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    Thermal radiation emmitted by a surface of a material.

    An electromagnetic radiation that may include both visible radiation (light) and infrared radiation, which is not visible to human eyes. 

    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. 
    • 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.
    • 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.
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