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The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is not visible to human eyes.
Thermal emittance or thermal emissivity is the ratio of the radiant emittance of heat of a specific object or surface to that of a standard 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 ...
Low emissivity (low e or low thermal emissivity) refers to a surface condition that emits low levels of radiant thermal (heat) energy. All materials absorb, reflect, and emit radiant energy according to Planck's law but here, the primary concern is a special wavelength interval of radiant energy, namely thermal radiation of materials.
Emissivity can in general depend on wavelength, direction, and polarization. However, the emissivity which appears in the non-directional form of the Stefan–Boltzmann law is the hemispherical total emissivity, which reflects emissions as totaled over all wavelengths, directions, and polarizations. [3]: 60
Kirchhoff's law of thermal radiation has a refinement in that not only is thermal emissivity equal to absorptivity, it is equal in detail. Consider a leaf. It is a poor absorber of green light (around 470 nm), which is why it looks green. By the principle of detailed balance, it is also a poor emitter of green light.
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The thermal inertia of a terrestrial planet such as Mars can be approximated from the thermal effusivity of its near-surface geologic materials. In remote sensing applications, thermal inertia represents a complex combination of particle size, rock abundance, bedrock outcropping and the degree of induration (i.e. thickness and hardness). [9]
Hemispherical emissivity: ε — Radiant exitance of a surface, divided by that of a black body at the same temperature as that surface. Spectral hemispherical emissivity: ε ν ε λ — Spectral exitance of a surface, divided by that of a black body at the same temperature as that surface. Directional emissivity: ε Ω —