Emissivity

prefLabel
  • Emissivity
definition
  • The ratio of the radiation emitted by a surface to the radiation emitted by a perfect blackbody radiator at the same temperature.
inScheme
broader
Abstract from DBPedia
    The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that 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 perfect black body (with an emissivity of 1) emits thermal radiation at the rate of approximately 448 watts per square metre at room temperature (25 °C, 298.15 K); all real objects have emissivities less than 1.0, and emit radiation at correspondingly lower rates. Emissivities are important in several contexts: Insulated windowsWarm 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 collectorsSimilarly, 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.Thermal shieldingFor the protection of structures from high surface temperatures, such as reusable spacecraft or hypersonic aircraft, high-emissivity coatings (HECs), with emissivity values near 0.9, are applied on the surface of insulating ceramics. This facilitates radiative cooling and protection of the underlying structure and is an alternative to ablative coatings, used in single-use reentry capsules.Planetary temperaturesThe 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 measurementsPyrometers 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.

    放射率(ほうしゃりつ、英: emissivity)は、物体が熱放射で放出する光のエネルギー(放射輝度)を、同温の黒体が放出する光(黒体放射)のエネルギーを 1 としたときの比である。0 以上 1 以下の値(無次元量)であり、物質により、また、光の波長により異なる。 キルヒホッフの法則によると、放射率εとαは等しい: ε = α また、エネルギー保存則から、ある波長の光が物体に当たった時、反射率ρ、透過率τ、吸収率αの和は 1 になる: ρ + τ + α = 1 もしも、物体が十分に厚ければ、透過率τは 0 になる。すると ρ + α = 1 となる。この式に上記のキルヒホッフの法則を使うと ρ = 1 - ε となる。すなわち、放射率εが大きければ反射率は小さく、逆に小さければ反射率は大きい。このことから、光をなるべく反射するには、放射率の小さな素材で物体表面を覆えばよいということがわかる。 例えば、消防士の着る耐熱服の表面が金属でコーティングされているのは、金属の放射率が広範囲の波長において低い(反射率が高い)ためである。高温な物体から照射される熱放射を反射することにより、消防士の体を高温から守るのである。

    (Source: http://dbpedia.org/resource/Emissivity)

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