Electromagnetic Radiation. Physical Principles of Remote Sensing

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Alban Cannon
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1 Electromagnetic Radiation Physical Principles of Remote Sensing

2 Outline for 4/3/2003 Properties of electromagnetic radiation The electromagnetic spectrum Spectral emissivity Radiant temperature vs. kinematic temperature

3 Energy Transfer Energy is the ability to do work Energy transfer: Conduction: transfer of kinetic energy by contact between atoms or molecules Convection: transfer of kinetic energy by physically moving the mass that contains the energy Radiation: propagation via waves/particles through a vacuum (or through a medium)

4 Electromagnetic Radiation EMR is the source for most types of remote sensing Sun and Earth are both passive sources of EM radiation Lasers and radar are active sources Generated by transformation of energy from other forms Kinetic: thermal motion of atoms and molecules (heat) Electrical: radio frequency (dipole antenna) Magnetic: electron tube (microwave) Radioactive: decay of radioactive substances Chemical/Laser: molecular excitation

5 Electrical (E) and magnetic field (B) are orthogonal to each other Direction of each field is perpendicular to the direction of wave propagation.

6 Electromagnetic Waves Described by: Wavelength Amplitude

7 Electromagnetic Radiation Its harmonic wave form can be described according to the Maxwell equations: E x = E 0 cos(wt - kz) Where, E is the electric field w= angular frequency (2pn), n = c/l, l = wavelength c = speed of light in a vacuum (300,000 kms -1 ) k = wavenumber (2p/l) z = distance t = time

8 Frequency vs. Wavelength The product of wavelength and frequency is a constant: n l=c l = distance of separation between two successive wave peaks n = number of wave peaks passing in a given time c = speed of light in a vacuum (300,000 kms -1 )

9 Energy vs. Frequency When considering the particle form of energy, we call it a photon The energy of a photon is proportional to frequency: Q = h n n = c/l Q = hc/l where, h = Planck s constant = Js Thus, Q ~ 1/l

10 The EM Spectrum

11 Polarization E and B fields are perpendicular to each other but their orientation can change If both remain in their respective planes, the radiation is called plane polarized If they rotate around the axis of propagation, the radiation is called circularly polarized or elliptically polarized If their orientation changes randomly, it is called randomly polarized or unpolarized

12 Polarization Plane polarized light can be either vertically polarized (E 0 is perpendicular to the plane of incidence) horizontally polarized (E 0 is parallel to the plane of incidence) Solar radiation is unpolarized (random) but can become polarized by reflection, scattering, etc. Lasers and radars produce polarized radiation

14 Spectral Emittance All bodies whose temperature are above absolute zero Kelvin ( o C) emit radiation at all wavelengths A blackbody is one that is a perfect absorber and perfect emitter (hypothetical, though Earth and Sun are close) Planck s Law describes how heat energy is transformed into radiant energy This is the basic law for radiation measurements in all parts of the EM spectrum

15 Planck s Blackbody Equation M l = C 1 [ ] l 5 e C 2 lt -1 M l = spectral radiant exitance (emittance), units are W m -2 mm -1 l = wavelength T = the blackbody s temperature in Kelvin (K) C 1 = W m -2 mm 4 C 2 = mm K

16 Blackbody Radiation According to Planck s law, a blackbody will emit radiation in all wavelengths but not equally Stefan-Boltzmann Law: Emittance is proportional to physical temperature s = W m -2 K -4 Graybody: M = est 4 M = st 4 Object that reflects part of incident radiation e < 1.0

17 Emissivity Describes the actual absorption and emission properties of real objects ( graybodies ) Is wavelength dependent Emissivity = graybody emittance/blackbody emittance Emissivity establishes the radiant temperature T rad of an object

18 Radiant Temperature vs. Kinematic Temperature Two objects can have the same kinematic temperature but different radiant temperatures Object Emissivity Kinematic Temperature Radiant Temperature Blackbody Water, distilled Basalt, rough Basalt, smooth Obsidian Mirror

19 Wien s Law l max = a /T a = 2898 mm K The wavelength of peak emittance is inversely proportional to the kinematic temperature Sun s temperature = 6000 K 2898/6000 = 0.48 mm Earth s temperature = 300 K 2898/300 = 9.6 mm

22 Sun s Radiant Energy Distribution Name of Spectral Region Gamma and X-rays Far Ultraviolet Middle Ultraviolet Near Ultraviolet Visible Near Infrared Middle Infrared Thermal Infrared Microwave Radio Waves Wavelength Range, mm < > 1000 > 1000 Percent of Total Energy Negligible Negligible Negligible

23 Emission spectrum of a 6000K blackbody Solar Emittance Curve Radiation leaving the surface of the sun Solar radiation at sea level

24 For terrestrial remote sensing, the most important source is the sun Reflected solar energy is used mm The Earth is also an energy source >6 mm for self-emitted energy

Dr. Linlin Ge The University of New South Wales

Dr. Linlin Ge The University of New South Wales GMAT 9600 Principles of Remote Sensing Week2 Electromagnetic Radiation: Definition & Physics Dr. Linlin Ge www.gmat.unsw.edu.au/linlinge Basic radiation quantities Outline Wave and quantum properties Polarization