Single Glazing Optical & Thermal Property Testing Procedures
Single glazing glasses are glasses without gas spaces. Laminated glasses or glasses with window film are treated as single glazing in the lab, as they do not have gas spaces, despite that there are multiple layers.
The testing procedures for all single glazing glasses are the same. This article aims to explain the step-by-step procedures for single glazing optical & thermal property testing.
- Testing procedures
- More information
Step 1. Sample preparation
Upon receiving a single glazing test sample, the sample is cleaned and labelled.
For sample labelling, each sample is assigned a unique sample ID. The sample facing is identified and labels. The side facing outdoors is labelled “Front”.
The sample dimensions are measured. The glass thickness affects the thermal property results and it is measured with 0.1 mm resolution.
The photos of the sample are taken and the essential information of the sample is recorded in the list of test samples.
Shown below are a sample with labels and the information recorded in the list of test samples.
Step 2. Spectral reflectance measurement with FTIR
The second step is to measure the spectral reflectance of the sample in the 5 micrometer – 25 micrometer range. This range is the Infra-Red (IR) range. The results are for glass emissivity calculation.
The instrument, Fourier Transform Infra-Red (FTIR) spectrometer, does not measure emissivity directly. The spectral reflectance measured needs to be processed further to get the emissivity. Shown below is the FTIR used at OTM.
Step 3. Spectral transmittance/reflectance measurement with UV/VIS/NIR
This step is the most important measurement step, as most optical & thermal properties are highly dependent on the results obtained in this step.
The spectral transmittance/reflectance of the glass sample in the 300 nm – 2500 nm range is measured with a UV/VIS/NIR spectrophotometer.
Natural sunlight consists of Ultra-Violet (UV), VISible light (VIS) and Near-Infra-Red (NIR) radiation. UV/VIS/NIR is the instrument that covers the entire solar spectrum.
The results measured by the UV/VIS/NIR are for solar energy transmittance/reflectance and visible light transmittance calculations. However, the instrument does not report these results directly. The results measured need to be further processed to get these results. Shown below is the UV/VIS/NIR used at OTM.
Step 4. IGDB file generation with IGDB@OTM
The result files generated by the FTIR and UV/VIS/NIR instruments cannot be directly used by the glass performance calculation software. They need to be formatted to the LBNL IGDB format.
The International Glazing DataBase (IGDB) format is a format developed by the Lawrence Berkeley National Laboratories (LBNL). Glass optical data in the IGDB format can be easily processed with the software tools popular in the glass industry.
At OTM, we use the in-house software, IGDB@OTM, to automate this process. Shown below are the screenshots of the software and an example IGDB format file.
Step 5. Result calculation with LBNL Optics/WINDOW or Glazing@OTM
With the IGDB format file, it is straightforward to calculate the final results to be reported.
If the NFRC methods are used, the Optics/WINDOW software by LBNL is used. If the ISO/EN methods are used, the in-house software Glazing@OTM is used.
Shown below are the screenshots of the software tools.
Step 6. Report writing
The last step is to write the test report according to the software calculation results.
Shown below is the result table based on the NFRC methods.
Steps 1 – 6 are the common steps for single glazing optical & thermal property testing. It is important to note that all results reported in Step 6 cannot be directly tested. The instrument measured results always need to be processed to get the final results.
If you are interested in more in-depth information, please continue reading this article. If your concerned questions are not explained, please feel free to leave a comment at the end of this page. This article will be reviewed and updated regularly.
What are the processing time and lead time?
The typical processing time for testing a single glazing is 4 – 6 hours.
We have robust procedures to ensure result accuracy. To fully comply with the procedures, our processing time is 2 – 3 times of that by a typical in-house lab.
Thanks to the extensive efforts, we are confident that our results are always accurate and reliable.
The typical lead time is 5 working days. We may schedule the activities on different days, due to operational reasons.
What are the meanings of the test results?
Essentially, the test results are the glass performance in terms of optical & thermal transmission/reflection.
The first 3 results are the solar properties, the performance of the glass in terms of solar radiation:
- Solar energy transmittance: the fraction of solar energy transmitted through the glass;
- Solar energy reflectance, front: the fraction of solar energy reflected by the front side of the glass;
- Solar energy reflectance, back: the fraction of solar energy reflected by the back side of the glass.
The second 3 results are the luminous properties, the performance of the glass in terms of light visible to human eyes:
- Visible light transmittance: the fraction of visible light transmitted through the glass;
- Visible light reflectance, front: the fraction of visible light reflected by the front side of the glass;
- Visible light reflectance, back: the fraction of visible light reflected by the back side of the glass.
For the 6 results listed above:
- OTM typically reports them as decimals between 0 – 1. They can be converted to percentages by multiplying 100%. Example, solar energy transmittance = 0.347 = 34.7%.
- The front side and back side reflectances could be different, particularly for glasses with coating. However, the front side and back side transmittances are always the same. That is the reason that there is only one transmittance value.
The last 4 results are the thermal properties, the performance of the glass in terms of indoor/outdoor heat exchange through the glass:
- Solar heat gain coefficient (SHGC): the fraction of solar energy transmitted through the glass as heat;
- Shading coefficient: the SHGC scaled by 0.87;
- Summer condition U-value: the amount of heat transmitted through the glass under the summer conditions;
- Winter condition U-value: the amount of heat transmitted through the glass under the winter conditions.
For the 4 results listed above, all 3 heat transfer modes, i.e. conduction, convection and radiation, are involved. Therefore, they are thermal properties.
For the first 8 results, i.e. 3 solar properties, 3 luminous properties and 2 emissivity results, only radiation is involved. Therefore, they are optical properties.
The 2 emissivity results are important to the thermal properties. They indicate the radiative heat exchange capability of the glass surface. Lower emissivity means less radiative heat exchange. Many glasses are coated with low-emissivity (low-e) coating, for better thermal performance.
Shown below is the categorization of the optical & thermal properties.
How to calculate glass optical properties from spectral transmittance/reflectance results?
The instruments, FTIR and UV/VIS/NIR, measure the spectral transmittance/reflectance of a glass only.
Spectral transmittance/reflectance is the transmittance/reflectance at a specific wavelength, e.g. 550 nm (narrowband).
The 3 solar properties, 3 luminous properties and 2 emissivity results can also be called the broadband properties, as they are in terms of a wavelength range (broadband).
In comparison, the spectral transmittance/reflectance is in terms of a specific wavelength, whereas the broadband transmittance/reflectance is in terms of a wavelength range.
If the spectral transmittance/reflectance values in a wavelength range are known, it is straightforward to calculate the broadband transmittance/reflectance with weighted averaging. Weighted averaging is used because the spectral distribution of solar radiation is not uniform.
Shown below is an illustration of the solar radiation spectrum. The different irradiance levels of solar radiation at different wavelengths are the weights used in the weighted averaging.
The actual weights to be used are defined in the relevant NFRC, EN or ISO methods. There are different weights for solar properties and luminous properties.
With the weighted averaging method, it is not necessary to simulate solar radiation in the lab. Standard instruments, i.e. FTIR and UV/VIS/NIR, can be used for glass optical & thermal property testing. It is more cost-effective and it is also easier to control the measurement accuracy.
Last update: 30/07/2023
Work with the expert in Optical & Thermal Measurement solutions.