emissivity

(noun)

The energy-emitting propensity of a surface, usually measured at a specific wavelength.

Related Terms

Examples of emissivity in the following topics:

  • Lasers

    • It does so through a process of optical amplification based on the stimulated emission of photons.
    • This is the mechanism of fluorescence and thermal emission.
    • This "induced" decay process is called stimulated emission.
    • In stimulated emission, the decaying atom produces an identical "copy" of the incoming photon.
    • Identify process that generates laser emission and the defining characteristics of laser light

  • Emission

    • Generally material has two routes for the emission of radiation: stimulated emission and spontaneous emission.
    • The spontaneous emission is independent of the radiation field.
    • Let's define the spontaneous emission coefficient, $j$.
    • Often the emission is isotropic and it is convenient to define the emissivity of the material per unit mass

  • Calculating the Emission and Absorption Coefficients

    • We can write the emission and absorption coefficients in terms of the Einstein coefficients that we have just examined.
    • The emission coefficient $j_\nu$ has units of energy per unit time per unit volume per unit frequency per unit solid angle!
    • The Einstein coefficient $A_{21}$ gives spontaneous emission rate per atom, so dimensional analysis quickly gives

  • Problems

    • What is the synchrotron emission from a single electron passing through a magnetic field in terms of the energy density of the magnetic field and the Lorentz factor of the electron?
    • What is the inverse Compton emission from a single electron passing through a gas of photons field in terms of the energy density of the photons and the Lorentz factor of the electron?
    • What is the total inverse Compton emission from the region if you assume that the synchrotron emission provides the seed photons for the inverse Compton emission?

  • Gamma Decay

    • Gamma decay is a process of emission of gamma rays that accompanies other forms of radioactive decay, such as alpha and beta decay.
    • For example, cobalt-60 decays to excited nickel-60 by beta decay through emission of an electron of 0.31 MeV.
    • In certain cases, the excited nuclear state following the emission of a beta particle may be more stable than average; in these cases it is termed a metastable excited state if its decay is 100 to 1000 times longer than the average $10^{-12}$ seconds.
    • The process of isomeric transition is therefore similar to any gamma emission; it differs only in that it involves the intermediate metastable excited states of the nuclei.
    • Excited levels for Ni-60 that drop to ground state via emission of gamma rays are indicated

  • Emission Topography

    • Positron emission tomography is a nuclear medical imaging technique that produces a three-dimensional image of processes in the body.
    • Positron emission tomography (PET) is a nuclear medical imaging technique that produces a three-dimensional image or picture of functional processes in the body.
    • PET acquisition process occurs as the radioisotope undergoes positron emission decay (also known as positive beta decay), it emits a positron, an antiparticle of the electron with opposite charge.
    • A technique much like the reconstruction of computed tomography (CT) and single-photon emission computed tomography (SPECT) data is more commonly used, although the data set collected in PET is much poorer than CT, so reconstruction techniques are more difficult.
    • Discuss possibility of uses of positron emission tomography with other diagnostic techniques.

  • Infrared Waves

    • The concept of emissivity is important in understanding the infrared emissions of objects.
    • This is a property of a surface which describes how its thermal emissions deviate from the ideal of a black body.
    • As stated above, while infrared radiation is commonly referred to as heat radiation, only objects emitting with a certain range of temperatures and emissivities will produce most of their electromagnetic emission in the infrared part of the spectrum.
    • Infrared radiation can be used to remotely determine the temperature of objects (if the emissivity is known).
    • Distinguish three ranges of the infrared portion of the spectrum, and describe processes of absorption and emission of infrared light by molecules

  • Fluorescence and Phosphorescence

    • Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation.
    • However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; this two-photon absorption can lead to emission of radiation having a shorter wavelength than the absorbed radiation.
    • Once in a different spin state, electrons cannot relax into the ground state quickly because the re-emission involves quantum mechanically forbidden energy state transitions.

  • Oscillator Strengths

    • In an earlier equation, the final term could be important if the $E_i-E_f \approx -\hbar \omega$ this corresponds to stimulated emission of radiation.
    • Except for the degeneracy factors for the two states, the Einstein coefficients will be the same, so we can define an oscillator strength for stimulated emission as well,
    • We can also separate the emission from absorption oscillator strengths
    • Because the second term is for stimulated emission.

  • Scattering

    • Couldn't you think about scattering as the absorption and re-emission of a photon and include the process in the absorption coefficients and source functions?
    • This yields an emission coefficient of
    • Notice that the emission rate depends on the radiation field through $J_\nu$ and not solely on the properties of the scatterer through $\sigma_\nu$.