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:
Step 1. Fresh SRI measurement: a fresh sample is measured before weathering
Step 2. Weathering: the fresh sample is returned to the customers for weathering
Step 3. Aged SRI measurement: an aged sample is measured after weathering
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:
User weathering: the customers perform the weathering following their in-house methods (which are determined internally or are mutually agreed by all relevant parties). For all aged SRI testing conducted by us so far, this option was employed.
Weathering according to ANSI/CRRC S100: in ANSI/CRRC S100, detailed natural weathering (field exposure) and laboratory soiling and weathering methods are defined. The natural weathering requires a 3-year weathering duration. The laboratory soiling and weathering is based on ASTM D7897.
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:
For spectral transmittance testing: a glass of the size 1.0 m × 1.9 m
For spectral reflectance testing: a solar panel of the size 1.1 m × 2.3 m
Due to the extra effort required to handle such large samples, some oversize surcharge is applicable to samples larger than 300 mm × 300 mm.
It is well known that there are three heat transfer modes:
Conduction
Convection
Radiation
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):
The surface emits radiation to the surroundings (characterized by its emissivity)
The surface absorbs radiation emitted by the surroundings (characterized by its absorptivity)
The surface reflects radiation emitted by the surroundings (characterized by its reflectivity)
There are two relationships:
Emissivity = Absorptivity (Kirchhoff’s law of thermal radiation)
Absorptivity + Reflectivity = 1 (conservation of energy)
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).
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.
Two examples: opaque black glass and opaque white glass
Listed in the table below are the optical & thermal properties of two opaque glasses:
Opaque black glass: a perfect black glass with 0% transmittance and 0% reflectance
Opaque white glass: a perfect white glass with 0% transmittance and 100% reflectance
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.
The theory: secondary solar heat gain
As shown in the sketch below, there are two components in the glass solar heat gain:
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.
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.
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:
Measure the thermal conductivity of each layer individually (k1 and k2). Note: the samples need to be prepared individually too, as the method can measure the top layer only.
Calculate the overall thermal resistance: R = d1/k1 + d2/k2, where d1 and d2 are the thickness of each layer.
Calculate the apparent thermal conductivity: k = (d1 + d2)/R
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.
