with silicon cells.
4.1.2 Dark room imaging studio or environment
A darkened environment is favored for high quality images. Precautions should be taken to
eliminate stray light entering the imaging studio, such as with use of hard walls, curtains,
baffles, and sealing of any gaps with material that are of light absorbing nature (black). If a
filter is used on the camera, then LED lighting may be used that emits light only in the
spectrum that is cut by the filter. For non-laboratory measurements, minimize extraneous light
when possible. For example, perform measurements at night. If stray light is present, an
image subtraction procedure will be required, as discussed in 4.1.5.2.
– 10 – IEC TS 60904-13:2018 © IEC 2018
Fixed mounting of the camera and a mount for the module(s) to be imaged are required so
that the camera and the module positions are absolutely stable.
Laboratory measurements, for consistency in achieving qualitative or quantitative
comparisons, should be performed with the module maintained between 20 °C to 30 °C.
Temperature should be obtained with a temperature sensor accurate to within 1 °C placed on
the module rear (the side not being imaged) and installed in a manner that does not interfere
with the imaging. It may however be necessary to obtain EL images with the module at
temperatures outside of the prescribed range to evaluate the effect of temperature or when it
is not possible to maintain the module temperature within the prescribed range. For such
measurements, the module temperature shall be noted and indicated as being performed
“outside of the standard testing condition” (See Clause 6). The final temperature, measured
within 15 s of the end of the image capture, shall be recorded for module temperature
reporting requirements.
For highly accurate work, the module temperature may be stabilized by passing current until
the temperature reaches equilibrium. The comparison of two images taken in sequence may
be performed to see if both the module temperature and the camera detector (also affected by
temperature) is stable by employing image histograms defined in Clause 5.3.8 and Formula
C.1. The module may be considered stabilized if the absolute difference of the image intensity
histogram of sequentially captured images is below 0,02 in each bin, where for this analysis,
each bin width is 5 % of the EL intensity range captured from the module area (not including
the background, area with no cells, or defective pixels).
For repeated measurements on a single module type under condition where the room
temperature is maintained within a range of 5 °C, the module stabilization time may be
determined by passing current until the temperature reaches equilibrium. EL imaging shall
commence after thermal stabilization, and the module temperature shall be recorded. The
waiting time required for the module to thermally stabilize shall be validated on at least one
module of the same type, after which stabilization may be based on waiting time (and not
direct temperature measurement) for future measurements of the module type. Images with
the module temperature outside of the range of 20 °C and 30 °C shall be noted as being
performed “outside of the standard testing condition.”
NOTE 1 EL images obtained at different temperatures, including within the range of 20 °C and 30 °C, lead to
different visibility of defects, such as those due to shunting and partially disconnected regions of broken cells
because of thermal coefficient of expansion mismatches.
NOTE 2 Due to factors including module positioning and poor connections in the cell (e.g. cracks), grid fingers
and interconnects, some EL signal may change, even when measurements are repeated at the same temperature.
4.1.3 Power supply
of the module or a series string of cells
An electric DC power supply capable of applying I
SC
or modules to be imaged is required. The power supply shall be able to provide sufficient
voltage to achieve I . Depending on the module technology, the required voltage may be
SC
approximately equal to the open circuit voltage V of the module, but it may be significantly
OC
higher for some PV modules, such as those based on thin-film technology.
Measuring voltage during application of DC current through the module for EL imaging gives
additional information about the condition of the module including the existence of shunt
resistance reduction (lower voltage is measured), series resistance (higher voltage is
measured), and correct connection to the module, but its measurement is optional. For
accurate voltage measurement reporting, cabling from the module leads shall be of sufficient
gauge to maintain less than 2 % voltage drop over the leads, or alternatively, a four-wire
configuration shall be used to separately supply current and measure voltage at the
connectors of the module(s) under test.
4.1.4 Computer interface with camera and power supply for image capture
Computer control of the power supply and camera so that pre-programmed currents can be
quickly applied and coordinated with image capture are optional equipment that will provide
speed and improve accuracy of the imaging of module EL. Further if the image and its capture
settings are programmatically transferred into a file on the computer, it can aid in
automatically recording image parameters for reporting requirements described in Clause 6.
4.1.5 Image processing and displaying software
4.1.5.1 Assignment of image colours
The image is transferred electronically from the camera to a computer for saving and
maintaining in raw image format for subsequent display and image post-processing. Computer
software should load EL image files, assign colours or a grey scale to each signal level
measured within the PV module and any regions of interest. In the case of a colour image, a
legend to indicate the meaning of the colours or levels shall be provided. Lowest EL signal
shall be represented by black and the highest EL signal in the image should be represented
by white; however, the image data of the active cell area shall not exist in the upper extreme
to avoid detector saturation except where unavoidable (see 4.2.2.3). The colours in the scale
between these extremes are not defined herein, but there should be no possibility of
misinterpretation by re-use of similar colours to represent multiple signal levels or by the
highlighting of areas where there are in fact no features; i.e., the number of colors should be
minimized.
4.1.5.2 Software capabilities
Software should produce histograms in counts versus EL signal level bin to quantitatively
interpret the images for features that are observed.
Basic software features that will be helpful, depending on the nature of the original image, for
post-processing of images in the application of this document include:
• Level range adjustment
• Cropping the image to the region of interest
• Determination of EL signal level at any given point on the image
• Frame subtraction: Uniform subtraction of noise signal including from dark current or stray
light, such as by subtracting the signal when the module is unpowered. An example of
results from this procedure is given and explained in Figure 3. This may be performed with
image processing software or in signal processing software, including with pulses of
forward bias current applied cyclically.
• Dead pixel removal
• Single time effects removal
• Dark current variations (variation in CCD sensitivity and offset)
• Barrel distortion
• Vignetting
Fundamentals of image processing may be found in published literature. [4] [5]
When programming for operations on images involving matrix calculations (e.g., signal-to-
noise ratio calculations, vignetting corrections), cast the data to double-precision floating
point variables to prevent numerical errors. For saving images after performing calculations,
image files may be reconverted to their original bit depth.
– 12 – IEC TS 60904-13:2018 © IEC 2018
IEC IEC IEC IEC
IEC IEC
a) Image of powered b) Image of unpowered c) pixel-by-pixel d) Image of module in
module with stray light module with stray light subtraction of Figures 3a) ideal dark room with
-3b) same camera settings as
a) and b)
Figure 3 – Example of frame subtraction given in Figure 3a) to Figure 3c),
with images taken in ideal dark room conditions given in Figure 3d)
Under stray light, areas where the pixels become saturated shall be discarded. The exposure
duration is preferably limited such that the pixels do not approach saturation. Image quality for
subtraction of stray light purposes is significantly improved with cameras having greater
dynamic range. The noise level is however greater in the case of images taken with stray light
after frame subtraction than images taken without stray light.
4.1.6 Safety and handling
EL imaging of photovoltaic modules with insulated cables and connectors (as is recommended)
does not generally involve risk of exposure to live electrical wiring hazards, however any
electrical safety protocols should be taken according to the specific circumstances.
Handle PV modules and laminates with care when moving them into position for imaging as to
not cause damage that will introduce new features or artifacts in the module image.
4.2 Procedure
4.2.1 Camera settings and positioning
4.2.1.1 Calibration
The camera should be in calibration according to any procedures specified by the
manufacturer. Time and date should be properly entered in the camera or image recording
computer if the functionality exists so that images may be later related to the time and date
captured.
4.2.1.2 Angle from normal of module plane
Angle of view relative to surface is preferably normal with respect to the module surface to be
imaged. The maximum angle of view from module normal should be less than 50°. Emissivity
adjustment for angle is required if it is greater than 50°.
The principal light facing side of the module is imaged. However, it may be advantageous to
image bifacial modules from the opposite side as well.
4.2.2 Camera settings
4.2.2.1 General practice
For routine measurements, the image intensity for the camera at each forward bias current
level may be optimized by adjusting the total exposure time, aperture f-number), or by gain
adjustment after a survey of modules of the type to be examined is made, and then kept
consistent. If changes need to be made to achieve the desired image intensity, exposure time
shall be adjusted and the change recorded. See 4.2.2.3 for guidelines regarding image
intensity.
4.2.2.2 Recommended camera settings (focus, gain f-number)
A first, rough focus, may be performed by viewing in the visible light regime, but fine focus
shall be optimized to the wavelength of the EL signal to be imaged. This can be simplified by
using IR-corrected lenses. In this case the focus setting is the same for the EL and visible
images. Focus shall be sufficient to resolve the features according to the desired level of
sharpness defined in 4.2.3. An algorithm is given in Annex B that may be used for computing
the optimum lens focus position.
Other recommended settings are as follows:
Gain setting shall be set to obtain optimum pixel depth resolution of module to be imaged.
f-number shall not be changed between images to be compared. If an adjustable f-number is
available, choose the lowest f-number by default when imaging samples that are centered in
front of the camera with the optical axis passing through the center of the module. A different
f-number may be selected and fixed if it is deemed to produce optimized results, such as to
decrease vignetting, increase sharpness, and increase the depth of field for imaging with an
oblique view of the module face.
4.2.2.3 Image intensity
Optimized images will have less than 5 % area around the perimeter of the module not
producing luminescence. The exposure shall be optimized (increased) to achieve maximum
signal to noise ratio, but limited such that pixels in the image are not saturated. Single time
effects and random noise are not counted in this analysis of pixel saturation.
4.2.2.4 Comparisons of modules
Various module types and degradation processes may show differing EL behavior. For easy
comparison of a degraded module to an undegraded module, image the degraded module at
the original condition and optionally, with settings reoptimized for the degraded condition.
Only the exposure time may be used for the readjustment when comparing undegraded and
degraded modules in this way. The current shall not be changed. Perform any required image
frame subtraction (4.1.5.2) before additional post processing. When visual comparison is
desired for modules imaged with different exposure times, the intensities of the pixel may be
scaled in post-processing by inversely scaling the pixel intensities by the exposure time and
the modified image labeled as such. The brightest image intensity in such comparisons shall
be set according to 4.2.2.3.
– 14 – IEC TS 60904-13:2018 © IEC 2018
4.2.3 Sharpness determination and classification
4.2.3.1 General
The sharpness (S [mm]) is used as the index of the resolvable object size in the image
obtained by a camera, its settings, and the WD. It is the minimum real dimension that still
provides a contrast of 50 %. Sharpness is dependent on the pixel dimension, the linear
distance on the module sampled by a pixel, and it includes the effects of image blurriness. A
method for determining and classifying image sharpness is defined here. Comparisons for
quality of module(s) shall be made between images of similar sharpness classes.
4.2.3.2 Sharpness measurement
The hereinafter described ‘V-cut’ measurement involves calculation of the intersection angle α
and distance r from two edge lines L , L created by an opaque mask on top of the
50 1 2
luminescent surface. To convert pixels into mm, which will be necessary to obtain the value of
r , a conversion factor between millimeters of length on the module and pixels in the image
shall be obtained. It can be obtained from an object of known dimension in the image such as
a cell or the module. The formulas needed for the sharpness determination are detailed in
Formula (1) through Formula (3).
NOTE This method is a modified version of the spoke target, also known as the Siemens star and sector star
target, based on a pair of lines (black-white-black group). Here it is modified such that only the bright field is
examined.
Two edges are created on an EL image of a monocrystalline silicon PV module. This can be
achieved with thin metal plates or opaque (e.g. aluminum film) tape over the luminescing
region as illustrated in Figure 4. The angle α between both edge lines should be between 3°
and 6°. For an example edge length of 10 cm, this corresponds to a distance of about 0,5 cm
to 1,0 cm between both edges at the open end. The edge lines run through the local maxima
of an edge gradient image G (Figure 5). The Sobel operator discussed in Annex B may be
Edge
used to determine G , though the Find Edges function in the public domain software
Edge
ImageJ, or simple linear derivatives may also be used. The middle line L goes from
intersection p of L and L to point p , in the middle of p and p . From all plotted EL image
0 1 3 2 1 3
intensity values along L , the distance 𝑟𝑟 is measured from 𝑝𝑝 to that point on L where the
50 0
2 2
image intensity equals the mean between the dark and bright EL intensity plateau
(𝐸𝐸𝐸𝐸 ,𝐸𝐸𝐸𝐸 ) as in the example in F
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