The thermal conductivity of agar gel (with 5-gram agar powder per liter of water) is tested, as part of the quality control measures when we test soil or similar gel-like materials.
Both quantities are about the reflectance of material surfaces. However, they are different.
- Daylight reflectance is about the reflectance of a surface to visible light.
- Solar reflectance is about the reflectance of a surface to solar energy.
Shown below is the solar radiation spectrum (red color part is for the sunlight at sea level). The solar radiation consists of 3 parts: 1) ultraviolet (UV) radiation; 2) visible light and 3) infrared (IR) radiation.
- Daylight reflectance is in terms of visible light only.
- Solar reflectance is in terms of UV radiation, visible light, and IR radiation, i.e. the entire solar radiation spectrum.
Additionally, in the lab, the daylight reflectance test and solar reflectance test are two different tests:
- Daylight reflectance test: only the spectral reflectance in the 380 nm – 780 nm range (the visible light range) is measured. Typically, we also separate the total, diffuse and specular components of it.
- Solar reflectance test: the spectral reflectance in the 300 nm – 2500 nm range (the entire solar spectrum, including UV radiation, visible light, and IR radiation) is measured. Typically, we also measure the material thermal emittance and calculate the solar reflectance index (SRI).
The optical properties of a thermochromic glass change with the glass temperature. For example, the glass is tinted with increased glass temperature.
The optical properties of an electrochromic glass change with the electrical control. For example, the glass is tinted with an electrical voltage applied.
The optical and thermal properties (e.g. visible light transmittance or VLT, solar heat gain coefficient or SHGC, and shading coefficient) of a thermochromic glass or an electrochromic glass can be tested as an ordinary glass, as described in this article, except that the optical states need to be properly controlled in the lab.
Thermochromic glass optical state control
For thermochromic glasses, the glass temperature needs to be controlled. Shown below is the setup available at OTM for thermochromic glass temperature control.
A rubber pad heater with a 1-inch circular opening at the center (the amber color part) is attached to the thermochromic glass surface. The power output to the heater is regulated by a temperature controller, based on a thermocouple attached to the glass surface.
With this setup, we’ve successfully measured one thermochromic glass in the temperature range from room temperature to 65 °C, with temperature stability better than 1 °C and temperature accuracy better than 2 °C.
Electrochromic glass optical state control
For electrochromic glasses, an electrochromic glass controller needs to be provided by the customer. It can be a simple device with a constant DC voltage output. Shown below is the setup.
We’ve measured a few electrochromic glasses, with customer’s controller.
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What is haze?
As illustrated above, for the incident light (black color ray) transmitted through a transparent material, there are two components:
- Specular transmission (blue color ray): the component in the same direction of the incident light
- Diffuse transmission (red color ray): the component scattered in all directions
The fraction of the visible light transmitted through a transparent material is its luminous transmittance. There are total, diffuse and specular luminous transmittances:
- Specular luminous transmittance: the fraction of specularly transmitted visible light
- Diffuse luminous transmittance: the fraction of diffusely transmitted visible light
- Total luminous transmittance: the sum of the above two, total luminous transmittance = specular luminous transmittance + diffuse luminous transmittance
If the diffuse transmittance of a transparent material is large, the material appears hazy when viewing objects through it. The haze of a transparent material is defined as the ratio of the diffuse luminous transmittance to the total luminous transmittance:
Haze = 100 × diffuse luminous transmittance / total luminous transmittance
To have a clear view through a transparent material, it is preferred that the material is with high luminous transmittance and negligible haze.
How is haze measured?
- Total luminous transmittance measurement: in this mode, the reflectance port is covered and the specular transmission component is included (SCI).
- Diffuse luminous transmittance measurement: in this mode, the reflectance port is uncovered and the specular transmission component is excluded (SCE).
With the two measurement modes, the total and diffuse luminous transmittances can be measured and the haze can be calculated.
The haze measurement results are dependent on the instrument integrating sphere geometry. When the measurements are performed with the same instrument, most error sources are cancelled out and the haze measurement results are very accurate.
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We’ve helped a few customers in determining the thermal conductivity of thin materials, such as paint and coating, according to ASTM D5930.
In general, the thermal resistance of thin materials is negligible. In case it is necessary to determine the thermal conductivity of paint and coating. A pair of special samples, with thick paint or coating, need to be prepared, as illustrated below.
- Substrate (the blue color part): the substrate needs to be flat and rigid. The substrate type does not affect the measurement result. Typical substrate types include glass, metal plate, and wood plate.
- The preferred substrate size is 50 mm x 50 mm; The minimum size is 30 mm x 30 mm and the maximum size is 100 mm x 100 mm.
- Thick paint or coating (the yellow color part): the paint or coating needs to be applied onto one side of the substrate, with large enough thickness
- The preferred thickness is 3 mm or larger; the minimum thickness is 1 mm.
- Sample quantity: two samples in a pair are needed.
Shown below is the arrangement during testing.
The measurement probe is a thin film (less than 0.1 mm in thickness, the red color part) with a tiny wire inside (refer to our thermal conductivity test page for details). The probe is sandwiched between the two test samples, next to the paint or coating material.
Because the probe is in contact with the paint or coating material and the measurement duration is very short, it is equivalent to insert a tiny wire into a bulk material block made of the paint or coating. Only the thermal conductivity of the paint or coating is measured and the result is not affected by the substrate material.
We are often requested to test the solar reflectance index (SRI) of membrane products, particularly liquid applied membrane products.
Unlike paints, membrane products can be free-standing. A membrane product sample can be prepared either as a standalone membrane layer or with a substrate. The question from many customers is whether the SRI of membrane products can be tested without substrate.
To answer this question, two examples are illustrated below:
The sample on the left is translucent (with large transmission). For such samples, a substrate is required.
The sample on the right is opaque (with zero transmission). For such samples, a substrate is not required.
For a material, we have the following relationship:
Solar transmittance + solar reflectance + solar absorptance = 1
In SRI calculations, the solar transmittance is assumed 0 (solar transmittance = 0). The solar reflectance is directly measured by the instrument. Therefore, we use the following relationship to calculate the solar absorptance:
Solar absorptance = 1 – solar reflectance
However, if the material is not opaque (solar transmittance ≠ 0, for example, the left example), the equation above is not valid.
For such translucent samples, the calculated solar absorptance is higher than the actual solar absorptance, as the solar transmittance is counted as part of the solar absorptance, and the resultant SRI is therefore lower.
In order to eliminate this error, translucent samples shall not be used for SRI testing. If a membrane product is translucent, it shall be applied onto a suitable substrate for testing.
To determine if a membrane sample is translucent, a simple method is to check if the flashlight from a phone can transmit through the sample, as shown in the examples above.
In our website, we explain that the solar reflectance index (SRI) is the surface temperature in a 0 – 100 scale. In the same section, we also mention that It is possible for SRI to be negative or larger than 100. So, what is the theoretical range of SRI? What is the possible minimum SRI and maximum SRI?
We can easily find the minimum and maximum SRIs with our online SRI calculator.
- Minimum SRI: the minimum SRI is the SRI of a surface with solar reflectance = 0 (perfect black color in the solar radiation spectrum) and emittance = 0 (perfect white body in the infrared radiation spectrum)
- Mininum SRI = -244.6 (low-wind), -99.7 (medium-wind), or -44.9 (high-wind)
- Maximum SRI: the maximum SRI is the SRI of a surface with solar reflectance = 1 (100%, perfect white color in the solar radiation spectrum) and emittance = 1 (perfect black body in the infrared radiation spectrum)
- Maximum SRI = 133.0 (low-wind), 129.6 (medium-wind), or 128.0 (high-wind)
Therefore, the theoretical range of SRI is:
- Low-wind: -244.6 ~ 133.0
- Medium-wind: -99.7 ~ 129.6
- High-wind: -44.9 ~ 128.0
Shown below are the calculation screenshots.
With the theoretical SRI range presented above, is it still valid to say that the SRI is the surface temperature in a 0 – 100 scale?
Yes, it is still valid. As most natural surfaces are with high emissivity (emittance > 0.8), the SRIs of most natural materials are in the range of 0 – 100.
Surfaces with low emissivity (low-e, e.g. emissivity < 0.2) are typically bare metal surfaces (e.g. aluminum or stainless steel). In practice, they are rarely directly used as the top layer of roof or pavement materials. When such low-e surfaces are painted, the emissivity is high (as the paint layer becomes the top layer).
Why the SRI of a low-e surface is low?
There are 3 heat transfer modes: conduction, convection, and radiation. For a low-e surface, the radiative heat exchange between the surface and the ambient environment is weak (i.e. less heat transfer via radiation). More heat is kept on the low-e surface and it results in higher surface temperature and, therefore, lower SRI.
In the low-wind condition, the convection is weak and the radiation is more dominant. This is the reason that the dependence of SRI on emittance is stronger at the low-wind condition.
We are sometimes requested to test the thermal conductivity of thin materials, such as paint, coating and metal sheet.
Negligible thermal resistance of thin materials
For a wall (or a roof) system, the influence of such thin materials to the overall wall system U-value is negligible.
Shown below is an example calculated with our online ETTV U-value calculator. it is obvious that the thermal resistance of a 0.2 mm thick paint layer with 0.2 W/(m⋅K) thermal conductivity is only 0.001 (m2K)/W, which is negligible comparing to the thermal resistances of other layers (e.g. concrete, plaster, or insulation wool).
For thin metal sheets, e.g. 0.7 mm thick aluminium plates, the thermal resistance is further smaller, as the thermal conductivity of metal is much larger.
The reason is that thermal resistance is dependent on both thermal conductivity and thickness, with the following relationship:
Thermal resistance = Thickness / Thermal conductivity
In practice, due to the small thickness of thin materials (typically less than 1 mm), it is not practical to reduce the thermal conductivity of thin materials to achieve better insulation.
In wall/roof U-value calculations, the thin materials can be simply ignored. It is not meaningful to get the thermal conductivity of thin materials.
Thin material thermal conductivity testing
It may be still necessary to determine the thermal conductivity of thin materials. For example, the thin material is not used in a wall/roof system, but in a system with low thermal resistance.
For such scenarios, we can test the thermal conductivity of thin materials according to ASTM D5930, with the following practices:
- For low thermal conductivity paint and coating, the paint/coating can be applied onto flat substrates for testing.
- There is no specific requirement on the substrate type, as long as the substrate surface is flat. We recommend flat metal plates or glass plates.
- The preferred substrate size is 50 mm x 50 mm (minimum size: 30 mm x 30 mm; maximum size: 100 mm x 100 mm). There is no requirement on the substrate thickness, as long as it is strong enough.
- The thickness of paint or coating should be thick enough (3 mm or thicker preferred, 1 mm minimum)
- 2 pieces of painted/coated samples are requried
- We cannot test materials with large thermal conductivity [i.e. > 10 W/(m⋅K)], for example, aluminium or stainless steel.
Please refer to our thermal conductivity page for more details.