1. Introduction toIntroduction to Physics fof Remote Sensing anjum m@nrsc gov inanjum_m@nrsc.gov.in

2. Electromagnetic Radiation Motion of charges produces EM waves. Changing Electric fields set up by oscillation of charged particles. Changing electric fields induce changing magnetic fields. Changing magnetic fields set up morechanging magnetic fields. Changing magnetic fields set up more changing fields and so on. 3. Wavelength, Frequency and Amplitude c = where c = 3 x 108 ms-1 in vacuum 4. THE ELECTROMAGNETIC SPECTRUM 5. Electromagnetic Spectrum Violet: 0.4 - 0.446 m Blue: 0.446 - 0.500 m Green: 0.500 - 0.578 m Yellow: 0 578 - 0 592 mYellow: 0.578 0.592 m Orange: 0.592 - 0.620 m Red: 0.620 - 0.7 m 6. Microwaves P band 0.3 1 GHz (30 100 cm) L band 1 2 GHz (15 30 cm) S band 2 4 GHz (7 5 15 cm)S band 2 4 GHz (7.5 15 cm) C band 4 8 GHz (3.8 7.5 cm) X band 8 12.5 GHz (2.4 3.8 cm) Ku band 12.5 18 GHz (1.7 2.4 cm) K band 18 26 5 GHz (1 1 1 7 cm)K band 18 26.5 GHz (1.1 1.7 cm) Ka band 26.5 40 GHz (0.75 1.1 cm) 7. MotivationMotivation Are physical principles important for application Remote Sensing scientists?pp g Why does vegetation reflect more strongly in a particular wavelength region?p g g Is thermal infrared panacea for measurement of all temperatures? 8. TERMINOLOGY Radiant energy: Energy carried / transferred by electromagnetic waves (Joule)e ect o ag et c a es (Jou e) Radiant energy density: Radiant energy per unit volume (J/m3)(J/m3) Radiant flux: Rate at which radiant energy is emitted, transferred or received in the form of ER. (Watt) Spectral Radiant flux : Radiant flux per unit wavelengthSpectral Radiant flux : Radiant flux per unit wavelength. Radiant emittance: Radiant flux emitted per unit area of aRadiant emittance: Radiant flux emitted per unit area of a source 9. TERMINOLOGYTERMINOLOGY Radiant Quantities Radiant intensity: Radiant flux (power) leaving a source per unit solid angle in a given direction. (W Sr-1 ) Irradiance: Radiant flux incident per unit area (W m-2 ) Radiance: Radiant flux per unit solid angle in a given direction per unit projected source area in thatp p j direction. (W m-2 Sr-1 ) 10. TERMINOLOGYTERMINOLOGY Spectral Quantities If the previous quantities are measured per unit wavelength interval at a particular wavelength, then the quantities are called spectral quantitiesthe quantities are called spectral quantities . Ex: Spectral Radiance (W m-2 sr-1 m-1) 11. ENERGY INTERACTION Wh EM i i id t i ENERGY INTERACTION Conservation of Energy When EM energy is incident on any given earth surface feature, three fundamental energy interactions are possible. A fraction ofgy p incident energy is reflected, absorbed and / or transmitted. Energy is neither created nor destroyed. Incident energy = reflected energyIncident energy reflected energy + transmitted energy + b b dabsorbed energy 12. Three forms of interaction I=A+R+T or A/I+R/I+T/I=1 (100%) 13. Energy InteractionEnergy Interaction Conservation of Energy T i t b t th ti f l ti hiTwo points about the conservation of energy relationship: The proportions of energy reflected,absorbed and transmitted will vary for different earth features depending on their material type and condition. The wavelength dependency. That is, even within a given feature type the proportion of reflected absorbedgiven feature type, the proportion of reflected, absorbed and transmitted energy will vary at different wavelengths. Two features may be distinguishable in one spectral range but not in another wavelength region. 14. Energy InteractionEnergy Interaction Reflection Many remote sensing systems operate in visible and NIR regions in which reflected energy is more Hence the reflectanceenergy is more. Hence, the reflectance properties of objects are more important. The reflectance is a function of surfaceThe reflectance is a function of surface roughness (or smoothness) of object. Based on surface roughness objects areg j categorized into two classes. 15. Energy Interaction Specular reflectorsp Objects which produce mirror like reflection are called Specular reflectors. 16. Energy Interaction Diff fl tDiffuse reflectors Rough surfaces that reflect uniformly in all di ti i d d t f th l f i iddirections independent of the angle of incidence are called Diffuse or Lambertian reflectors. 17. Energy InteractionEnergy Interaction Smooth and Rough Smoothness or roughness of a surface depends on the wavelength of the incident di ti d id tiradiation under consideration. According to Rayleigh, a surface is smooth if the surface height variations are less thanif the surface height variations are less than /8, where is the wavelength of incident radiation. Otherwise, surface is considered to be rough. 18. TERMINOLOGY Ratio Quantities Emissivity: Ratio of radiant emittance of a source to that a blackbody at the same temperaturey p Reflectance: Ratio of reflected radiant flux to incident radiant flux Absorptance: Ratio of absorbed radiant flux to incident radiant flux Transmittance: Ratio of transmitted radiant flux to incident radiant flux 19. Types of Remote Sensing SensorsTypes of Remote Sensing Sensors Active Passive 20. Introduction to Principles ofp thermal Remote Sensing 21. Thermal Radiation Any object above absolute zero, emits EMR. Objects around and we ourselves are thermal radiators. Ideal thermal radiator Black body. Emission is according to the Plancks lawEmission is according to the Planck s law M()= C1-5/[exp(C2/T) - 1] 22. Plancks lawPlanck s law Plancks Law: The most general lawPlanck s Law: The most general law Planck's Law allows us to calculate total energy di t d i ll di ti f bl kb dradiated in all directions from a blackbody (radiator) for a particular temperature and wavelength.g M()= C1-5/[exp(C2/T) - 1] hwhere C1(2hc2) = 3.74 x 10-16 W m-2, C2 (hc/k)= 1.44 x 10-2 m K, = wavelength (m), T = temperature (K), M() = spectral exitance (W m-2 m-1)M() = spectral exitance (W m m ), k = 1.38 x 10-23 W s K-1, h = 6.625 x 10-34 J s 23. TERMINOLOGY Blackbody An ideal thermal emitter is called a Blackbody. (Also known as Planckian radiator.) A black body is an ideal f h th tsurface such that Its emissivity is equal to 1. In other words it radiates the entire energy whatever it absorbedthe entire energy whatever it absorbed. For a given temp and wavelength, no body can emit more energy than a black bodymore energy than a black body. Emission from a black body is independent of direction, i.e. it is a diffuse emitter.d ect o , e t s a d use e tte 24. RADIATION LAWSRADIATION LAWS Stefan-Boltzmann Law The total energy radiated by an object at a particular temperature is given by M = T4 where M is total radiant exitance from the surface of the material (W m-2), is Stefan-Boltzmann constant (5.67 x 10-8 W m-2 K-4), T is absolute temperature in KT is absolute temperature in K. The higher the temperature of the radiator, the greater the total amount of radiation it emits 25. RADIATION LAWS It is continuous. Single maximum for any temp A T i hift tAs T increase, max shifts to the shorter wavelength. Exitance value is higher than That for the lesser T. Curves do not intersect. Beyond max radiationBeyond max, radiation Decrease monotonically with Increasing wavlength. I h t i ti di tiImp characteristic radiation Even in microwave region. Spectral distribution of energy radiated by blackbodies at various temperatures 9.6 m 26. Blackbody Radiation A blackbody is a perfect emitter and absorber of EM radiation. Two laws explaining the emission characteristics of the body are:characteristics of the body are: (a) Weins law (b) R l i h J l(b) Rayleigh-Jeans law 27. Weins law Thi l h ld d f hi h f i This law holds good for high frequencies M()=C1-5/exp(C2/T) M is the spectral Exitance With C1 and C2 as constantsWith C1 and C2 as constants Gives the wavelength at which the exitance is maximum and is related toexitance is maximum and is related to temperature. T constant maxT = constant If max is in micrometer and T in 0K, the constt= 2897 max = 2897/T 28. For Earth T 300 0KFor Earth, T ~ 300 0K = 9 66 micron max = 9.66 micron. Hence 8 15 micrometer region thermal IR regionHence 8 15 micrometer region thermal IR region 29. TERMINOLOGY Graybody A graybody is one for which emissivity value is constant but less than unity. A l ti di t i f hi hA selective radiator is one for which emissivity value varies with wavelength. 30. TERMINOLOGY Radiant exitances for a blackbody, graybody and a selective radiator 31. TERMINOLOGYTERMINOLOGY Spectral emissivities for a blackbody, graybody andp y g y y a selective radiator 32. l h lRayleigh-Jeans law This law explains blackbody emission at high wavelengths:g g M()=C -4T/C M()=C1 4T/C2 33. PARTICLE THEORY The particle theory suggests that electromagnetic radiation is composed of many discrete packets f ll d Ph t Q t of energy called Photons or Quanta. The energy of each quantum is given by Q = h where Q is energy of quantum (J), h is Plancks constant (6.626 x 10-34 J-s) and is frequency Also, Q = hc/, implies the longer the wavelength involved, the lower its energy content. 34. RELEVANCERELEVANCE Wiens Displacement Law Is Thermal Infrared suitable for measurements Is Thermal Infrared suitable for measurements of all ranges of temperatures? ---- NO! Glacier at 20 C (~253 K): m =2898/253=11.45 m (TIR) Room Temperature (300 K): m =2898/300=9.66 m (TIR) Forest fire (~800K): m =2898/800=3.66 m (TIR)m Volcano (~1200 C): m =2898/1473=1.97 m (Mid IR) Sun (6000 K): m =2898/6000=0.48 m (Green)Su (6000 ) m 898/6000 0 48 (G ee ) 35. ConclusionsConclusions In remote sensing, we study reflective/emissive/scattered properties F ifi l d diff For a specific target, specular and diffuse interactions depend on the incoming wavelengthwavelength In thermal remote sensing, emission is the key and thermal (far) infrared is not always the answer Physical principles are important for understanding processes in remote sensing!understanding processes in remote sensing!