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Contract No. DE-AC36-08GO28308

Examination of an Optical 

Transmittance Test for 

Photovoltaic Encapsulation 

Materials

Preprint

David C. Miller,a Jaione Bengoechea,b Jayesh G. Bokria,c

Michael Köhl,d Nick E. Powell,e Michael E. Smith,f

Michael D. White,g Helen Rose Wilson,d 

and John H. Wohlgemuth a

a

National Renewable Energy Laboratory

b

Fundación CENER-CIEMAT

c

Specialized Technology Resources, Inc.

d

Fraunhofer-Institut für Solare Energiesysteme ISE

e

Dow Corning Corp.

f

Arkema Inc. g

The Dow Chemical Company

Presented at SPIE Optics & Photonics 2013

San Diego, California

August 25–29, 2013

Conference Paper

NREL/CP-5200-60029

September 2013
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*David.Miller@nrel.gov; phone: +1-303-384-7855; fax: +1-303-384-6490; www.nrel.gov/pv/performance_reliability/

Examination of an Optical Transmittance Test 

for Photovoltaic Encapsulation Materials

David C. Miller,*a Jaione Bengoechea,b Jayesh G. Bokria,

c

 Michael Köhl,d Nick E. Powell,

e

 

Michael E. Smith,

f Michael D. White,g Helen Rose Wilson,d

 and John H. Wohlgemuth a

a

National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO, USA 80401

b

Fundación CENER-CIEMAT, C/Ciudad de la Innovación 7, Sarriguren, Spain 31621 c

Specialized Technology Resources, Inc., 18 Craftsman Road, East Windsor, CT, USA 06088

d

Fraunhofer-Institut für Solare Energiesysteme ISE, Heidenhofstr. 2, 79110 Freiburg, Germany e

Dow Corning Corp., 2200 W. Salzburg Rd, Midland MI, USA 48686

f

Arkema Inc., 900 First Avenue, King of Prussia, PA, USA 19406

g

The Dow Chemical Company, 1605 Joseph Drive, 200 Larkin, Midland, MI, USA 48667

ABSTRACT

The optical transmittance of encapsulation materials is a key characteristic for their use in photovoltaic (PV) modules. 

Changes in transmittance with time in the field affect module performance, which may impact product warranties. 

Transmittance is important in product development, module manufacturing, and field power production (both immediate 

and long-term). Therefore, an international standard (IEC 62788-1-4) has recently been proposed by the Encapsulation 

Task-Group within the Working Group 2 (WG2) of the International Electrotechnical Commission (IEC) Technical 

Committee 82 (TC82) for the quantification of the optical performance of PV encapsulation materials. Existing 

standards, such as ASTM E903, are general and more appropriately applied to concentrated solar power than to PV. 

Starting from the optical transmittance measurement, the solar-weighted transmittance of photon irradiance, yellowness 

index (which may be used in aging studies to assess durability), and ultraviolet (UV) cut-off wavelength may all be 

determined using the proposed standard. The details of the proposed test are described. The results of a round-robin 

experiment (for five materials) conducted at seven laboratories to validate the test procedure using representative 

materials are also presented. For example, the Encapsulation Group actively explored the measurement requirements 

(wavelength range and resolution), the requirements for the spectrophotometer (including the integrating sphere and 

instrument accessories, such as a depolarizer), specimen requirements (choice of glass-superstrate and -substrate), and 

data analysis (relative to the light that may be used in the PV application). The round-robin experiment identified both 

intra- and inter-laboratory instrument precision and bias for five encapsulation materials (encompassing a range of 

transmittance and haze-formation characteristics).

Keywords: material characteristics, quality assurance, accelerated stress testing, reliability, polymer

1. INTRODUCTION

Optical transmittance is a key performance characteristic for photovoltaic (PV) encapsulation materials. The 

discoloration of encapsulation (and corresponding reduction in transmittance) has also been identified as a key 

contributor to the long-term performance degradation of fielded PV modules [1]. Existing standards—including ISO 

13468 [2], ASTM E903 [3], ASTM E1175 [4], and ASTM E424 [5]—do not examine the relevant parameter (the photon 

irradiance, Epλ, {m-2·s-1·m-1}) or make use of analysis that may be used to further assess PV encapsulation, including 

yellowness index (YI, {unitless}) and UV cut-off wavelength (λcUV, {m}). The Encapsulation Committee within the 

International Electrotechnical Commission (IEC) Technical Committee 82 (TC82) on PV Working Group 2 (WG2) on 

PV modules has created a standard material-level test to assess the expected optical performance of encapsulation at its 

interface with the PV cell. The protocol, colloquially known as the “transmittance standard,” describes the measurement 

of optical transmittance and subsequent analysis of Epλ, YI, and λcUV. The purpose of the standard is to aid material 

manufacturers and module manufacturers in performing material acceptance, material or process development, design 

analysis, and failure analysis. No “pass” or “fail” criteria are assigned for the proposed test procedure; rather, it is 

intended to be used for datasheet reporting, quality control, and durability assessment. 
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The standard specifies to measure the transmittance (τ, {unitless}) from 300 to 2500 nm in a 1-nm increment using a 

double-beam spectrophotometer, as in [2],[3],[4],[5]. The standard uses the photon irradiance (Equation 1), which 

considers number of photons rather than the raw incident energy. This is done because one photon can produce at most 

one electron-hole pair in a typical PV cell. The solar-weighted transmittance of photon irradiance (Equation 2) considers 

the measured τ relative to the Epλ of the solar source, defined in Ref. [6]. Following Ref. [7], new parameters in the 

equations include: λ, the wavelength {m}; Eλ, the spectral irradiance {W•m-2•m-1}; h, Planck’s constant {6.626⋅10−34

W⋅s

2

}; c, the speed of light in a vacuum {2.998⋅108 m⋅s

-1}; and τw, the weighted transmittance {unitless}.

[ ] [ ] λ λ

λ λ Eλ hc

Ep = (1)

[ ] [ ]

[ ] ∫

∫ = λλ

λλλτ

τ

λ

λ

dE

dE

p

p

w (2)

The “solar-weighted” transmittance (τsw,{unitless}), obtained using Equation 2, is defined for 300 ≤ λ ≤ 2500 nm. τsw is 

intended to be used as a figure of merit that may be applied to all PV, regardless of the cell technology. The 

“representative solar-weighted” transmittance (τrsw,{unitless}) is also determined from Equation 2, but is defined for 

300 ≤ λ ≤ 1250 nm. τrsw considers the typical maximum operating wavelength range for contemporary flat-panel PV 

devices. Figure 1 shows the external quantum efficiency (EQE) profiles for champion cell technologies, documented in 

the publication series of Ref. [8]. The figure identifies spectral performance for cell technologies including 

InGaP/GaAs/InGaAs multijunction (3J), gallium arsenide single-junction (GaAs), monocrystalline silicon (m-Si), 

copper-indium-gallium-selenide (CIGS), polycrystalline silicon (p-Si), cadmium-telluride (CdTe), and copper-zinc-tinsulfur-selenide (CZTSS), relative to the AM1.5 solar spectrum [6]. The λcUV and infrared (IR) regions are examined to 

the left and right of Figure 1, respectively. The insets of the figure confirm that the spectral bandwidth for the cell 

technologies occurs within 300 ≤ λ ≤ 1250 nm (except for 3J, which extends up to 1800 nm). 

Figure 1: Representative EQE profiles for popular PV device technologies shown relative to the global terrestrial solar 

spectrum (AM1.5 in IEC 60904-3). 

YI is determined in the standard for a D65 illuminant (mid-day outdoor sun, as in Ref. [9]) for a human observer (CIE 

1964 XYZ color space with 10° field of view, as in Ref. [10]). The illuminant and observer data are tabulated in the ISO 

standards for a 1-nm increment, rather than the 5-nm tabulated data in the comparable ASTM standards, i.e., [11],[12]. 

YI is a metric of how much absorption occurs in the blue wavelengths and is weighted by the D65 spectrum and the 

efficacy of the cone receptors of the human eye. A yellowness of 0.00, considered neutral, indicates no optical 
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absorption between 360 and 830 nm. A positive value indicates yellowing from absorption in the blue region of the 

spectrum, and a negative value indicates a shift toward blue due to absorption in the red region of the spectrum. The 

calculation of YI may be performed using numerical integration, which provides the three tri-stimulus values 

(corresponding to the three cone types), a normalizing factor, and two other coefficients [10],[12]. The YI of polymers 

tends to increase with age with the development of chromophore species, which would be perceived as yellow or brown 

(in worst cases) by a human observer.

Like YI, λcUV may be used to assess the weathering of polymers. The critical consideration for the standard is whether a 

low, medium, or high threshold (e.g., τ = 10, 50, or 90%) is preferred for λcUV. The steps in the protocol recommended 

by Ametek, Inc., include: 1) determine the value of the maximum measured τ; 2) determine the λ at the τ = 72% of the 

maximum τ; 3) determine the λ at the τ = 50% of the maximum τ; 4) linearly extrapolate λcUV (defined at the τ = 5%) 

from τ = 72% and τ = 50%. The Ametek method is advocated because rapid transitions in optical transmittance were

found to be difficult to replicate between different instruments, e.g., limited by the finite rates of scanning and detection. 

The Ametek method to determine λcUV was therefore established experimentally, based on the measured reproducibility 

within and between separate laboratories.

Several criteria for the determination of λcUV were identified early in the creation of the test standard, including: the 

absolute τ of 10%; the Ametek method (described above for the relative τ, projected to 5%); the absolute τ of 1%; and 

the absolute τ of 50%. The criterion of 10% (which corresponds to an optical absorbance of 1) was the most widely cited

in the literature. It comes from the biological and nanosciences, where it is often applied to liquid specimens measured in 

a 1-cm cuvette. The criterion of 1% is used in some of the physical sciences, where it is considered as the threshold for 

biologically affecting solar radiation. The criterion of 50% is common in applications such as photographic filters. 

The standard specifies to use silica/polymer (for datasheet reporting) or silica/polymer/silica specimens (for durability 

studies). The silica/polymer geometry simulates the application (up to the interface with the PV cell) when the polymer 

surface faces the entrance of the integrating sphere. The silica/polymer/silica geometry is anticipated to contain both 

aerobic (at the edges) and anaerobic (at the interior) regions during aging in air, for specimens ~50-mm x~50-mm or 

greater in size. Users may wish to verify this assumption. The glass is specified to be 3±1mm thick (including the 

popular 3.2-mm “double-strength” glass found in flat-panel PV). The glass used for datasheet reporting cannot have 

texture, coatings, or antireflective (AR) layer(s). Any glass may be used in tests performed for other purposes. 

Figure 2: Measured τ for two silica glass specimens relative to commercial “quartz” and soda-lime glass products, as well 

as the global terrestrial solar spectrum (AM1.5 in IEC 60904-3).
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The measured τ for two appropriate silica glass specimens is shown in Figure 2, relative to commercial “quartz” and 

soda-lime glass products. Figure 2 shows examples of soda-lime glass including both low-iron (“low-Fe”) and PVspecific glass (“PV S-L”). The examples of “quartz” glass, 7980 (Corning Inc.) and Lithosil (Schott AG), are included 

along with the AM1.5 solar spectrum [6]. The direct transmittance (τd, obtained without an integrating sphere, unlike in 

the standard) is shown in Figure 2, because of the greater wavelength range that can be obtained (175 ≤ λ ≤ 3300 nm) for 

material verification. The measured τ for the specimens of different thickness were compensated to the thickness (z, 

{m}) of 3.2 mm, as described in Ref. [13] and indicated in Figure 2.The inset left of the figure shows the transmittance

at short wavelengths, including the UV, where the “UV-A” and “UV-B” bands are labeled. The same specimens shown 

in Figure 2 are summarized in Table 1. The values for the specimen thickness (measured using a micrometer), τsw, τrsw, 

YI, and λcUV (determined for 10% of the absolute measured τ) are given in the table. The data averages (x) for the silica 

specimens are also provided in Table 1. As in Figure 2, the data in Table 1 have been analyzed to compensate the results 

to z = 3.2 mm. In general, the silica specimens have a τsw of 93%, τrsw of 93%, YI of 0.2, and λcUV of 179 nm. λcUV is 

listed; however, the wavelength limit below which it can be accurately measured in an ambient atmosphere containing 

moisture was not examined further. The seven silica specimens all demonstrate optical transmittance intermediate to the 

Lithosil (considered an “optical”-grade quartz) and 7980 (a “fused silica”). In particular, silica C and 7980 exhibit a

minor–OH absorption band at 2750 nm, suggesting minimal residual water content. The soda-lime glasses have a 

reduced τ relative to the silica specimens, which are purer in content. In Figure 2, the low-Fe soda-lime specimen 

uniquely demonstrates the Fe absorption band about 1075 nm. The PV-specific soda-lime glass in Figure 2 transmits 

UV-B radiation, whereas the low-Fe soda-lime specimen transmits UV-A only. The characteristics of silica include: UV 

transparency—it will minimally affect the transmittance measurements of intermediate polymer layers (compare λcUV in 

Table 1 to λcUV in Table 6); immunity to corrosion in hot-damp aging environments; an outer surface may be readily 

cleaned with solvents (e.g., toluene, isopropyl alcohol, water) to remove contamination or residual polymer.

Table 1: Summary of the glass specimens (“silica A” – “silica G”) used in the round-robin experiment, described below.

SPECIMEN

z, SPECIMEN

THICKNESS 

{mm}

τsw {%} τrsw {%} YI 

{unitless}

λcUV

{nm}

A 3.2 92.86 92.78 0.17 178

B 3.2 92.56 92.84 0.20 177

C 2.9 92.79 92.67 0.26 180

D 1.6 92.78 92.81 0.22 181

E 1.5 92.74 92.75 0.16 181

F 3.1 92.85 92.82 0.18 178

G 3.2 92.79 92.76 0.17 178

x, Avg N/A 92.77 92.77 0.19 179

S-L (Low Fe) 5.6 86.53 86.46 -0.06 320

PV S-L 3.2 91.87 91.80 0.29 289

7980 5.3 92.88 92.82 0.12 176

Lithosil 5.9 92.48 92.77 0.14 176

Many polymeric materials are birefringent, having a refractive index (n) that depends on the polarization and 

propagation direction of light. These materials may generate measurement error in some spectrophotometers because 

they modify the polarization state of transmitted light passing through the instrument optics and onto the detectors, even 

after measurement baselines have been performed. To mitigate this effect, a depolarizer, included in the baseline, can be 

placed before the sample. Figure 3 shows the τ for a glass/polymer/glass specimen, measured on the same 

spectrophotometer, with and without a depolarizer present. A section view of the measurement and specimen geometry 

is shown in the inset of Figure 3. The individual measurements were obtained well after the instrument had been 

activated, so that the lamp emission and instrument detection had stabilized to their steady-state operation. A 
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discontinuity occurs at 900 nm in the measurement where no depolarizer is present. Discontinuities, such as that shown 

in Figure 3, may result from changes in the optics (e.g., the detector and grating changeovers) or source used within the 

spectrophotometer. As shown in Figure 3, the effect of birefringence is minimized when a depolarizer is used. A change 

in stress, which also affects the polarization state in a birefringent specimen, occurring over time (e.g., with aging) would 

further misconstrue the transmittance measurement. The effects of residual moisture absorbed during aging, which can 

scatter and attenuate light in a τ measurement, are described in Ref. [14]. An optical image (obtained using crosspolarization photography) is shown in the inset of Figure 3. Cross-polarization photography may be used on materials 

like ethylene-co-vinyl acetate or polyolefin to identify polarization sensitivity or to qualitatively visualize stress within a 

specimen, based on color or brightness contrast within the image.

Figure 3: τ for a glas/polymer/glass specimen, measured on the same spectrophotometer, with and without a depolarizer

present. 

The goal of the described experiments was to support the development of a standardized test procedure that can be used 

to evaluate the optical transmittance of encapsulation products intended for use in PV modules. Discovery experiments 

were used to examine issues, including: the variability of representative silica glass specimens; the use of a single-beam 

or double-beam spectrophotometer instrument; and the variability of transmittance for specimens using the same 

encapsulation material, but constructed in different laboratories. A round-robin experiment was conducted to determine 

the intralaboratory repeatability and interlaboratory reproducibility for the test standard, as well as to develop the test 

procedure, e.g., select the criteria for the UV cut-off wavelength. 

2. EXPERIMENTAL

2.1 Details of the Round-Robin Experiment

A round-robin experiment (R-R) was conducted according to Ref. [15] (which describes the design and implementation) 

and Ref. [16] (which describes the analysis and interpretation of the results). Table 2 summarizes the spectrophotometer 

instruments used in the R-R. Characteristics listed in the table include the make and model of the instrument, inner 

diameter of the integrating sphere, beam size at the specimen (given to the nearest mm), incident angle (at the reflector 

port), presence/use of a depolarizing optic, ratio of the area of the open ports to that of the sphere walls (plus all the 

ports), material for the walls of the integrating sphere, and type of detectors. All of the instruments use a deuterium lamp 

as a UV light source and a halogen lamp for the visible and IR wavelengths. Although the maximum active range is 

listed in Table 2 for the detectors, the active range is typically reduced (e.g., 200–2600) by the range of reflectance of the 

sphere material. Practically speaking, a nitrogen gas purge is required to improve the quality of optical measurements for 

λ < 250 nm. All of the instruments were double-beam spectrometers, except the single-beam CENER instrument. Use of 

a depolarizing optic may reduce the measurement range to ~275 nm, depending on its composition. Six different 
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instruments, with some replicates (two Lambda 950’s and two Cary 5000’s) were used. A photomultiplier tube (PMT) 

was used for UV-Vis detection, except at CENER, where CCD arrays were used. 

Four types of material were examined in the R-R: ethylene-co-vinyl acetate (EVA), ionomer, poly(dimethylsiloxane) 

(PDMS), and thermoplastic polyolefin (TPO). For one of the materials, both a hazy and non-hazy formulation was used 

to specifically examine the results for a material prone to light-scattering. The material types will not be identified in the 

R-R results. The material types are omitted to prevent misinterpretation of the transmittance results (the rank of materials 

examined), which can depend highly on the specific formulation of the materials examined. Both glass/polymer and 

glass/polymer/glass specimen geometries (three replicates each) were examined for each material. Specimens were 

cleaned on their exposed glass surfaces with isopropyl alcohol prior to measurement, except for the PDMS specimens, 

which were first cleaned with toluene.

Table 2: Summary of the spectrophotometer instruments used in the round-robin experiment.

LABORATORY INSTRUMENT

MAKE

INSTRUMENT

MODEL

DIAMETER,

SPHERE

{mm}

BEAM SIZE

{mm x mm}

INCIDENT

ANGLE {°} 

DEPOLARIZER

? 

RATIO: 

PORTS/

SPHERE{%}

SPHERE

MATERIAL

TYPE

OF

DETECTORS

(ACTIVE RANGE {nm})

Arkema Perkin-Elmer Lambda 950 150 10 x 16 8 CBD present, 

not used 12.8 Spectralon PMT (175 – 860)PbS [cooled] (861 – 3300)

CENER Instrument 

Systems GmbH Custom 150 Ø 20 8 No 1.4 BaSO4

CCD Arrays, Si (200-850)

InGaAs (800-1650)

Dow Chemical Perkin-Elmer Lambda 950 60 12 x 15 O CBD present,

not used 12.8 Spectralon PMT (175 – 860)PbS [cooled] (861 – 3300)

Dow-Corning Agilent Cary 5000 110 11 x 13 3.33 Yes 3 PTFE PMT (175 – 800)

PbS [cooled] (800 – 3300)

Fraunhofer Perkin-Elmer Lambda 900 220 10 x 16 8 CBD present, 

not used 1.9 (1.4) Sintered PTFE

PMT (175 – 860)

PbS [cooled] (861 – 3300)

NREL-1 Agilent Cary 5000 150 4 x 12.5 7 CBD present, 

not used 4.9 PTFE PMT (175 – 800)PbS [cooled] (800 – 3300)

NREL-2 Shimadzu UV-3600 60 3 x 10 8 No 4.6

BaSO4

(Barium 

Sulfate)

PMT (175-740)

InGaAs (740-1650) 

PbS (1650-3300)

STR Perkin-Elmer Lambda 1050 150 8 x 16 8 No 2.5 Spectralon PMTTube (175-860)

InGaAs (861-3300) 

2.2 Details of the Discovery Experiments

Several discovery experiments were conducted in conjunction with the R-R. First, the silica (procured separately from 

different sources by the participating laboratories) was all measured at NREL. The results, presented in Figure 2 and 

Table 1 quantify the characteristics of silica glass appropriate for the transmittance test. A single-beam 

spectrophotometer was used at one laboratory to examine typical instrument performance and identify possible issues 

relative to double-beam instruments. This second experiment used the same five specimen sets circulated in the R-R. 

The variability of specimen fabrication was also examined in addition to the R-R. Here, the laboratory participants were 

supplied uncured EVA sheet and instructed upon its intended processing by the material manufacturer. The laboratory 

participants also had to procure silica glass from their chosen vendor. The variability of specimens constructed from the 

same EVA was compared by measuring all the EVA specimens at NREL. Representative glass/polymer (g/p) and 

glass/polymer/glass (g/p/g) EVA specimen sets (all specimens within each set made at the same laboratory) were then 

chosen for the R-R. In the fourth experiment, the same single glass/EVA/glass specimen was measured in each 

laboratory ten times without replacement (the specimen was not moved between measurements) and ten times with 

replacement (the specimen was removed from and then replaced into the instrument before each measurement).

3. RESULTS AND DISCUSSION

The use of a single-beam (SB) spectrophotometer is examined in Figure 4, where representative data profiles (one of the 

three replicates, for both g/p and g/p/g specimens) are compared to those obtained using a double-beam (DB) instrument. 

The baseline for the SB measurements in Figure 4 was performed with no specimen present at the entrance port of the 

integrating sphere. The SB measurements were then performed with the specimen placed over the entrance port of the 

integrating sphere. A section view of the specimen geometry and uncompensated measurement is shown in the inset of 

Figure 4. The AM1.5 solar spectrum [6] is also shown in the figure for reference. A substantial (~1.3% difference in τw) 

occurs in Figure 4 between SB and DB measurements. An additional ~0.6% difference in τw occurs in Figure 4 between 

g/p/g and g/p specimens for both the SB and DB measurements. Separately, measurement noise (associated with the 

detector and grating used below ~900 nm is observed in Figure 4, particularly for the DB measurements. 
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There is a notable difference between the SB and DB measurements that is attributed to port effects. When a baseline is 

performed without a specimen in place and then a specimen is placed for measurement at the sample port, the specimen 

increases the reflectivity of the port and total light-trapping of the sphere. This increases the measured signal relative to 

the baseline, resulting in erroneously high transmittance measurements. To facilitate proper SB measurements, the 

baseline was first performed with a specimen on the reflectance port of the integrating sphere, but with the beam first 

striking the sphere wall (requiring a double-beam sphere or sphere designed for off-axis use) rather than the sample. 

Measurements were then performed with the specimens on entrance port of the integrating sphere, enabling automatic 

measurement correction of the port effects. Using that measurement procedure, the SB data could not be readily 

distinguished from DB data (outside of the measurement variability between laboratories, not shown). Regarding the 

difference between the g/p/g and g/p specimens, the τw was typically greater for the g/p/g specimens. Here, the 

difference is attributed to the index mismatch between the back surface of the specimen and air. The refractive index of 

most of the PV encapsulation materials examined is greater than that of silica glass [13]. The least reflectance loss (and 

corresponding greatest transmittance) is therefore expected for the g/p/g specimens, where n of the silica back surface is 

best matched to air. It should be noted that a very different index mismatch will occur in air (n = 1.0003) than at the 

polymer/cell interface, because the n of the cell is greater, e.g., the n of a Si3N4 antireflective surface layer is ~ 2.0. 

Lastly, the greater noise observed in the UV-Vis data for all the materials is attributed to the limitations of the instrument 

technology (e.g., detectors) as well as the UV scattering from airborne moisture. 

Figure 4: Comparison of single-beam (uncorrected) and double-beam instrument measurements (representative profiles) for 

glass/polymer and glass/polymer/glass specimens.

The measured transmittance for the hazy material (averaged for the three replicate specimens) is shown in Figure 5. All 

specimen data are from the glass/polymer/glass configuration, as indicated in the figure inset. The haze {unitless}, 

calculated as haze = (τh-τd)·τh

-1, is also shown where it may be applied in Figure 5. The haze was determined for the 

NREL-1 instrument. The AM1.5 solar spectrum [6] is also shown in Figure 5. An increase in haze is observed as the 

wavelength is decreased, identifying increased optical scattering at shorter wavelengths. The τh profiles for the other 

materials corresponded well, essentially overlapping between instruments. There are considerable differences, however, 

between the measurements for the hazy material in Figure 5. In the figure, the NREL-2 τh profile is notably less than the 

other τh profiles. The NREL-2 results were repeatable between measurement sessions.

The disparity between the measurements in Figure 5 is thought to be related to the different angles of acceptance for the 

different makes of spectrophotometer instruments, including the position of the detector with respect to the entrance 

port. For example, the Dow-Corning and NREL-1 data (both obtained using a Cary 5000) nearly overlap, as would be 

expected for the same make of instrument. Haze may result in polymers from spherulite size and concentration. Because 

the haze depends on the refractive index of the crystallite and amorphous regions within the polymer, scattering will vary 
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with wavelength. The effect becomes critical to transmittance measurements if the scattered light falls outside of the 

acceptance angle of the spectrophotometer. Unfortunately, the acceptance angle is typically not specified by the 

manufacturer. Furthermore, the reflectance spectrum of the interior sphere wall is neither easily determined nor constant 

with time. The results in Figure 5 are not attributed to the aging of the specimen, e.g., no overt trend was observed over 

the course of the measurements and the NREL-2 measurements were performed first. The behavior in Figure 5 could be 

of concern in aging studies, however, if discolored/aged material were prone to haze formation. In such cases, the 

transmittance results would be instrument dependent.

Figure 5: Measured transmittance for the haze-generating material. The haze, determined from the hemispherical 

transmittance (τh) and direct transmittance (τd, where no integrating sphere is present) for the “NREL-1” instrument, is also 

shown, relative to the global terrestrial solar spectrum (AM1.5 in IEC 60904-3).

The results of the measurements for the EVA specimens made at the different laboratories are summarized in Table 3. 

Data in the table include the characteristics of τsw, the τrsw, YI, and λcUV (10% absolute criterion) for the seven sample

sets. The average (x), standard deviation (sx, 1σ), repeatability (sr, nominal), and reproducibility (sR, nominal) are 

identified in Table 3 for each of the four characteristics. As defined in Ref. [16], repeatability addresses the question, 

“What is the average of the variance for the laboratories, i.e., how tightly clustered are the data within each laboratory?” 

If each laboratory had a small variation, then sr would be minimal, even if the variation between the different 

laboratories was substantial. As defined in Ref. [16], reproducibility considers the deviation of the laboratory averages 

from the average of the experiment as well as the repeatability, weighted by the number of samples. Reproducibility 

therefore quantifies, “How well do the data sets overlap, and how tightly clustered are the data within each laboratory

(sr)?” sr and sR in Table 3 apply to specimens made at different laboratories, rather than measurements from different 

laboratories. The global average for the seven specimen sets is also provided in Table 3 for the glass/polymer and 

glass/polymer/glass specimen sets. As specified in Ref. [16], one additional significant figure is provided in Table 3 to 

aid the assessment of the experiment.

Table 3: Summary of base results for the different makes (performed at each laboratory) of EVA, measured on the NREL-2 

instrument. One additional significant figure is provided in the table as specified in [16]. 

τsw {%} τrsw {%} YI {unitless} λcUV {nm}

CONSTRUCTION x sx sr sR x sx sr sR x sx sr sR x sx sr sR

g/p 87.873 0.187 0.293 0.293 90.562 0.068 0.289 0.289 0.602 0.049 0.088 0.088 357.95 1.11 1.27 1.27

g/p/g 88.509 0.114 0.157 0.157 91.138 0.065 0.119 0.119 0.456 0.025 0.037 0.037 357.19 0.87 1.07 1.07
9 

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Regarding the results for the different makes of EVA specimens, the g/p EVA-B, g/p EVA-E, g/p/g EVA-B, and g/p/g 

EVA-C specimens showed a variation about twice that of the other specimen sets (not shown). This suggests that only 

one laboratory (EVA-B) had a notable difference (variation) in the make of EVA specimens. As in Figure 4, the greater 

τ for the g/p/g specimens in Table 3 is attributed to the lesser n (and corresponding reduced reflectance loss) of the silica 

substrate for the g/p/g specimens. The maximum sr and sR values of ±0.29%, ±0.29%, ±0.09, and ±1.3nm were identified 

for τsw, τrsw, YI, and λcUV, respectively. As indicated by orange and red shading in Table 3, the maximum sr and sR

values occur for the g/p specimens in all cases. The maximum sr and sR values in Table 3 will be shown in Table 4 to be 

~3x greater than the practical values that may be obtained from a particular specimen using a trusted instrument, but 

~1/3x the variability observed for the different materials examined and different instruments used in the R-R. This 

suggests that although there is an increased variability associated with the specimen make (including the silica glass and 

lamination process used), it does not exceed the variability associated with the choice of the encapsulation material and

the instrument used for measurement.

The results of the repeatability experiment (for the same EVA specimen, both with and without replacement) are 

summarized in Table 4. Data in the table include τsw, the τrsw, YI, and λcUV (10% absolute criterion) for the seven

measurement laboratories in Table 2. The average (x) and standard deviation (sx, 2σ) are identified in Table 4 for each of 

the four characteristics. The global average and global standard deviation for the eight laboratories is also provided in 

Table 4. The sx for the measurements taken with no replacement can be used to identify the best possible repeatability 

that may be obtained for the different instruments. Threshold values of ±0.03% and ±0.01 are identified for τ and YI

measurements, respectively, from the average sx of the same-instrument measurements. The average sx values are 

indicated in orange in Table 4. The average of 0.06 nm is less than the uncertainty of ±0.5 nm, taken for λcUV based on 

the 1-nm resolution specified in the test procedure. The aforementioned threshold values identify the minimum 

increment that can be determined by the instrument for each of the four characteristics, establishing the appropriate 

number of significant figures.

Table 4: The average (x) and standard deviation (sx, 2σ) for the repeatability experiment, obtained for the same glass/EVA/glass 

specimen measured 10x, with and without replacement. One extra significant figure is provided in the table as specified in [16]. 

NO REPLACEMENT WITH REPLACEMENT

τsw {%} τrsw {%} YI {unitless} λcUV {nm} τsw {%} τrsw {%} YI {unitless} λcUV {nm}

LABORATORY x sx x sx x sx x sx x sx x sx x sx x sx

Arkema 88.720 0.001 91.376 0.001 0.487 0.003 359.00 0.00 88.760 0.006 91.413 0.008 0.478 0.003 359.00 0.00

CENER N/A N/A 91.908 0.004 0.553 0.008 357.00 0.00 N/A N/A 91.927 0.005 0.567 0.010 356.90 0.19

Dow Chemical 88.690 0.018 91.190 0.019 0.442 0.005 358.00 0.00 88.539 0.042 91.012 0.045 0.481 0.008 358.00 0.00

Dow-Corning 88.855 0.001 91.547 0.001 0.509 0.003 359.00 0.00 88.894 0.009 91.592 0.010 0.475 0.006 359.00 0.00

Fraunhofer 88.675 0.004 91.389 0.006 0.465 0.018 359.00 0.00 88.692 0.005 91.404 0.007 0.461 0.020 359.00 0.00

NREL-1 88.668 0.003 91.313 0.003 0.476 0.002 359.00 0.00 88.468 0.062 91.110 0.064 0.466 0.005 359.00 0.00

NREL-2 89.001 0.163 91.446 0.151 0.431 0.047 355.80 0.47 88.516 0.280 90.920 0.278 0.472 0.043 355.20 1.44

STR 88.853 0.009 91.544 0.009 0.430 0.002 359.00 0.00 88.831 0.009 91.535 0.012 0.428 0.006 358.20 0.25

x, Avg 88.780 0.029 91.464 0.024 0.474 0.011 358.23 0.06 88.672 0.059 91.364 0.054 0.478 0.013 358.04 0.23

sx 0.126 0.215 0.042 1.22 0.167 0.335 0.039 1.37 

The measurements taken with replacement can be used to identify the best practical repeatability that may be obtained 

between different instruments (or laboratories). The within-laboratory measurements (performed using the same 

instrument) have a variability 5x (for YI) to 10x (for τsw and τrsw) less than the intralaboratory variability (performed 

using different instruments). λcUV was found to be extremely repeatable between measurements. Practical values of 

±0.28%, ±0.04, and ±1.4 nm are identified for τ, YI, and λcUV measurements, respectively, from the maximum sx of the 

measurements with specimen replacement. The variability in the λcUV measurements, however, identifies the UV-3600, 

custom CENER, and Lambda 1050 instruments as the most variable instruments used. The variability of the UV-3600 

notably exceeded that of the other instruments. Therefore, if the UV-3600 instrument is not considered, the values of 

±0.06% and ±0.02 are identified for τ and YI measurements, respectively, from the maximum sx of the measurements 

with specimen replacement. The maximum sx values are indicated in red in Table 4. The maximum of 0.25 nm is less 

than the uncertainty of ±0.5 nm, taken for λcUV based on the 1-nm resolution specified in the test procedure. These latter 
10 

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values are considered the minimum increment that can be determined for a particular specimen, for each of the four 

characteristics.

The R-R data comparing different materials at the different laboratories was examined to aid the selection of the 

criterion for λcUV. Using the transmittance profiles, various criteria for λcUV were analyzed based on the corresponding 

sr and sR. The λcUV was determined for 10, 25, 50, 72, and 90% transmittance. Both absolute and relative criteria were 

examined, i.e., x% of a chosen maximum transmission, or a transmission of x%. For the relative criteria, the 

transmittance at ~450 nm was used to define the maximum τ. Table 5 summarizes the relative results in addition to the 

Ametek and 10% absolute criteria. sr (nominal) and sR (nominal) are identified in Table 5 for each of the λcUV criteria.

In the table, the minimal sr and sR is seen among the relative transmittance criteria at 50% for both the g/p and g/p/g 

specimens. Several of the criteria approach or exceed the expected ±0.5-nm resolution for λcUV (based on the 1-nm 

measurement increment), particularly if the variable data for material C are neglected.

Table 5: Summary of the λcUV results of the round-robin experiment. The repeatability and reproducibility are provided for several 

criteria used to evaluate λcUV. One extra significant figure is provided in the table as specified in [16]. 

90%

relative

72%

relative

50%

relative

25%

relative

10%

relative

5% relative

(Ametek method)

10%

absolute

MATERIAL CONSTRUCTION sr sR sr sR sr sR sr sR sr sR sr sR sr sR

A g/p 0.22 0.25 0.31 0.39 0.22 0.27 0.31 0.36 0.31 0.49 0.92 0.92 0.31 0.36

B g/p 0.38 0.38 0.31 0.33 0.31 0.38 0.58 0.63 0.49 0.59 1.13 1.16 0.58 0.63

C g/p 2.77 3.20 2.46 2.62 0.97 1.07 0.58 0.72 0.58 0.73 2.47 2.51 0.58 0.72

D g/p 0.87 1.04 0.62 0.64 0.65 0.65 0.76 0.76 0.87 0.92 1.19 1.21 0.76 2.28

E g/p 0.49 0.49 0.49 0.49 0.62 0.62 0.98 0.98 2.37 2.37 1.25 1.25 0.98 0.98

x, Avg g/p 0.95 1.07 0.84 0.89 0.55 0.60 0.64 0.69 0.92 1.02 1.39 1.41 0.64 0.99

A g/p/g 0.31 0.35 0.00 0.28 0.00 0.29 0.38 0.51 0.00 0.51 0.00 0.45 0.31 0.47

B g/p/g 0.00 0.13 0.31 0.37 0.00 0.29 0.38 0.49 0.44 0.57 0.63 0.78 0.31 0.54

C g/p/g 9.89 9.89 3.09 3.39 2.22 2.22 2.00 2.00 2.30 2.30 1.33 1.78 2.44 2.44

D g/p/g 1.98 1.98 1.86 1.86 1.70 1.70 2.01 2.01 2.28 2.28 1.79 1.79 2.28 2.28

E g/p/g 0.44 0.44 0.49 0.49 0.22 0.29 0.44 0.52 1.05 1.37 1.43 1.43 0.69 0.90

x, Avg g/p/g 2.52 2.56 1.15 1.28 0.83 0.96 1.04 1.11 1.21 1.41 1.04 1.24 1.20 1.33

The Ametek method was found to be the least repeatable and reproducible. Furthermore, it was observed during the 

λcUV analysis that the Ametek method (because it extrapolates results for τ = 5%) could render results that were not 

located on the measured data profile. Note that the presence of airborne or specimen absorbed moisture can introduce 

variability in optical measurements below the UV-B wavelengths.

To interpret the relative results in Table 5, the most repeatable and reproducible data generally correspond to that 

obtained at steep slopes in the transmittance profile: 20% < τ < 70%. For λcUV, one could be concerned that the initial (τ

< ~10%) and final portions (τ > ~80%) of the optical transition profile have a shallow slope, and corresponding reduced

repeatability and reproducibility. Furthermore, the aging and soiling of transparent polymeric materials typically results 

in the rounding of the final portion (τ > ~80%) of the transition profile [17],[18],[19]. The profile rounding from the 

aging of polymers often occurs with the formation of discoloring chromophore species [17],[18]. The profile rounding 

from soiling typically results from increased optical scattering as the size of the particulate contamination becomes 

comparable to light of shorter wavelength [19]. Both aging and soiling typically affect PV materials at wavelengths that 

are examined by YI. In contrast, the initial portion of the transition profile typically retains its slope, even with aging and 

soiling. The location of the initial portion of the transition profile may shift, however, if the UV absorber or UV 

stabilizer additives in the specimen become depleted by aging [18]. A shift in the onset of transmittance of PV materials 

is not typically examined in the YI. 
11 

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The R-R participants chose to identify λcUV using the 10% absolute τ criterion. This criterion has a specific meaning 

(optical absorbance of 1) that is readily understood in the literature. Furthermore, the 10% definition distinguishes 

between the loss of UV absorber or UV stabilizer additives and the formation of chromophore species. That is, λcUV and 

YI are not redundant characteristics.

One additional pitfall in the determination of λcUV occurs for spectra with one or more UV-transmitting wavebands. For 

example, it is not uncommon for UV absorber additives in polymers to exhibit a doublet profile, with two absorbance 

peaks separated by a relatively transmitting waveband. This dilemma can be addressed by the algorithm used to 

determine λcUV, if the search is made to start in the visible and continued into the UV wavelengths, until the first λcUV is 

found. 

The results of the R-R are summarized in Table 6, for τsw, τrsw, YI, and λcUV. The average (x), standard deviation (sx, 

1σ), repeatability (sr, nominal), and reproducibility (sR, nominal) are identified in Table 6 for the four characteristics

examined in the standard. For the R-R analysis, sr now emphasizes repeatability within the same laboratory, whereas sR

emphasizes reproducibility between different laboratories. As identified in orange, the maximum sr values of ±0.78%, 

±0.63%, ±0.27, and ±2.4 nm are identified for τsw, τrsw, YI, and λcUV, respectively. The maximum sR values of ±0.88%, 

±0.90%, ±0.46, and ±2.4 nm are similarly identified in red for τsw, τrsw, YI, and λcUV. For τ and YI, the sr and sR of the 

hazy polymer, material D, approaches 10x that of the other materials. This identifies that haze can increase sr and sR. 

Material C, which had a λcUV < 250 nm, also had the greatest sr and sR values for λcUV. The sr was generally less than 

the sR, i.e., the between laboratory variation is larger than the internal laboratory variation. As in Table 3 and Table 4, the 

τsw in Table 6 is typically less than the τrsw. As in Figure 4, the τ for the g/p specimens is typically less than that of the 

g/p/g specimens, which most likely results from refractive index difference between the polymers and the glass. 

Table 6: Summary of the round-robin experiment results. One extra significant figure is provided in the table as specified in [16].

τsw {%} τrsw {%} YI {unitless} λcUV {nm}

MATERIAL CONSTRUCTION x sx sr sR x sx sr sR x sx sr sR x sx sr sR

A g/p 89.178 0.088 0.145 0.161 91.148 0.078 0.117 0.134 0.528 0.020 0.029 0.034 356.58 0.31 0.31 0.36

B g/p 87.505 0.056 0.167 0.167 90.098 0.055 0.081 0.093 0.765 0.032 0.034 0.046 381.33 0.41 0.58 0.63

C g/p 92.573 0.143 0.227 0.254 93.701 0.141 0.241 0.266 0.325 0.062 0.029 0.067 206.76 0.45 0.58 0.72

D g/p 84.525 0.818 0.257 0.852 85.743 0.858 0.217 0.882 2.422 0.370 0.128 0.388 356.13 0.37 0.76 2.28

E g/p 86.834 0.043 0.264 0.264 89.186 0.058 0.155 0.156 2.895 0.082 0.234 0.234 365.63 0.63 0.98 0.98

x g/p 88.12 N/A 0.21 0.34 89.98 N/A 0.16 0.31 1.39 N/A 0.09 0.15 333.3 N/A 0.6 1.0

A g/p/g 89.509 0.111 0.058 0.123 91.627 0.102 0.063 0.118 0.490 0.019 0.024 0.030 357.42 0.37 0.31 0.47

B g/p/g 88.158 0.073 0.080 0.104 90.668 0.072 0.050 0.086 0.704 0.023 0.026 0.033 381.13 0.46 0.31 0.54

C g/p/g 91.215 0.101 0.777 0.777 93.089 0.084 0.149 0.162 0.468 0.091 0.177 0.178 210.19 0.41 2.44 2.44

D g/p/g 84.700 0.641 0.643 0.875 85.844 0.680 0.626 0.897 2.732 0.378 0.272 0.456 360.54 0.28 2.28 2.28

E g/p/g 87.762 0.152 0.595 0.595 89.893 0.072 0.104 0.121 2.556 0.068 0.075 0.097 360.63 0.62 0.69 0.90

x g/p/g 88.27 N/A 0.43 0.49 90.22 N/A 0.20 0.28 1.39 N/A 0.11 0.16 334.0 N/A 1.2 1.3

The range of optical transmittance for the materials in Table 6 is at least 5%, several times greater than the sr or sR. The 

different performance intrinsic to the materials examined in the R-R is readily observed if the transmittance profiles are 

superimposed. Because the thickness of the polymer layer is nominally the same (z= 0.5 mm), the different performance 

is attributed to the base materials and the formulations. A manufacturer may also wish to consider characteristics in 

addition to τ, including: electrical resistivity; elastic modulus; moisture permeability; and UV-durability, when selecting 

an encapsulation for use in PV modules. The maximum sr and sR values in Table 6 are taken as the conservative estimate 

for error in τsw, τrsw, YI, and λcUV. To explain, if the 1σ average in Table 6 were used to evaluate the 95% limits, the 

maximum sr and sR values slightly exceed the confidence limits identified in Ref. [16], where r = 2.8·sr and R = 2.8·sR. 

The τ profiles varied the most between the laboratories for the hazy material D. The λcUV of material C approaches the 

limitations of measurement for PMT detectors in a moisture-containing atmosphere, corresponding to the greatest sr and 

sR values for λcUV. As in Figure 3, Figure 4, and Figure 5, and further examined in Ref. [13], all of the encapsulation 

materials have absorbing bands in the IR wavelengths; therefore, it is not surprising that the τsw is less than the τrsw

(which only extends to 1250 nm). Lastly, the lesser τ for the g/p specimens is consistent with Figure 4, where the 
12 

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difference was attributed to the lesser n (better matched to air) of silica. In contrast, because the n of material C is 

uniquely less than that of silica, a greater reflectance loss is expected for the g/p/g specimens, as observed in Table 6. 

4. CONCLUSIONS

A round-robin experiment (R-R) was conducted to support a recently proposed standardized test method for the 

measurement of PV encapsulation materials. Key results include: (1) Threshold values of ±0.03%, ±0.01, and ±0.5 nm

were identified for transmittance (τ), yellowness index (YI), and UV cut-off wavelength (λcUV) measurements as the 

minimum increment that can be determined by contemporary spectrophotometer instruments for each of the 

characteristics. Practical values of ±0.06%, ±0.02, and ±0.5 nm are identified for τ, YI, and λcUV as the minimum 

increment that can be determined for a particular specimen using a trusted instrument, for each of the characteristics. (2) 

The criterion 10% of the absolute transmittance was shown to provide a reasonable repeatability (sr) and reproducibility 

(sR), with respect to other relative or extrapolated criteria. The 10% criterion for λcUV also helps to distinguish between 

the loss of UV additives (absorbers and stabilizers) and the formation of chromophore species. (3) The maximum sr

values of ±0.78%, ±0.63%, ±0.27, and ±2.4 nm were identified for τsw, τrsw, YI, and λcUV, respectively in the R-R. The

maximum sR values of ±0.88%, ±0.90%, ±0.46, and ±2.4 nm were identified for τsw, τrsw, YI, and λcUV. (4) The

maximum sr and sR values of ±0.29%, ±0.29%, ±0.09, and ±1.3 nm were identified for τsw, τrsw, YI, and λcUV, 

respectively, for specimens of the same material made at the different laboratories. This suggests that although there is 

an increased variability associated with the specimen make (including the silica glass and lamination process), it does 

not exceed the variability associated with the choice of encapsulation material and/or instrument used for measurement.

Additional experiments demonstrated the benefit of using a depolarizer, the representative performance for silica 

substrates and superstrates, and a baselining protocol to correct port effects in single-beam spectrophotometer 

instruments. Although haze-prone materials were found to limit τ and YI, it is uncertain whether material discoloration 

would similarly limit transmittance measurements.

ACKNOWLEDGMENTS

The authors are grateful to Mikel Ezquer Mayo of Fundación CENER-CIEMAT, Johannes Hanek of Fraunhofer ISE, 

and Scott Deibert and Dr. Mike Kempe of the National Renewable Energy Laboratory for their help with measurements 

and discussion of the results. This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-

08GO28308 with the National Renewable Energy Laboratory. Instruments and materials are identified in this paper to 

describe the experiments. In no case does such identification imply recommendation or endorsement by NREL.

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