Thermal emittance or thermal emissivity (
) is the ratio of the
radiant emittance of
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
of a specific object or surface to that of a standard
black body
A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body ...
. Emissivity and emittivity are both
dimensionless quantities given in the range of 0 to 1, representing the comparative/relative emittance with respect to a blackbody operating in similar conditions, but
emissivity refers to a
material property (of a homogeneous material), while emittivity refers to specific samples or objects.
For building products, thermal emittance measurements are taken for wavelengths in the
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...
. Determining the thermal emittance and
solar reflectance of building materials, especially
roofing materials, can be very useful for reducing heating and cooling
energy costs
The following articles relate to the price of energy:
*Carbon price
*Energy crisis
* Price of oil
* Gas prices (disambiguation)
*Hubbert peak theory, or peak oil
* Energy economics
* Electricity market
* Electricity pricing
*Cost of electricity by ...
in buildings. Combined index Solar Reflectance Index (SRI) is often used to determine the overall ability to reflect solar heat and release thermal heat. A roofing surface with high solar reflectance and high thermal emittance will reflect solar heat and release absorbed heat readily. High thermal emittance material radiates thermal heat back into the atmosphere more readily than one with a low thermal emittance. In common construction applications, the thermal emittance of a surface is usually higher than 0.8–0.85.
High thermal emittance materials are essential to
passive daytime radiative cooling, which uses surfaces high in thermal emittance and solar reflectance to lower surface temperatures by dissipating heat to
outer space
Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, pred ...
. It has been proposed as a solution to
energy crises and
global warming
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...
.
References
Physical quantities
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