Lecture 6: Thermal Radiation

1 Satellite Remote SensingSIO 135/SIO 236 Lecture 6: Thermal RadiationHelen Amanda FrickerHow is EMR generated?Radio frequency generated by periodic currents of electric charges generated by electron tubes that use the motion of highspeed electrons in specially designed structure to generate E/B fieldswhich is than guided by a wavegarde to Radiation structureVisible high frequency wave in the infrared and visible are generated bymolecular excitation (vibrational or orbital) followed by decay. Theemitted frequency is exactly related to the energy difference betweenthe 2 energy level of the kinetic energy of random motion of particles of matter.

2 The randommotion results in excitation (electronic, vibrational, rotational) due tocollisions followed by random emission of EM wave during the of its random nature this type of energy transformation leadsto emission over a wide spectral (LRadiance (L!!) is the radiant flux per) is the radiant flux perunit solid angle leaving an extendedunit solid angle leaving an extendedsource in a given direction per unitsource in a given direction per unitprojected source area in thatprojected source area in thatdirection and is measured in wattsdirection and is measured in wattsper meter squared per steradianper meter squared per steradian(W m(W m-2-2 sr sr-1-1 ).)

3 We are interested in the radiant fluxWe are interested in the radiant fluxin certain wavelengths (Lin certain wavelengths (L!!) leaving) leavingthe projected source area (A) withinthe projected source area (A) withina certain direction (a certain direction ("") and solid) and solidangle (angle (##):):RadianceJensen2005 Jensen2005units W m-3 sr-1 TerminologyConsider a 60 W light bulb. An electric current passes through thetungsten filament and heats it to about 3000 K. Our bulb is perfect in thesense that it radiates all of this energy, perhaps as a gray SpectrumVisible: Small portion of the EMS thathumans are sensitive to:blue ( m); green ( m);red ( m) infrared : Three logical IR: reflected, can be recordedon film emulsions ( - m).

4 infrared : reflected, can bedetected using electro-opticalsensors ( - m). infrared : emitted, can onlybe detected using electro-opticalsensors ( - and 8 - 14 m).MicrowaveRadar sensors, wavelengths range from1mm - 1m (Ka, Ku, X, C, S, L & P) spectral bandsThree important spectral bands in remote sensing: visible light infrared Radiation microwave radiationImage from NASA 1987. SAR: Synthetic Aperture Radar. Earth Observing System, Vol. channels of the EM spectrum Absorption is the process by which radiant energy is absorbed andconverted into other forms of energy. An absorption band is a range ofwavelengths (or frequencies) in the electromagnetic spectrum withinwhich radiant energy is absorbed by substances such as water (H2O),carbon dioxide (CO2), oxygen (O2), ozone (O3), and nitrous oxide (N2O).

5 The cumulative effect of the absorption by the various constituents cancause the atmosphere to close down in certain regions of the is bad for remote sensing because no energy is available to In certain parts of the spectrum such as the visible region ( - mm),the atmosphere does not absorb all of the incident energy but transmits iteffectively. Parts of the spectrum that transmit energy effectively are called atmospheric windows . Absorption occurs when energy of the same frequency as the resonantfrequency of an atom or molecule is absorbed, producing an excited , instead of re-radiating a photon of the same wavelength, the energy istransformed into heat motion and is reradiated at a longer wavelength,absorption occurs.

6 Transmission is inversely related to the extinction coefficient times thethickness of the layer. Certain wavelengths of Radiation are affected farmore by absorption than by scattering. This is particularly true of infraredand wavelengths shorter than visible molecules absorb different wavelengths of Radiation : O2 and O3 absorb almost all wavelengths shorter than 300 nm. Water (H2O) absorbs many wavelengths above 700 nm, but this dependson the amount of water vapor in the a molecule absorbs a photon, it increases the energy of themolecule. We can think of this as heating the atmosphere, but theatmosphere also cools by emitting of EMR by atmosphereJensen 2005 Jensen 2005 Jensen 2005 Major subdivisions of the atmosphere and the types of moleculesand aerosols found in each Layers and ConstituentsJensen 2005 Jensen 2005 Jensen 2005windowAbsorption of the Sun's incident electromagnetic energy in the Region from to 30 mm by various atmospheric gases When you combine the absorption spectra of the gasses in theatmosphere, you are left with "windows" of low opacity, allowing thetransmission of only certain EMR.

7 Optical window runs from around 300 nm (UV-C) up the visible spectrum(commonly called light), at roughly 400 700 nm and continues to theinfrared to around 1100 nm. There are also infrared and radio of EMR by atmosphere Let us think of a box with a cavity and hot walls giving off and receivingphotons at an equal rate If we drill a small hole through the side, the Radiation coming out willhave the same spectrum, and this is what we measure with ourspectrometer to determine the black body Radiation spectrum. The spectral radiancy is given by Planck's Radiation law, which wasinitially : UNSW websiteBlack body lab experiment An object of controlled temperature T contains a cavity, joinedto the outside by a small hole.

8 If the hole is very small, the Radiation in the cavity comes toequilibrium with the walls. The hole allows a small fraction of the Radiation to pass to body lab experimentBlack body lab experiment Plot of Planck's Radiation law for two temperatures. Note that thepeak of the curve moves to the left as the temperature increases:hotter objects output a larger fraction of their EMR at shorterwavelengths. Note also the strong dependence on temperature of the totalemission. The radiancy is the power emitted per unit area perincrement of wavelength and so has units of W body lab experiment The spectral radiance from the hole is independent of the materialused and only depends on the temperature This was a remarkable observation at the turn of the century and manyphysicists strived to explain it Lord Rayleigh and Sir James Jeans developed a classical theory usinga principle of equipartition of energy Considered all possible standing waves in the cavity and counted thenumber of standing waves that could occur in a frequency interval d$they noted that each

9 Standing wave could have 2 degrees of freedom sothey assigned an energy of kT to each, where k is the Boltzmannconstant The result is the Rayleigh-Jeans approximation for long wavelengths(more later on that)What is a black body ? A black body is an idealized objectthat absorbs all EMR that falls on it --no EMR passes through it and none isreflected. The term "black body" wasintroduced by Gustav Kirchhoff in1860. Because no light ( visible EMR) isreflected or transmitted, the objectappears black when it is is a black body ? A black body emits a temperature-dependent spectrum of light. Thisthermal Radiation from a black body istermed black-body Radiation .

10 At room temperature, BBs emitmostly infrared light, but as thetemperature increases past a fewhundred C, BBs start to emit visiblewavelengths, from red, throughorange, yellow, and white beforeending up at blue, beyond which theemission includes increasing amountsof in nmColour of BB Radiation depends onthe temperature of the : is an ideal source that transforms heat energy into radiantenergy with the maximum rate permitted by thermodynamic laws at agiven temperature T and for a given !We can also say: any object that is a perfect emitter and a perfectabsorber of Radiation -- object does not have to appear "black"-- Sun and Earth behave approximately as black bodiesThe spectral emittance S(!)