Theoretical range of 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.

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