Lab test of Solar Reflectance Index (SRI), SAC-SINGLAS accredited & internationally recognized
We are a Singapore-based third-party test laboratory, providing lab test services of solar reflectance index (SRI). Our lab is SAC-SINGLAS (ISO 17025) accredited and our customers are from around 30 countries.
Solar reflectance index (SRI) is an indicator of material surface temperature under solar radiation. It is useful in urban heat island (UHI) effect mitigation.
The SRI of a material is dependent on its
- Solar reflectance
- Emittance
Shown on the right is the heat transfer mechanism affecting the surface temperature under solar radiation. refer to our detailed explanations: what is solar reflectance index (SRI)?
We test the following important properties of your materials:
- Solar reflectance and absorptance
- Emittance
- Solar reflectance index (SRI) at three wind conditions (low, medium and high)
Online SRI calculator
Interested in performing your own SRI calculations? We have a user-friendly online calculator:
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Solar reflectance index (SRI) is an important material property in urban heat island mitigation applications.
SRI: surface temperature in a 0 – 100 scale
Layman’s experiences tell us that, under sunlight, a black color surface is much hotter than a white color surface.
Cooler surfaces are preferred for urban heat island mitigation. However, it is troublesome to directly compare surface temperatures in material selection, as the surface temperature is dependent on many factors, such as solar radiation level and wind speed.
SRI is a standardized method for material surface temperature comparison, with the following features:
- The temperatures are calculated under 3 standard conditions
- The SRIs are calculated with reference to 2 standard materials
Surface temperature under standard conditions
Shown on the right is the heat transfer mechanism of a material under solar radiation.
The surface temperature is determined by the following two factors:
- Absorbed solar radiation: the amount of solar radiation absorbed by the material
- Dissipated heat: the amount of absorbed solar radiation dissipated to the ambient air or space
Absorbed solar radiation
The absorbed solar radiation is dependent on the solar absorptance of the material.
Solar absorptance is the fraction of solar radiation that is absorbed by a surface. For example, out of 500 W of incident solar radiation, 300 W of it is absorbed by a surface. The solar absorptance of the surface is then 300 W / 500 W = 0.60.
The absorbed solar radiation can also be understood as dependent on the solar reflectance of the material.
Solar reflectance is the fraction of solar radiation that is reflected by a surface. For opaque materials, the incident solar radiation is either absorbed or reflected. Therefore, the sum of solar absorptance and solar reflectance is 1. For the example above, the solar reflectance of the surface is 1 – 0.60 = 0.40.
Solar reflectance is also called total solar reflectance (TSR). TSR is commonly used by the industry, but in the ASTM standards, solar reflectance is the standard name used, instead of TSR.
Dissipated heat
The dissipated heat is dependent on the emittance of the material.
After absorbing some solar radiation, the surface is hotter than the ambient environment and it transfers heat to the air and the surroundings by convection and radiation.
- Convection is the heat transfer due to airflow. It is dependent on airflow speed near the sample.
- Radiation is the heat transfer due to infra-red radiation. It is dependent on the emittance of the material.
For convection, the airflow speed is not a material property and it is defined in the standards. There are 3 standard airflow speeds defined in the SRI calculation model, i.e. low-wind, medium-wind and high-wind. Therefore, all calculations of surface temperatures and SRIs are in terms of one of the standard airflow speeds and there are always 3 sets of results.
Emittance is the ratio of infra-red radiation emitted by a material to that by a perfect black body. The emittances of most general materials are around 0.90.
Emittance is also called emissivity. In practice, emissivity is more commonly used, but emittance is the standard name used in the ASTM standards.
Surface temperature calculation
With the two material properties, solar reflectance (or solar absorptance) and emittance, and the three sets of environmental conditions, at the low-wind, medium-wind and high-wind conditions, the surface temperatures of a material can then be calculated.
For example, as given in the SRI test sample report, for a material with solar reflectance = 0.090 (9.0%) and emittance = 0.90, its temperature under the medium-wind condition is 353.7 K (80.7 °C).
It is easy for a layman to understand that a surface with 80.7 °C temperature is very hot and we even cannot put our hands on such a surface. The reason is that the solar reflectance of the surface is very low and its color is black.
However, it is still not convenient to directly compare the temperature values. SRI is then defined to express the temperature values in a 0 – 100 scale, for easy comparison.
SRI calculation
SRI is the relative surface temperature of a material, with reference to the reference white surface and reference black surface.
- Reference white surface: a surface with solar reflectance = 0.80 and emittance = 0.90
- Reference black surface: a surface with solar reflectance = 0.05 and emittance = 0.90
The SRI of a sample surface is calculated as: SRI = (temperature of the reference black surface – temperature of the sample surface) / (temperature of the reference black surface – temperature of the reference white surface) x 100.
Essentially, the SRI of the reference black surface is SRI = 0; the SRI of the reference white surface is SRI = 100. For most materials, the SRI is between 0 – 100. It is possible for SRI to be negative or larger than 100.
Shown below is the result table extracted from the SRI test sample report.
The results in the green color box are the material properties measured in the lab:
- Emittance = 0.90
- Solar reflectance = 0.090 (9.0%)
- Solar absorptance = 1 – solar reflectance = 0.910 (91.0%)
The results in the red color box are the calculated temperatures and SRI, for the medium-wind condition:
- Reference black surface temperature = 355.6 K = 82.6 °C
- Reference white surface temperature = 317.8 K = 44.8 °C
- Sample surface temperature = 353.7 K = 80.7 °C
- SRI = (355.6 K – 353.7 K) / (355.6 K – 317.8 K) x 100 = 5.0
Apparently, the other two columns of results in the table below are for the low-win and the high-wind conditions, respectively.
How to improve SRI?
The SRI of a material is dependent on two material properties, solar reflectance and emittance.
The emittance of most general materials is close to 0.90 and the room for further improvement is little and technically challenging.
The only practical way to improve SRI is to increase the solar reflectance:
- Option A: use a light color material, as naturally the solar reflectance of light color materials is high.
- Option B: use a material with the same color, but with high near-infrared reflection.
Basically, for solar radiation, there is around half of the energy distributed in the visible light spectrum (i.e. the color we can see) and another half distributed in the near-infrared spectrum (i.e. the invisible radiation).
Option A is to increase the reflectance in the visible light spectrum, whereas Option B is to increase the reflectance in the near-infrared spectrum. It is more technically challenging to use Option B.
How is SRI tested in the lab?
The calculation part is introduced in detail in the texts above.
Please refer to page 35 of our lab profile for some additional details on the test methods.
UV/VIS/NIR spectrophotometer with 150 mm integrating sphere
The instrument used at OTM for solar reflectance test is PerkinElmer Lambda 950 UV/VIS/NIR spectrophotomer, with 150 mm integrating sphere.
For solar reflectance test, the instrument produces monochromatic light in the wavelength range of 300 nm – 2500 nm, at every 5 nm interval. An optical device called intergrating sphere is used to measure the light reflected by the test sample. Shown below is the interior of the integrating sphere. The bright spot in the center is the sampling beam.
Emissometer with small port adapter
The instrument used by OTM for emittance measurement is Devices and Services emissometer with scaling digital voltmeter, model AE1 RD1, and small port adapter. The instrument is able to measure surface hemispherical emittance at room temperature.
Calculation software
OTM also developed a calculation software for SRI calculation, shown below is the software interface.
Materials for SRI testing
We can test nearly all types of roof and non-roof materials. Shown below are some examples:
Sample requirements
The test samples to be tested shall be flat (at least in a small patch) and meet the following size requirements:
- Minimum size: 40 mm x 40 mm
- Preferred size: 100 mm x 100 mm
- Maximum size: 300 mm x 300 mm
It is still possible to test samples larger than 300 mm x 300 mm, but surcharge is applicable. The largest sample tested by us so far is a full size solar PV module, of the size around 0.5 m x 1.5 m
Only 1 sample is needed and it is optional for the client to provide additional samples as the backup.
Additional notes to sample requirements
- Sample thickness: there is no specific requirements on sample thickness. However, for membrane materials, they shall not be too thin to be translucent. If a membrane material is translucent, it should be tested with a substrate.
- Sample flatness: the test sample needs to be flat. It is possible to be rough. However, curvatures and grooves shall be avoid.
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