Samiksha Jaiswal (Editor)

Emissivity

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Emissivity

The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation and it may include both visible radiation (light) and infrared radiation, which is not visible to human eyes. The thermal radiation from very hot objects (see photograph) is easily visible to the eye. Quantitatively, emissivity is the ratio of the thermal radiation from a surface to the radiation from an ideal black surface at the same temperature as given by the Stefan–Boltzmann law. The ratio varies from 0 to 1. The surface of a black object emits thermal radiation at the rate of approximately 448 watts per square meter at room temperature (25 °C, 298.15 K); real objects with emissivities less than 1.0 emit radiation at correspondingly lower rates.

Contents

Emissivities are important in several contexts:

  • insulated windows. – Warm surfaces are usually cooled directly by air, but they also cool themselves by emitting thermal radiation. This second cooling mechanism is important for simple glass windows, which have emissivities close to the maximum possible value of 1.0. "Low-E windows" with transparent low emissivity coatings emit less thermal radiation than ordinary windows. In winter, these coatings can halve the rate at which a window loses heat compared to an uncoated glass window.
  • solar heat collectors. – Similarly, solar heat collectors lose heat by emitting thermal radiation. Advanced solar collectors incorporate selective surfaces that have very low emissivities. These collectors waste very little of the solar energy through emission of thermal radiation.
  • planetary temperatures. – The planets are solar thermal collectors on a large scale. The temperature of a planet's surface is determined by the balance between the heat absorbed by the planet from sunlight, heat emitted from its core, and thermal radiation emitted back into space. Emissivity of a planet is determined by the nature of its surface and atmosphere.
  • temperature measurements. – Pyrometers and infrared cameras are instruments used to measure the temperature of an object by using its thermal radiation; no actual contact with the object is needed. The calibration of these instruments involves the emissivity of the surface that's being measured.

    • Hemispherical emissivity

    Hemispherical emissivity of a surface, denoted ε, is defined as

    ε = M e M e ∘ ,

    where

  • Me is the radiant exitance of that surface;
  • Me° is the radiant exitance of a black body at the same temperature as that surface.

    • Spectral hemispherical emissivity

    Spectral hemispherical emissivity in frequency and spectral hemispherical emissivity in wavelength of a surface, denoted εν and ελ respectively, are defined as

    ε ν = M e , ν M e , ν ∘ , ε λ = M e , λ M e , λ ∘ ,

    where

  • Me,ν is the spectral radiant exitance in frequency of that surface;
  • Me,ν° is the spectral radiant exitance in frequency of a black body at the same temperature as that surface;
  • Me,λ is the spectral radiant exitance in wavelength of that surface;
  • Me,λ° is the spectral radiant exitance in wavelength of a black body at the same temperature as that surface.

    • Directional emissivity

    Directional emissivity of a surface, denoted εΩ, is defined as

    ε Ω = L e , Ω L e , Ω ∘ ,

    where

  • Le,Ω is the radiance of that surface;
  • Le,Ω° is the radiance of a black body at the same temperature as that surface.

    • Spectral directional emissivity

    Spectral directional emissivity in frequency and spectral directional emissivity in wavelength of a surface, denoted εν,Ω and ελ,Ω respectively, are defined as

    ε ν , Ω = L e , Ω , ν L e , Ω , ν ∘ , ε λ , Ω = L e , Ω , λ L e , Ω , λ ∘ ,

    where

  • Le,Ω,ν is the spectral radiance in frequency of that surface;
  • Le,Ω,ν° is the spectral radiance in frequency of a black body at the same temperature as that surface;
  • Le,Ω,λ is the spectral radiance in wavelength of that surface;
  • Le,Ω,λ° is the spectral radiance in wavelength of a black body at the same temperature as that surface.

    • Emissivities of common surfaces

    Emissivities ε can be measured using simple devices such as Leslie's Cube in conjunction with a thermal radiation detector such as a thermopile or a bolometer. The apparatus compares the thermal radiation from a surface to be tested with the thermal radiation from a nearly ideal, black sample. The detectors are essentially black absorbers with very sensitive thermometers that record the detector's temperature rise when exposed to thermal radiation. For measuring room temperature emissivities, the detectors must absorb thermal radiation completely at infrared wavelengths near 10×10−6 meters. Visible light has a wavelength range of about 0.4 to 0.7×10−6 meters from violet to deep red.

    Emissivity measurements for many surfaces are compiled in many handbooks and texts. Some of these are listed in the following table.

    Notes:

    1. These emissivities are the total hemispherical emissivities from the surfaces.
    2. The values of the emissivities apply to materials that are optically thick. This means that the absorptivity at the wavelengths typical of thermal radiation doesn't depend on the thickness of the material. Very thin materials emit less thermal radiation than thicker materials.

    Emissivity and absorptivity

    There is a fundamental relationship (Gustav Kirchhoff's 1859 law of thermal radiation) that equates the emissivity of a surface with its absorption of incident radiation (the "absorptivity" of a surface). Kirchhoff's Law explains why emissivities cannot exceed 1, since the largest absorptivity - corresponding to complete absorption of all incident light by a truly black object - is also 1. Mirror-like, metallic surfaces that reflect light will thus have low emissivities, since the reflected light isn't absorbed. A polished silver surface has an emissivity of about 0.02 near room temperature. Black soot absorbs thermal radiation very well; it has an emissivity as large as 0.97, and hence soot is a fair approximation to an ideal black body.

    With the exception of bare, polished metals, the appearance of a surface to the eye is not a good guide to emissivities near room temperature. Thus white paint absorbs very little visible light. However, at an infrared wavelength of 10x10−6 meters, paint absorbs light very well, and has a high emissivity. Similarly, pure water absorbs very little visible light, but water is nonetheless a strong infrared absorber and has a correspondingly high emissivity.

    Directional spectral emissivity

    In addition to the total hemispherical emissivities compiled in the table above, a more complex "directional spectral emissivity" can also be measured. This emissivity depends upon the wavelength and upon the angle of the outgoing thermal radiation. Kirchhoff's law actually applies exactly to this more complex emissivity: the emissivity for thermal radiation emerging in a particular direction and at a particular wavelength matches the absorptivity for incident light at the same wavelength and angle. The total hemispherical emissivity is a weighted average of this directional spectral emissivity; the average is described by textbooks on "radiative heat transfer".

    Emissivity and emittance

    Emittance (or emissive power) is the total amount of thermal energy emitted per unit area per unit time for all possible wavelengths. Emissivity of a body at a given temperature is the ratio of the total emissive power of a body to the total emissive power of a perfectly black body at that temperature.

    The term emissivity is generally used to describe a simple, homogeneous surface such as silver. Similar terms, emittance and thermal emittance, are used to describe thermal radiation measurements on complex surfaces such as insulation products.

    References

    Emissivity Wikipedia
    (Text) CC BY-SA