Solar reflectance index

Solar absorptance and thermal emittance of transparent/translucent materials

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Related services Solar reflectance index (SRI), Emissivity / emittance

We had a post on the sample requirements of translucent membrane products for SRI testing before: SRI testing: Can membrane products be tested without substrate? Essentially, if a transparent/translucent material is to be laid on top of an opaque substrate, it is required to test the transparent/translucent material together with the substrate.

There are also scenarios that transparent/translucent materials are with standalone installations and without substrates, .e.g. canopies made of fabric materials. In such cases, it is sometimes necessary to determine the solar absorptance and thermal emittance of transparent/translucent materials.

Solar absorptance of transparent/translucent materials

According to ASTM E903, the solar absorptance of a transparent/translucent material can be calculated as:

Solar absorptance = 1 – solar transmittance – solar reflectance

The solar transmittance and solar reflectance can be measured with a UV/VIS/NIR spectrophotometer.

Thermal emittance of transparent/translucent materials

It is out of the scope of ASTM C1371 to measure the thermal emittance of a transparent/translucent material. A manufacturer’s technical note is available to measure the IR transmittance and thermal emittance of transparent/translucent materials, with a portable emissometer.

SRI of transparent/translucent materials

It is out of the scope of ASTM E1980 to determine the solar reflectance index (SRI) of transparent/translucent materials. As suggested by its title, ASTM E1980 is for “horizontal and low-slope opaque surfaces”.

Are solar reflectance, TSR, and albedo the same?

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Related services Solar reflectance index (SRI)

In the context of material solar reflectance index (SRI), solar reflectance, TSR (total solar reflectance) and albedo are the same.

Particularly, solar reflectance is the standard term used in all relevant ASTM standards. TSR and albedo are not used in the ASTM standards, but they are often used by the industry or some literature.

In our test report, we only report the solar reflectance. In case the TSR or albedo values are needed, the solar reflectance results can be directly used, without conversion.

Are daylight reflectance and solar reflectance the same?

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Related services Daylight reflectance, Solar reflectance index (SRI)

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).

SRI testing: Can membrane products be tested without substrate?

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Related services Solar reflectance index (SRI)

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.

Two examples

To answer this question, two examples are illustrated below:

A translucent membrane sample
An opaque membrane sample

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.

The theory

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.

The practice

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.

Theoretical range of solar reflectance index (SRI)

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Related services Solar reflectance index (SRI)

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.

Minimum SRI calculation results
maximum SRI calculation results

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.

Online solar reflectance index (SRI) calculator: V1.1.0

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Related services Solar reflectance index (SRI)

We are pleased to introduce our online solar reflectance index (SRI) calculator. The current version is V1.1.0. Click the screenshot below to access this online calculator.

Online SRI calculator

Features:

  • No download or installation required.
    • The calculator works in all mainstream browsers with javascript enabled.
  • User friendly and responsive
    • In fact, you don’t need to click the “Calculate SRI” button and the results are updated instantly after your changes.
  • Full compliance to ASTM E1980
    • Accuracy has been validated with other independent codes. Eq. 1 of ASTM E1980 is solved iteratively.
  • Informative
    • Surface temperatures and SRIs of all 3 conditions are displayed side-by-side and with options of 3 temperature units

Feedback and comments

We will improve this calculator regularly. If you have feedback and comments, please let us know.

New release of lab profile 2020 edition

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We are pleased to release our 2020 edition of laboratory profile, which is a 56-page comprehensive document, with rich information on our capabilities and many technical insights.

What are in the laboratory profile?

  • A comprehensive overview of the laboratory
  • Detailed introduction of 10 popular tests
  • 14 informative technical insights articles

You may download a soft copy of our lab profile.