La emisividad, llamada antiguamente emitancia, es la proporción de radiación térmica emitida por una superficie u objeto debida a su temperatura. La emisividad direccional espectral se define como la razón entre la intensidad emitida por la superficie en una dirección particular y la intensidad que sería emitida por un cuerpo negro a la misma temperatura y longitud de onda. La emisividad total se obtiene por integración sobre todo el espectro electromagnético y todo el espacio. Cuanto más pequeño sea el valor de la emisividad, mejor aislante por reflexión será dicha superficie, siendo 1 el valor máximo. Una cantidad relacionada es la absortividad, definida como la fracción de irradiancia recibida que es absorbida por un cuerpo. Toma valores entre 0 y 1. Para un cuerpo negro, la absortividad espectral es 1. Si la absortividad de un cuerpo es menor que 1, pero se mantiene constante para todas las longitudes de onda, éste se denomina cuerpo gris.

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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 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. Emissivities are important in several contexts: \n* 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. \n* 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. \n* 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. \n* 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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