# Emissivity

emissivitiesEmissivity of Earth's atmosphereemittedgreybodylow-emissivitysection of "absorptivity" in "emissivitythermal emissionsthermal emissivity
The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation.wikipedia
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The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation.
The ratio of any body's emission relative to that of a black body is the body's emissivity, so that a black body has an emissivity of unity.

### Stefan–Boltzmann law

Stefan-Boltzmann lawStefan's lawStefan–Boltzmann
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.
A body that does not absorb all incident radiation (sometimes known as a grey body) emits less total energy than a black body and is characterized by an emissivity, :

### Low emissivity

low-emissivityLow-Elow-emissive
Emissivity is the value given to materials based on the ratio of heat emitted compared to a perfect black body, on a scale from zero to one.

### Heat shield

heatshieldheat shieldsthermal shielding
To achieve good functionality the three attributes required of a heat shield are low thermal conductivity (high thermal resistance), high emissivity and good thermal stability (refractoriness).

### Infrared

IRnear-infraredinfra-red
Thermal radiation is electromagnetic radiation and it may include both visible radiation (light) and infrared radiation, which is not visible to human eyes.
The concept of emissivity is important in understanding the infrared emissions of objects.

### Pyrometer

pyrometrypyrometricoptical pyrometer
The output signal of the detector (temperature T) is related to the thermal radiation or irradiance j^{\star} of the target object through the Stefan–Boltzmann law, the constant of proportionality σ, called the Stefan–Boltzmann constant and the emissivity ε of the object.

### Thermography

thermal imagingimaging infraredinfrared thermography
The ability of objects to emit is called emissivity, to absorb radiation is called absorptivity.

### Leslie cube

Leslie's cube
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.
In contemporary terms, the "emissivities" of shiny metals are low.

The material is translucent to sunlight and has infrared emissivity of 0.93 in the infrared atmospheric transmission window.

### Molybdenum disilicide

disilicideMolybdenum silicidemolybdenum(IV) silicide
The thermal stability of MoSi 2 alongside its high emissivity make this material, alongside WSi 2 attractive for applications as a high emissivity coatings in heat shields for atmospheric entry.

### Black body

blackbodyblack-bodyblack bodies
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.
Real materials emit energy at a fraction—called the emissivity—of black-body energy levels.

### Optical depth

optical thicknessoptically thickoptically thin

### Planck's law

Following Plancks law, the total energy radiated increases with temperature while the peak of the emission spectrum shifts to shorter wavelengths.
The passage of radiation across an interface between media can be characterized by the emissivity of the interface (the ratio of the actual radiance to the theoretical Planck radiance), usually denoted by the symbol

The amount of energy radiated depends on the surface temperature and a property called emissivity (also called "emittance").

### Tungsten disilicide

tungsten silicideWSi 2 tungsten(IV) silicide
In particular, in similarity to Molybdenum disilicide, MoSi 2, the high emissivity of tungsten disilicide makes this material attractive for high temperature radiative cooling, with implications in heat shields.

### Reflectance

reflectivityspectral reflectancereflection

### Albedo

albedosreflectivitygeometrical albedo

### Light

visible lightvisiblelight source
Thermal radiation is electromagnetic radiation and it may include both visible radiation (light) and infrared radiation, which is not visible to human eyes.

### Insulated glazing

double glazingdouble glazeddouble-glazed

### Solar thermal collector

solar heatingsolar collectorsolar collectors

### Spacecraft

spaceshipspaceshipsspace ship