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 Coherent and monochromatic radiation Doppler effects Blackbody radiation Solar radiant energy Definition of Basic Radiation Quantities Electromagnetic Radiation Quantity Radiant Energy Radiant Flux Incident Flux Reflected Flux Symbol Q F F i F r Unit Joule, J Watt, W (J/s) Watt, W Watt, W Quantity Irradiance Exitance Radiant Intensity Radiance Symbol E M I L Unit Watt / metre 2, W/m 2 Watt/metre 2 W/m 2 Watt/Steradian W/SR Watt/Steradian/ metre 2 W/(SR * m 2 ) Most important medium for remote sensing Only form of energy transfer that can take place through free space, with velocity C = 3 * 10 8 m/s Source to sensor Directly Indirectly : Reflection or Re-radiation
Nature of Radiation Energy = Ability to do work. Exist in variety of forms: chemical, electrical, heat, mechanical etc. In the course of work being done energy transferred by: Conduction atomic or molecular collisions Transfer by contact, RS? Convection corpuscular mode of transfer. Molecules physically moved. Radiation only form of energy transmitted without intervening medium. Primary concern in Remote Sensing Generation of Radiation Either: Electron: Atomic transition - Short wave: infrared, visible, ultraviolet. Molecule: Vibrational or Rotational - Electron orbital changes -> shortest wavelengths - Vibrational -> short and intermediate infrared - Rotational -> long wavelength Infrared and short wavelength microwave Wave Model of E.M.R Electromagnetic Radiation (E.M.R) is conceived of as wave motion due to regular variations in both electric and magnetic fields surrounding a charged particle. Two force fields, electrical and magnetic, are inseparable. If E.M. waves intercepted by matter, result will depend on both magnetic and electric properties of the matter. E.R.M cont. E and B are mutually orthogonal and perpendicular to direction of wave advancement.
E.R.M cont. Wave propagatethrough through empty space (i.e., vacuum) C =? 0 ƒ where C = 3 x 10 8 m/s? 0 = wavelength ; ƒ = frequency Waves propagate through a material V =?ƒ where ƒ does not change? changes with V V = C / n, n refractive index Note: Maxwell s equations govern all propagation of E.M. waves. Describe time and space inter-relationship relationship of electric and magnetic fields. Complex Waves Principle of superposition: amplitude of combined waves = sum of separate waves Complex wave made up of sinusoidal components - spectral components. Complex Waves cont. Diffraction Effects Change in direction of some radiation due to propagation of wave past edge. (explained by wave theory) Diffraction MUST always occur when wave is cut off by sensor aperture. One limiting factor to measurement of E.M.R.
Particle or Quantum Properties of E.M.R. Wave theory fails to account for certain significant phenomena at low levels of radiation. Particle model treats radiation as comprised of discrete packages. Quanta = Photon = Short wave trains or bursts of energy, smallest package of energy possible. Q = h*f = h * c /? where h is Planck s constant = 6.625 x 10-34 J. sec X-ray vs radar/radio Quantum Properties cont. Energy delivered on a probabilistic basis. Probability of full delivery proportional to flux density at that place. Flux Density = time rate with which radiation passes a spatial position Large number of photons time rate of energy delivered, as wave theory predicts. Instant to instant fluctuating at receiver. Limit to precision of measurements photon /noise limited. Wave Particle Duality E.R.M cont. Photon : Quantised, statistical properties of radiation. Wave : Overall average effects. Atomic particles have wave properties. Light waves have particle properties. E and B are mutually orthogonal and perpendicular to direction of wave advancement.
Polarization Polarization - Plane Direction of electric field conventionally used to define direction of wave polarization. Plane, circular and elliptical polarizations. Random polarization particularly in visible spectrum. Note: (both Plane and circular polarizations are special cases of elliptical polarization) Transverse waves oscillating (a) in the z-direction, and (b) at an angle in the y-z plane Polarization - Plane y x z Phase difference = 0 and Ey = Ez Image copyright: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polclas.html#c4 David M. Harrison, Dept. of Physics, Univ. of Toronto
Polarization - Circular Polarization - Elliptical Phase difference = 90 o Ey = Ez Phase difference = 90 o Ey? Ez Left-handed and right-handed Left-handed and right-handed Image copyright: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polclas.html#c4 Image copyright: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polclas.html#c4 Polarization - Summary Plane polarized radiation incident on surface will reflect in different amounts depending upon the direction of polarization relative to the surface (depolarize). Horizontal and vertical polarizations are used in microwave radar (HH, VV, HV, VH). Parallel and perpendicular polarizations used by optics allow prediction of effect of any other direction of polarization. EMR & polarisations Longitudinal: sound waves Transverse: EMR Plane Circular Elliptical Random/unpolarised
Week 3 Coherent Radiation Coherent if there is a regular or systematic relationship between amplitudes (i.e. highly correlated). The coherent waves can be in phase or out of phase. Receiving detector will indicate more power at some locations and less power at others. In Phase v.s.. Out of Phase Incoherent Radiation Two waves are in phase Two waves are 180 o out of phase Amplitudes related in random fashion. Power of combined radiation is sum of each separately. Diffusely reflected solar radiation is incoherent measured flux received at a sensor from a heterogeneous surface will be simple sum due to each point surface. Detector will indicate average power at any position.
Monochromatic Radiation Microwave Radar and Lasers Reflected radiation can be highly coherent Range from no power to 4 x average power, depending on sensor position relative to the objects. Characteristic speckle pattern. Pattern, function of geometric arrangement of point source reflectors, not generally resolvable. Revealed through appropriate analysis. Doppler Effect Alteration of E.M.R. frequency due to relative motion between observer and source. Doppler Effect cont. When relative velocity is much less than c then change in frequency,? ƒ, is given by? ƒ = ƒ u cos? / c where u = relative velocity between source and receiver? = angle between direction of motion of source and a line connecting source and observer Airborne or spaceborne microwave radar has twice Doppler shift (source and detector re-radiation). radiation). Applications of Doppler Effects In astronomy: to predict the rate of expansion of the universe; Positioning: to determine the location of a point on the Earth by the change in frequency of an emitted signal from an overpassing satellite; and SAR: to improve the ground resolution of imaging radar (more in GMAT9606 Microwave Remote Sensing).
Emission of Radiation Blackbody Radiation Temperature scales Celsius ( 0 C): named after A. Celsius, a Swedish astronomer; Kelvin( 0 K): named after W. Kelvin, a British physicist; Celsius ( 0 C) Kelvin( 0 K) Absolut Ice & e zero water -273.15 0 0 273.15 Room temperat ure 27 300 Boiling point 100 373 Sun 5727 6000 Emission of Radiation All bodies with temperatures above absolute zero generate or emit energy in radiant form. Blackbody is a perfect radiator and absorber. Real material cannot emit thermally at a rate in excess of a blackbody. Blackbody absorbs and converts all incident radiant energy into heat energy.
Four Laws of Concern Stefan-Boltzman s Law Planck s Law Wien s Displacement Law Kirchhoff s Law Stefan-Boltzman s Law The total emitted radiance, M or Exitance, in Watts per square metre (W/m 2 ) is proportional to the fourth power of its absolute temperature in degrees Kelvin (for entire wavelength spectrum), M = Power/Area= s T Where s = 5.669 x 10-8 W/m 2 / o K 4 4 Planck s Law Planck s Law cont. It gives the intensity radiated by a blackbody at a temperature T as a function of wavelength or frequency. Where M 2 2πc h 5 λ e λ = hc/ λkt 1 1 C is the speed of light = 3 x 10 8 m/s T is the absolute temperature in degrees Kelvin h is Planck s constant = 6.6262 x 10-34 J.s k is Boltzmann s constant = 1.3807 x 10-23 J/K The equation can be simplified to: M C 1 1 λ = 5 C2 / λt λ e 1 Where C 1 = 3.747 x 10-16 W m 2 C 2 = 0.0144 m o K M? is spectral exitance at wavelength?, Watts/ m 3
Planck s Law cont. Spectral exitance of a blackbody at a given temperature is not the same at all wavelengths. For every long and very short wavelengths, M? is low. Example Radiation from the Sun Example Red Hot Object (Lillesand and Kiefer, 2000, Remote sensing and image interpretation.)
Wien s Displacement Law When the temperature of a blackbody increases, the overall radiated energy increases and the peak of the radiation curve moves to shorter wavelengths (why?). The wavelength,? M, for which spectral exitance is a maximum can be calculated by Wien s Displacement Law. Wien s Displacement Law cont.? M = c / T ; where c = 2.898 x 10-3 m o K Relationships Emissivity No real body is a perfect emitter, its exitance is less than a blackbody. The spectral emissivity, e, is defined as: Stefan- Boltzman slaw Integration Planck s Law: M? =f(t,? ) Differentiation Wien s Displacement Law ε λ = M M λ λ (material) (blackbody ) e nearly independent of temperature for most common materials and for terrestrial environment temperatures. Significant changes when material undergoes a change of state.
Emissivity cont. Kirchhoff s Law States that under conditions of thermal equilibrium, the spectral emissivity of a material must be equal to the spectral absorptance of that material (why?). Good approximation even when thermal equilibrium not present but temperature differences not extreme. Common practice to determine e by measuring spectral absorptance. Kirchhoff s Law cont. Kirchhoff s Law cont. Ideal Radiator = Perfect Absorber Object with zero exitance = perfect reflector or so- called White Body This relationship can only be applied in portion of spectrum where materials are opaque. Frequently occurs in thermal portion of spectrum.
Kirchhoff s Law cont. If surface opaque, T = 0, then p + a = 1 or a = 1 p a = e Radiation Summary In the shorter wavelength region where solar energy predominates transmission of radiation must be considered. At room temperature (~300 o K) spectral radiant exitance has a peak wavelength of ~10 µm between 7 and 15 micrometres, so called Thermal Infrared Region. Radiation in this band can be interpreted as temperature. Radiant Energy From The Sun Landsat MSS
Radiant Energy From The Sun cont. Emits radiation approximately as a blackbody at 5900 o K. Wavelength of maximum emission intensity is given by Wien Displacement Law as ~ 0.5 micrometres. Absorption by the sun s atmosphere and other effects means that radiation at top of earth s atmosphere not exactly equivalent to blackbody. Radiant Energy From The Sun cont. Values derived for spectral irradiance at the top of the atmosphere assume the mean distance of the earth from the sun. (at aphelion earth/sun distance ~ 1.034 distance at perihelion) By simple geometry represents an increase in solar irradiance between aphelion and perihelion of 6.8% or change from mean of 3.4%. Radiant Energy From The Sun cont. Spectral solar irradiance at the top of the atmosphere, July 1 st (aphelion) through January 1 st (perihelion) [W/m 3 ]. Band 4 5 6 7 July 1st 1710 1460 1195 802 Aug 1st 1730 1478 1209 811 Sept 1st 1750 1497 1223 821 Oct 1st 1770 1515 1237 830 Nov 1st 1790 1533 1251 839 Dec 1st 1810 1552 1265 849 Jan 1st 1830 1570 1279 858 (Average Oct values derived from P.N. Slater Remote Sensing )
Solar irradiance at the top of the atmosphere 2000 1800 1600 1400 1200 1000 Band 4 Band 5 Band 6 Band 7 800 July 1st Aug 1st Sept 1st Oct 1st Nov 1st Dec 1st Jan 1st Landsat MSS bands Summary Band 4 5 Wavelength (µm) 0.5-0.60.6 0.6-0.70.7 Coherent and monochromatic radiation Doppler effects Blackbody radiation Solar radiant energy 6 0.7-0.80.8 7 0.8-1.1