METO 621

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METO 621. LESSON 8. Thermal emission from a surface. be the emitted. energy from a flat surface of temperature T s , within the solid angle d w in the direction W. A blackbody would emit B n (T s )cos q d w. The spectral directional emittance is defined as. Let.

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METO 621

LESSON 8

Thermal emission from a surface

be the emitted

energy from a flat surface of temperature Ts , within the solid angle dw in the direction W. A blackbody would emit Bn(Ts)cosqdw. The spectral directional emittance is defined as

• Let
Thermal emission from a surface
• In general e depends on the direction of emission, the surface temperature, and the frequency of the radiation. A surface for which eis unity for all directions and frequencies is a blackbody. A hypothetical surface for which e = constant<1 for all frequencies is a graybody.
Flux emittance
• The energy emitted into 2p steradians relative to a blackbody is defined as the flux or bulk emittance
Absorption by a surface
• Let a surface be illuminated by a downward intensity I. Then a certain amount of this energy will be absorbed by the surface. We define the spectral directional absorptance as:
Absorption by a surface
• Similar to emission, we can define a flux absorptance
• Kirchoff showed that for an opaque surface
• That is, a good absorber is also a good emitter, and vice-versa
Collimated Incidence - Lambert Surface
• If the incident light is direct sunlight then
Collimated Incidence - Specular reflectance
• Here the reflected intensity is directed along the angle of reflection only.
• Hence q’=q and f=f’+p
• Spectral reflection function rS(n,q)
• and the reflected flux:
Absorption and Scattering in Planetary Media
• Kirchoff’s Law for volume absorption and Emission
• Consider conservative scattering - no change in frequency.
• Assume the incident radiation is collimated
• We now need to look more closely at the secondary ‘emission’ that results from scattering. Remember that from the definition of the intensity that