Expert in Optical & Thermal Measurement Solutions
We’ve just released our Q3/2023 OTM Insights newsletter, with the main article “Façade & roof material daylight reflectance testing procedures“.
Please click the image below to read the newsletter. For the past issues, please proceed to our branding & publications page.
Related services Solar reflectance index (SRI)
Typically, an solar reflectance index (SRI) test is performed on a fresh sample. Such SRI can also be called fresh SRI. It is often required to determine the aged SRI, to understand the decrease of SRI after weathering, due to soiling and material degradation.
We’ve helped a few customers determine the aged SRI of their materials. Below are the typical steps:
For the fresh and aged SRI measurement part, our usual SRI measurement practices are followed and there are no differences on the lab side, except that a sample is tested twice.
For the weathering part, there are 3 options:
Related services Spectral transmittance / reflectance
We typically advise our customers that the maximum sample size is 300 mm × 300 mm. Most of our customers can provide samples smaller than 300 mm x 300 mm and this size is convenient to handle for both us and our customers, but it is not a technical limit.
Technically, we can test samples greater than 300 mm × 300 mm. The maximum size we can test is limited by the weight (due to safety reasons, samples that are too heavy cannot be lifted and aligned properly) and our lab space (due to the space limit, samples that are too large cannot be handled in the lab).
The maximum sample sizes tested by us so far are:
Due to the extra effort required to handle such large samples, some oversize surcharge is applicable to samples larger than 300 mm × 300 mm.
Related services Emissivity / emittance
It is well known that there are three heat transfer modes:
Emissivity is the key material surface property related to radiative heat transfer. In radiative heat transfer, a surface exchanges heat with the surroundings via radiation (electromagnetic wave):
There are two relationships:
It is easy to calculate the absorptivity and reflectivity, when the emissivity is known.
Most natural surfaces are with high emissivity, around 0.9. Reflective metal surfaces are with low emissivity, around 0.05 or lower. Listed in the table below are the performances of high emissivity and low emissivity surfaces:
| High emissivity surface | Low emissivity surface | |
|---|---|---|
| Radiation emission to surroundings | Emits more radiation | Emits less radiation |
| Absorption/Reflection of radiation from surroundings | Absorbs more radiation Reflects less radiation | Absorbs less radiation Reflects more radiation |
| Overall radiative heat transfer with surroundings | Stonger radiative heat transfer | Weaker radiative heat transfer |
In summary, emissivity is a material surface property characterizing its radiative heat transfer ability. A surface with high emissivity has stronger radiative heat transfer with the surroundings; a surface with low emissivity has weaker radiative heat transfer with the surroundings.
For insulation applications, surfaces with low emissivity are preferred, due to the weaker radiative heat transfer (and therefore better insulation).
Related services Glass optical & thermal properties
As discussed in the article solar control window film and glass optical & thermal performances, tinted solar control window films are often used. Such window films darken the glasses and it results in lower solar heat gain (i.e. smaller SHGC & SC; SHGC: solar heat gain coefficient; SC: shading coefficient).
Some tinted solar control window films are with very low visible light transmittance (e.g. less than 5%). The glass looks almost opaque. However, the SHGC and SC of such glasses are not close to 0 (indeed they are still quite high). This post aims to explain why the SHGC & SC of opaque glasses are still very high.
Listed in the table below are the optical & thermal properties of two opaque glasses:
Bear in mind the relationship: transmittance + reflectance + absorptance = 100%
0% transmittance means that both glasses are opaque.
0% reflectance means the glass absorbs 100% incident radiation and reflects 0% back. The glass looks black.
100% reflectance means the glass absorbs 0% incident radiation and reflects 100% back. The glass looks white.
| Opaque black glass | Opaque white glass | |
|---|---|---|
| Visible light transmittance | 0% | 0% |
| Visible light reflectance, front | 0% | 100% |
| Solar energy transmittance | 0% | 0% |
| Solar energy reflectance, front | 0% | 100% |
| Solar energy absorptance, front | 100% | 0% |
| Solar heat gain coefficient (SHGC) | 0.323 | 0.000 |
| Shading coefficient (SC) | 0.371 | 0.000 |
As presented above, the SHGC & SC of the opaque black glass are still very high, whereas the SHGC & SC of the opaque white glass are 0.
The reason is that the opaque black glass absorbs 100% of solar radiation. This portion of solar heat can still be transferred to the indoor space and it is counted as part of the SHGC & SC.
In practice, most tinted solar control window films are in dark colors (close to the opaque black glass). Therefore, their SHGC & SC are still high, despite of the small transmittance.
As shown in the sketch below, there are two components in the glass solar heat gain:
For more details, please refer to our glass optical & thermal property testing page.
For the opaque black glass, its primary solar heat gain is 0, but the secondary solar heat gain is still high. Actually, the secondary solar heat gain of the opaque black glass is the highest, as its solar energy absorptance is 100%.
For the opaque whie glass, both its primary solar heat gain and secondary solar heat gain are 0. It is possible to archieve zero solar heat gain with the opaque white glass.
Related services Spectral transmittance / reflectance
The UV/VIS/NIR spectrophotometer used by us (PerkinElmer Lambda 950) allows spectral transmittance measurement in the wavelength range of 200 – 2500 nm.
The typical measurements performed by us for building materials are with the wavelength range of 300 nm – 2500 nm (solar spectrum). We’ve supported customers with spectral transmittance measurements in the 200 nm – 300 nm successfully. However, our internal tests showed that it is not possible to perform the measurement in the wavelength range of less than 200 nm. The results are noisy and non-physical.
We’ve just released our Q1/2023 OTM Insights newsletter, with the main article “Curtain wall system thermal property calculation procedures“.
Please click the image below to read the newsletter. For the past issues, please proceed to our branding & publications page.
We are a Singapore-based third-party test laboratory, providing lab test services of material optical & thermal properties. We use Konica Minolta CM-2500d portable spectrophotometer, with the following key information.
Related services Thermal conductivity
We have a post on the measurement of thin paint and coating thermal conductivity with the ASTM D5930 method. Many surfaces are coated with multiple layers of coatings. How to determine the apparent thermal conductivity of such multilayer coating systems?
The method described in the post is only applicable to one single layer of coating. For a surface coated with two or more layers of coatings, the method can only measure the top layer, but cannot determine the apparent thermal conductivity of multiple layers.
To determine the apparent thermal conductivity, the calculation method needs to be used. In this post, we use a two-layer coating system as an example:
For the calculation of the overall thermal resistance, our online ETTV U-value calculator can be used. The R-value result reported is the overall thermal resistance of all layers in the system.
Related services Glass optical & thermal properties
As an alternative to glasses, several transparent plastic materials are utilized as window glazing panels. Transparent polycarbonate sheets and transparent acrylic sheets are two examples of such transparent plastic materials.
Can the standard NFRC/EN/ISO glass test methods still be employed to determine the optical & thermal properties of such transparent plastic materials?
The NFRC/EN/SIO glass test methods are for glazing materials, which are not limited to glasses. Plastics are a type of glazing material. Transparent plastic sheets can be tested by the NFRC/EN/ISO methods when the following conditions are met:
In principle, the scope of the NFRC/EN/ISO glass optical & thermal property test methods is based on the optical characteristics, but not on the material type. Transparent plastic sheets with the same optical characteristics as transparent glasses are within the scope.