Why do we care about emissivity

1
Why do we care about emissivity?

• Dion L. Heinz
• University of Chicago

2
We use blackbody radiation for temperature
measurement

• Actually we use the greybody approximation (where
  we assume that the emissivity is constant)

3
Plancks Radiation Law

• I(l,T) e(l)C1l-5expC2/lT - 1-1

4
What is the physical interpretation of emissivity?

• For the solid to give up a photon, there has to
  be a physical mechanism of that energy in the
  solid e.g. a vibration, electron, etc
• Metals typically have electrons that can give up
  the appropriate energy, but the emissivity can
  easily have a wavelength dependence

5
Emissivity

• Assume absorption is equal to emissivity
  (Kirchoffs Law)
• Absorption is proportional to the complex part of
  the index of refraction (actually 4pi/lambda)

6
Fe
7
(No Transcript)
8
Fe Emissivity

• In Iron the d electron band is right at the Fermi
  level, thus there are occupied electronic states
  below unoccupied states
• Therefore, as you go to shorter wavelengths of
  light, you can excite more and more electrons
  with photons

9
(No Transcript)
10
(No Transcript)
11
How much does emissivity matter?
12
What about transparent materials?

• Take MgO for instance, it does not have any
  mechanisms in the visible region to give up
  photons
• The band gap is large (i.e. in the UV)
• The lattice vibrations are
• in the infrared

13
MgO
14
SiO2
15
SiO2 Glass
16
LiF
17
Why does MgO glow?

• The answer has to be surface states (in a
  polycrystal), defects or impurities.
• The problem is what does the absorption spectrum
  of an impurity or defect look like?
• Impurities are also very sample dependent!

18
How do we measure the wavelength dependence of
the absorption?

• Use three lasers to measure reflectivity or
  transmission of the sample in situ (you have to
  give up these wavelength bands for temperature
  determination)
• Some Problems with doing the above
• Reflectivity is highly surface dependent
• Transmission could be affected by scattering

19
(No Transcript)
20
The end!
21
(No Transcript)
22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
25
(No Transcript)
26
(No Transcript)