Solar reflectance

Solar spectra in ASTM E903 solar reflectance calculations

chen.fz@otm.sg Testing , 0 Comment
Related services Solar reflectance index (SRI)

Various solar spectra (i.e. solar spectral irradiance distributions) could be used in solar reflectance (of course, solar transmittance and absorptance too) calculations according to ASTM E903, as listed below:

  • ASTM E891:
    • Default solar spectrum used by the building industry and by OTM in SRI testing.
    • It is the default solar spectrum due to historical reasons and for compatibility with existing practices, data and instruments.
    • ASTM E891 is withdrawn and the same data set is available in ISO 9845-1.
  • ASTM E892:
    • Default solar spectrum for solar reflectance testing of non-roofing materials, according to LEED (note: solar reflectance only, not SRI)
    • Similarly, ASTM E892 is withdrawn and the data set is availble in ISO 9845-1.
    • ASTM E891 and E892 are different. ASTM E891 is in terms of direct normal solar radiation (no diffuse radiation); ASTM E892 is in terms of hemispherical solar radiation (with diffuse radiation).
  • ASTM G173:
    • Default solar spectrum included in ASTM E903, including both direct normal solar spectrum (equivalent to ASTM E891) and hemispherical solar spectrum (equivalent to ASTM E892)
    • It is not often used in the buidling industry, due to the historical reasons stated under ASTM E891.
  • ASTM E490:
    • Used in extraterrestrial applications, with AM0.
    • All other spectra mentioned above are with AM1.5 (refer to this article on air mass).

Solar absorptance and thermal emittance of transparent/translucent materials

chen.fz@otm.sg Testing , , , , 0 Comment
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 daylight reflectance and solar reflectance the same?

chen.fz@otm.sg Testing , , 0 Comment
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).