What is Solar Reflective 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.