#### 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.

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.

**Solar Reflective Index Calculator **

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.

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.

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.

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