ASTM E 1980 : Standard Practice for Calculating Solar Reflectance Index

The steady-state surface temperature (Ts) under the sun is strongly correlated to solar reflectivity and thermal emissivity of the surface. For equivalent conditions, the Ts of dark surfaces (with low solar reflectance) is higher than light-colored surfaces (with high solar reflectance); and surfaces with low thermal emissivity have higher Ts’s than surfaces with high thermal emissivity. The procedure recommended in this standard will allow a direct comparison of Ts of surfaces under the sun. The procedure defines a Solar Reflectance Index (SRI) that measures the relative Ts of a surface with respect to the standard white (SRI = 100) and standard black (SRI =0) under the standard solar and ambient conditions.

This practice covers the calculation of the Solar Reflectance Index (SRI) of horizontal and low-sloped opaque surfaces at standard conditions. The method is intended to calculate SRI for surfaces with emissivity greater than 0.1.

Convective coeffıcient (hc)—the rate of heat transfer from the surface to air induced by the air movement, expressed in watts per square metre per degree Kelvin, W·m–2·K–1.

Low-sloped surfaces—surfaces with a slope smaller than 9.5° from the horizontal.

Reference black surface temperature (Tb) —is the steady-state temperature of a black surface with solar reflectance of 0.05 and thermal emissivity of 0.9, under the standard solar and ambient conditions.

Reference white surface temperature (Tw) —is the steady-state temperature of a white surface with solar reflectance of 0.80 and thermal emissivity of 0.9, under the standard solar and ambient conditions. 

Sky temperature (Tsky)—is the temperature of a black body that would radiate the same power toward the earth as does the sky. 

Solar absorptance (α)—the fraction of solar flux absorbed by a surface. For an opaque surface α = 1 − a. 3.1.7 solar flux (I)—is the direct and diffuse radiant power from the sun received at ground level over the solar spectrum, expressed in watts per square metre, W·m–2 .

Solar reflectance (a)—the fraction of solar flux reflected by a surface.

Solar reflectance index (SRI)—is the relative Ts of a surface with respect to the standard white (SRI = 100) and standard black (SRI = 0) under the standard solar and ambient conditions. 

Solar spectrum—spectral distribution of typical terrestrial sunlight at air mass 1.5 as defined in Tables G173.

Standard solar and ambient conditions— for the purpose of this calculation, is defined as a solar flux of 1000 W·m–2, ambient air temperature of 310 Kelvin (K), and sky temperature of 300 K. Three convective coefficient of 5, 12, 30 W·m–2·K–1 , corresponding to low- (0 to 2 ms–1), medium- (2 to 6 ms–1), and high-wind (6 to 10 ms–1) conditions, respectively.

Steady-state surface temperature (Ts) —is the temperature of the surface, in K, under the standard solar and ambient conditions.

Thermal emissivity (ε)—the ratio of radiant flux emitted by a surface at a given temperature to that emitted by a black body radiator at the same temperature. For this calculation, the thermal emissivity is for a temperature below 150°C.

 Significance and Use 

Solar reflectance and thermal emittance are important factors affecting surface and near-surface ambient air temperature. Surfaces with low solar reflectance, absorb a high fraction of the incoming solar energy. A fraction of this absorbed energy is conducted into ground and buildings, a fraction is convected to air (leading to higher air temperatures), and a fraction is radiated to the sky. For equivalent conditions, the lower the emissivity of a surface the higher its steady-state temperature. Surfaces with low emissivity cannot effectively radiate to the sky and, therefore, get hot. Determination of solar reflectance and thermal emittance, and subsequent calculation of the relative temperature of the surfaces with respect to black and white reference temperature (defined as Solar Reflectance Index, SRI), may help designers and consumers to choose the proper materials to make their buildings and communities energy efficient. The method described here gives the SRI of surfaces based on measured solar reflectances and thermal emissivities of the surfaces.