TC10: Optical Properties of Glass

TC10 activities being based around different optical measurements of glass and coated glass products. However, it retains a sub-group to study basic optical properties.

Planned activities for 2024

Two hybrid meetings are planned for 2024:
March 22nd – Stazione Sperimentale del Vetro, Murano, Italy
October 11th – IFT Rosenheim, Rosenheim, Germany
Activity will focus on completion of the emissivity and Ug value round robins.
Further standardisation discussions are planned relating to prEN 410 Annex D; there may also be another calculation round robin which will include g value and absorption calculations.

Activities in 2023

TC annual meeting
TC10 held two hybrid meetings in 2023:
March 31st – Stazione Sperimentale del Vetro, Murano, Italy
October 6th – OMT Solutions BV, Eindhoven, Netherlands

Round robin studies
Work has continued on the 3 emissivity / Ug value round robins with measurement results to be presented at the next TC10 meeting in March 2024:
Heat treated glass – measurement of emissivity on non-flat test specimens using FT-IR spectrometers with direct reflection units and integrating spheres, alongside the use of emissometers and sheet resistance measurements. Samples delivered to 13 participants in October/November 2023.
Evaluation emissivity measurements – allows laboratories to separate the contributions to the uncertainty in the calibration standard from that of the measurement device and sample – initial characterization performed on 25 sample sets; samples delivered to participants in November 2023.
Round robin according to EN 674 – measurement of Ug value (Guarded hot plate method) using the same coatings as round robin 2). Measurements to be conducted by 3 laboratories and compared against U value calculations made using EN 12898 / EN 673.
There was also a 4th completed calculation round robin:
Modified Matrix Inter-comparison – 7 participants tested Annex D of prEN 410 (Matrix Method for non-scattering Incoherent Optical Systems) using 5 different calculations tools, and 3 different calculation methods (Modified Matrix, EN 410, EN 52022), for single, double and laminated glazing. Calculation results and revision comments from TC10 members were presented in March 2023. After providing feedback to the CEN/TC129/WG9 committee, this work allowed the equations, definitions and calculation examples in the annex to be significantly improved.

Other topics discussed throughout the year include:

NFRC updates relating to the IGSDB (International Glass and Shading Database), Diffuse Glazing, Bird Friendly Glazing and the pyWinCalc python engine
Photochromic glazing
TC10 website/file sharing solution – latest platform to be demonstrated in March 2024.

Planned activities for 2023

Two meetings are planned for 2023:

  • 31st March 2023 – Murano (Italy) – hybrid meeting
  • 6th October 2023 – venue to be decided

Activities will primarily focus on completion of the 3 emissivity round robins alongside supporting standard revision (particularly the on-going revision of prEN 410). There is also an action to revive the extensive TC10 archive and find a permanent hosting/website solution.

Activities in 2022

TC10 held two meetings in 2022, mainly focusing on the preparation of 3 interlaboratory comparisons for measurement of emissivity, and revision of various

European and North American standards:

  • April 8 th , virtual meeting
  • October 7 th , hybrid meeting hosted at HAWK Campus in Göttingen (Germany)

Ingrid Marenne completed her term of 9 years as chair, the TC10 committee once again thanks Ingrid for her leadership and dedication throughout the years. James Farmer was introduced as the new chair.

Emissivity Interlaboratory Comparisons
A sub-group of TC10 has spent significant effort in developing 3 proposals for measurement of emissivity:

  1. Round Robin for heat treated glass – the desired final optical properties of a thermally toughenable coating are achieved by the tempering process; this however leads to a non-flat test specimen which is difficult to characterize using traditional methods. The round robin will focus on measurement of non-flat samples using FT-IR spectrometers with direct reflection units and integrating spheres, alongside the use of emissometers and sheet resistance measurements. Low emissivity as-deposited/heat treated samples have been prepared and initial characterization measurements performed.
  2. Evaluation emissivity measurements – The objective is to allow labs to evaluate the quality of their emissivity measurements, separating the contributions to the uncertainty in the calibration standard from that of the measurement device and sample. Gold reference mirrors have been sourced and characterized.
  3. Round Robin according to EN 674 – The parallel planned round robin tests for emissivity measurements offer the possibility to validate Ug measurements according to EN 674 (Guarded hot plate method) via emissivity measurements according to EN 12898 and U value calculations according to EN 673. A validation of the measurement for inclined and horizontally oriented insulating glass panes as well as the measurements with a downward heat flow can also be carried out.
    Distribution of samples and measurement instructions to participants for all 3 round robins is expected in early 2023.

Optical Standard Development
TC10 members have made significant contribution to the prEN 410:2023 revision (Determination of luminous and solar characteristics of glazing), proposing two new annexes relating to a ‘Modified Matrix Method for non-scattering Incoherent Optical Systems’, and ‘Modifications to the formulae to permit calculation and declaration of the luminous and solar properties of BIPV glazing’.

A section of prEN 17940:2023 (Folio Interlayers for the Manufacturing of Laminated Glass) relating to the derivation of interlayer properties from a laminated glazing was proposed/discussed by TC10 members and accepted within the revision.

Other topics discussed throughout the year include:

  • NFRC Emissivity Task Group – efforts made to standardize emissivity calculation between EN 12898 and NFRC 301 standards (not possible due to EN rounding rules)
  • NFRC Diffuse Glazing Task Group – updates regarding the IGDB (International Glass Database), IGSDB (International Glass and Shading Database) and CGDB (Complex Glazing Database), including new submission rules and criteria
  • ICON Project – addressing complex fenestration systems including light- scattering, geometrically complex and switchable glazing, as well as active solar components and shading devices (Fraunhofer ISE)
  • EQWIN-P – a database network for glazing and other building envelope components (Fraunhofer ISE)
  • Thin-film measurements on glass substrates by imaging spectroscopic ellipsometry

Activities in 2019

In 2019, TC10 met twice, focusing mainly on diffusing glass and on revision of standards:

  • Venice, Stazione Sperimentale del Vetro, April 12th
  • Würzburg, at the Fraunhofer Institute für Silicatforschung ISC, October 25th 2019.

Diffusing sample

We completed a very long study on how to measure diffusing glass with a classical spectrophotometer using an adapted method. We demonstrated that using a diffuser for calibration and during measurement was very helpful if a correction method is applied after measurement. However, we have not found yet a correct commercially available diffuser.

New samples have been prepared by Berliner glass to be used as diffuser. Uppsala university investigated them to identify the most promising solution that could be commercially available. We showed that the diffuse-to-hemispherical ratio should be  as similar as possible for the reference and sample measurement for the method to be validly applied. We identified that the correction for compensation for multiple reflection between sample and diffuser must be done even if the correction is close to 1.

Advantages of method:

  • Avoids errors due to edge losses around direct transmittance exit port for small angle scattering samples
  • Makes anisotropic scattering by sample more isotropic
  • Makes scattering distribution inside sphere more equal for reference and sample


Rules have been proposed to select the right diffuser depending of the sample type (diffusing at small/large angle). We plan to publish those results.

Within the same context of diffusing sample, we had signficant interaction and communication with the committee of NFRC and LBNL. Those 2 groups are working on a new standard that will explain how to characterize diffusing sample correctly. Before publication of the new standard, an ILC will happen, using the equipment described in the document for validation. Our TC10 members having this equipment will take part in the ILC.


The committee of normalization CEN129 WG9 requested ICG TC10 to participate in a round robin test to test the new procedure of validation of simulation tools for optical properties. It will be used to determine the uncertainty which can be accepted and used as a reference for the future. Interaction between EN410 and ISO9050 groups happened and comments from ICG TC10 are expected before revision of the standard. Recommendations were sent.

A new standard ISO23237 (glass in building testing method of light transmission for glass photovoltaics) was suggested. ICG TC10 gave some comment about the utility of the standard (already covered by other standards) and recommended just to adapt EN410 and ISO9050 to cover all cases (area weighting for inhomogeneous sample such as transparent BIPV, screen-printed glass,…)

Low iron glass

We closed the round robin test on transmission of low iron glass. The last step included more devices, increasing the range of instruments. Differences are visible in the steep UV edge, change detector and NIR. All values are within 0.005 of the mean, most of them within 0.003.

Plans and deliverable for 2020

We prepared a new round robin test on tensile stress by optical instrument applying polarization to ensure that float glass can be cut. Samples are already delivered. We intend to compare the results of different instruments using different physical principle and methods next year. Samples were already prepared and sent in 2019.

We are considering the possibility of running a round robin test about measurement of fabrics as it has an impact on solar factor calculation.

Activities in 2018

TC10 hold 2 meetings in 2018. The first one was in April in Murano, at the “SSV”, Statione sperimentale del Vetro in Italy. The second one was in Mons, at “INISMA”, Institut Interuniversitaire des Silicates, Sols et Matériaux”, in Belgium.

RRT validation of measurement capacity

TC10 run a RRT on measurement and calculation of transmittance, reflectance and emissivity according to EN410 and EN673. Some labs should prove their ability to measure accurately for certification needs. In that context, 21 participants from 13 countries sent results. We determine the average value and the standard deviation for each property. In that condition, each participant was able to verify that his results was (or not) correctly positioned compared to other labs.


Measurement of diffusing samples

New light scattering samples were prepared by members of the group with different proportion of normal – diffused transmittance. Evaluation of the new diffusers, potentially available commercially, was presented.  It allowed to determine which was the best diffuser and to understand the direction to take to improve the diffuser. The next step will consist to improve those samples in order to get proper measurement for nearly all diffusing samples with the method developed within TC10 with the strong support of Uppsala University.

Activities and planned actions 2017

Summary of the activities of TC10 in 2017

TC10 held 2 meetings in 2017. The first one happed in Venice, Italy on April 28th, the second one in Rosenheim, Germany on October 20th.

Characterization of diffusing glazing

The main concern in 2017 (and will continue in 2018) of the TC10 is about characterization of diffusing glass product. This include frit glass, enamel, coated enamel or frit, translucent PVB, pattern glass, etched glass or combination of these products. Different projects are running around this subject.

As this subject is becoming more and more important in the glass industry business, a task group was created in the NFRC committee which is called “diffuse glazing task force”. The idea is to include recommendation in the standard to measure diffusing products. Jacob Jonsson is the convenor and Helen Rose Wilson is a member of this task force, (both being member of TC10 also) gave us the progress made in that group. Some members of TC10 participated also to the RRT organized by the NFRC task group in order to define a correct method to measure the sample.

It was shown that once the aperture size reaches 100 mm, light collection is stable and does not significantly increase by using larger apertures. Different configurations were tested on classical 150mm diameter sphere. The best configuration is when using a large beam size and a small aperture. Large spheres were also investigated. The reference being the 630mm sphere from Fraunhofer. A 270 mm sphere was designed by OMT solutions. It was compared with the reference. The study showed that some differences exist between both spheres. Larger steps were observed with the 270 mm sphere from OMT Solutions delivered to some labs. The problems are related to how the detectors see not only the sample but also the reference beam. Adding a diffusor at the reference sphere entrance port was a solution at least for some samples, but not all. Recommendations for the change of grating and detector were done. This lead to a redesign of the 270mm sphere. The upgraded version shown a good agreement between the Fraunhofer/LBNL pgll goniophotometer and Fraunhofer 630 mm sphere. Those results were obtained thanks to 2 RRT inside TC10 and in parallel with NFRC task group. The first included translucent laminated glass, the second one ceramic frit white, grey and black.

In parallel, Uppsala university is working in the development of a new method for 150mm sphere called pull back method. The history of this approach was presented. A round robin test was organized a few years ago which is used by Uppsala.  The principle of the diffusor method is to reduce the change in geometrical distribution of the light transmitted in the 100% measurement and by a light-scattering sample by introducing a diffuser. The diffuser should be strongly scattering over the entire spectrum, thin, have high transmittance and low reflectance. The pull-back method can be used to determine whether a diffuser is suitable for this purpose. However quantitative criteria need to be introduced.

The IR properties of those products are also difficult to collect. OMT solutions explained how emissometer measurements are done in their lab. The accuracy was showed to be around 1-2%. A comparison of the reflectance of glass only compared to fritted glass was shown. There was a decrease in signal in the fritted measurement for both the emissometer and FTIR, but the difference was small. However, the difference between coating and coated frit were much larger, with a 1-2% decrease in signal for the emissometer, and a huge 20% decrease for the FTIR. These results are a warning that large errors can occur when measuring the IR properties of rough samples

OMT solutions also presented new IR reflectance accessory for EN12898 emissivity measurement. VW reflectance principle was implemented.

A summary of the ASTM/NFRC/EN standers for emissivity were detailed. Information about the possibilities of the different labs were distributed.

Solar shading charaterisation

A presentation was given for Tnn (normal-normal transmittance) determination of solar shading devices and textile materials. BRDF technique was presented. Two hypotheses were proposed. The conclusion was that Tnn determination depends on the ratio between the area of the transmittance exit port to the internal area of the sphere and the use of the pullback method to determine if the sample is deemed ‘critical’. This methodology could also be applied to patterned glazing.

Laminate calculation exercise

The final result of a round robin test on how to calculate laminate glass properties based on component properties and based on annex B of EN410 were presented. There was a very good agreement for the first laminate. The second and third ones presented 3 differences noticeable. This presentation was the conclusion of this exercise. However, it was mentioned that a matrix method rather than the annex B equation might be preferred. It is requested to members participating to TC129 WG9 meeting to mention the opportunity to add the matrix method to the standard.

RRT EN410/EN12898

A round robin test for EN410 / EN12898 was initiated motivated by the need for round robin by EU Notified Bodies and laboratories accredited according to ISO17025. 25 sets of results were submitted by 21 laboratories (TC10 members and others) in 13 countries. An initial set of results was presented in April and additional information and data were communicated in October. The standard deviation for visible transmittance was observed to increase when the absolute value was lower. The standard deviation for emissivity increase when emissivity decreases. Apart from a few obvious outliers, there is good agreement.

It was also reported a request from CEN TC129/WG9 concerning a validation procedure for calculation procedure e.g. according to EN410 since the WIS program does not correspond anymore to current version of EN410 and EN673. It was suggested to draw on the previous ICG-TC10 round robin test for the procedure and criteria to define conformity.

RRT low iron glass

A RRT test was carried on transmittance of low iron glass. This interim analysis concentrates on the results at individual wavelengths used for redox calculations. Feedback from 8 members were received and compared to measurement by a metrological institure (PTB) in 2009.However, lower value were obtained than PTB. We suspect that this may be due to sample degradation as they were not kept in a controlled environment. Especially at 300 nm.

Planned activities for 2018:

Two meetings are planned for 2018. The first one will be in Murano (Italy) on April 27th and the second one will be in Mons (Belgium) on October 26th.

TC10 will continue to focus on diffusing product. Especially, we will focus on finding commercial product to apply the pullback method and to find the quantitative criteria for the diffuser. We still continue to follow the revision of standards but also to work proactively to produce input for new standards or revision of standard like certification of modeling tools, EN410, NFRC 300 series,…

We will produce the final report on angular round robin test and write the conclusion of the two last round robin test on low iron transmittance and measurement according to EN410 and EN673.

A general overview of the different methods to measure emissivity will be produced.

The TC website, that has been recently upgraded, will also be updated.


TC10 continued investigations into the measurements of scattering and patterned glass products. They introduced a diffuser that allowed a significant reduction in the divergence of their measurements. A report on variable angle transmission and reflection measurements was written. Finally, a calculation inter-laboratory comparison was achieved on standard EN410 and EN673 that showed that almost all participants had correctly integrated the new rules.

The subgroup continued investigations on the modeling of colouring ions in glass and the effect of the tin layer of float glass on the optical properties. A study on solarisation effects was started.

There were two meeting held in 2013 both for the entire TC and the subgroup.


Planning is as follows:

  • Verification of a predictive algorithm for variable angle transmission based on the RRT.
  • Redaction of the method and criteria for scattering and patterned glass products measurements.
  • Publication of results for the inter-laboratory comparison on variable angle measurements.
  • Collection and analysis of the results of a calculation inter-laboratory comparison for complex glazing.

Activities in 2016

2 meetings were organised in March in Venice and in September in Lanthom.

1. Patterned and light scattering glass

TC10 is interested in understanding the behaviour of pattern and light scattering glass. Indeed those glass with a diffused component are hard to caracterised correctly.

In our previous studies, we have underlighted a new method of measuring those glasses. It includes using a diffusor during the calibration phase as well during the sample measurement. However the diffusor must have some specific characteristic that have been difined previously like : having a high light transmission, be thin and same level of diffusion for all wavelengths. We have thus develop a method called “pull back” method to validate the diffusor. This method was described and send to all participants so each of us was eable to try it. The results will be presented in 2017.

Laminated glass calculation according to EN410

The annex B of standard EN410 (informative) explains the calculations needed for creating simulation of laminates. As the method is not simple, it was decided to check wether the information of the standard can be used in laboratories and exploit in simulations. A set of data was sent to participant so that everybody is using the same raw data. Base on those raw data, participants must create and simulate 3 different laminates with coating and evaluating the optical properties.

5 labs were able  up to now to send results. It appears quite complex to realise but new participants would like to test. Among the 5 results received,  there is a good agreement for the simple laminate. But discrepancies are observed when the laminates become more complexe. Additionnal analysis will be done in 2017 to find possible origin of the differences.

3. Complex glazing performances

Complex glazing is defined as any combination of a window glazing unit with a solar protection device, e.g. a shade, roller blind, shutter, Venetian blind etc. with respect to the glazing unit, the solar protection device may be located externally, internally or between the panes of the glazing unit (mid-pane).

The calculation intercomparison builds upon the ICG TC10 2013/14 comparison of the technical values for unshaded glazing units on the basis of spectral data undertaken to ensure competence in essential calculation procedures following the updating and re-issuing of the European Norms EN 410 and EN 673.

Calculations may be performed by the participants using European Norms and/or International Standard procedures and algorithms from in-house developed methods or nationally or internationally recognized window design tools, e.g. EN 13363-1, EN 13363-2, ISO 15099, WIS (Window Information Systems), WINDOW 7, .Parasol etc.

The objective of this round robin test  is to

  • compare calculated values of key technical values obtained by individual participants using common calculation procedures.
  • Compare the differences in key complex glazing performance parameters found using (i) different international standard calculation procedures (where possible to include simplified and detailed calculation methods) and (ii) different respective calculation tools.
  • Evaluate the suitability and/or limitations of the data file formats employed to characterize shading product and complex glazing performance.
  • Identify where possible measures to be considered to promote increased harmonization of calculation methods and procedures and improve the quality of the calculations.

Only 5 laboratories were able to present complete or partial results. There is a good agreement on light transmission except for venetian blinds at 45°. However U and solar factor show wide variation.

New results are expected in 2017. The project will be closed then. It is considered as interesting but too complex and people maybe notready now forsuch complex calculation.

RRT on emissivity

Certified body laboratories are required to participate to round robin test at regular frequencies. We took the opportunity to include ICG TC10 to the round robin test organised by INISMA. 2016 was the time of the organisation of the RRT. 6 samples were prepared, among them raw glass and coated glass, single, double and triple silver coatings. Samples were distrituted to 26 laboratories with instructions and workbook. The collection of results (transmission, reflexion and emissivity properties) will happen in 2017.

Committee Members

Akdemir, Merve
Anderson, Charles
Bretschneider, Joachim
Daneo, Antonio
Farmer, James (Chair)
Feurer, Thomas
Freinberger, Michael
Goussarov, Andrei
Hofmann, Thomas
Jonsson, Jacob
Kappertz, Olivier
Kermel, Christine
Kinoshita, Taito
Marenne, Ingrid
Mika, Martin
Olive, François
Roos, Arne
Rossi, Giuseppe
Saito, Takayoshi
Stemmler, Ivo
Timmermans, Serge
van Nijnatten, Peter (Vice Chair)
Wilson, Helen Rose

Committee Contact Details

TC10: Optical Properties