EE/Ge 157 a SYSTEMS FOR REMOTE SENSING FROM SPACE Week 3: Visible and Near IR 3-1

Transcription

1 EE/Ge 157 a SYSTEMS FOR REMOTE SENSING FROM SPACE Week 3: Visible and Near IR 3-1

2 TOPICS TO BE COVERED Source Characteristics Interaction Mechanisms Reflection, Scattering, Diffraction Gratings, Vibrational Processes, Crystal Field Effect, Charge Transfer, Conjugate Bonds, Energy Bands, Fluorescence Geologic Materials, Biologic Materials, Depth of Penetration Remote Sensing Systems Basic System, Basic Telescope, Resolution, Telescope Classification, Types of Imaging Systems, Imaging Terms 3-2

3 Basic Remote Sensing System Source Detector Waves Emitted Scattering Object Collecting Aperture 3-3

4 Source Spectral Characteristics From Elachi,

5 Reflection Snell s Law Incident Wave Reflected Wave Air θ i θ s k i sinθ i k s sinθ s Surface Interface k t sinθ t ε θ t Transmitted Wave Boundary conditions require the same phase velocity along the interface: ω k i sinθ i = ω k s sinθ s = ω k t sinθ t θ i = θ s sinθ i = ε sinθ t 3-5

6 Reflection Reflection Coefficient From Maxwell s equations, one finds that e=3, Rh e=3, Rv e=80, Rh e=80, Rv R h 2 = cosθ ε sin 2 2 θ cosθ + ε sin 2 θ R v 2 = 2 2 εcosθ ε sin θ 0.2 cos + sin 0 2 ε θ ε θ Incidence Angle 3-6

7 Reflection Brewster Angle For vertical polarization there is a special angle at which there is no reflection. This happens when tan θ = n = ε r This angle is called the Brewster angle. Typically used in gas lasers to allow oscillation in only one of the two polarizations Dielectric Constant 3-7

8 Reflection Restrahlen Effect At normal incidence, the reflection coefficient becomes R h = R v = n 1 n + 1 = n r 1+ in i n r + 1+ in i The magnitude squared is R R 2 ( ) n i ( ) n i = n 1 r n r +1 Near an absorption line n i >> n r R 2 = 1 ν 0 ν 3-8

9 Scattering by Particles Spheres From Radar Cross Section Handbook, by Ruck, et al.,

10 Reflection from Rough Particulate Surface R ν 0 ν 3-10

11 Reflectivity Minnaert Law In the general case of natural surfaces, the reflectivity is modeled by empirical expressions The Minnaert law models the reflection as Bcosθ s = B 0 ( cosθ i cosθ s ) κ B = Apparent surface brightness B 0 = Brightness of ideal reflection κ = Darkening parameter θ i,θ s = Incidence and scattering angles For a Lambertian surface κ = 1 B = B 0 cosθ 3-11

12 Diffraction Gratings θ d d k d = kd sinθ = n2π d λ = n sinθ 3-12

13 Vibrational Processes In a molecule with N atoms, there are 3N possible modes of motion since each atom has 3 degrees of freedom Of these, 3 constitute translation and 3 constitute rotation of the molecule as a whole The remaining 3N-6 constitute independent type of vibrations Each mode corresponds to a classical frequency ν The energy levels are 1 E = n + h n N h N ν ν Fundamental tones occur when only one is 1 Overtones occur when only one is an integer > 1 Combination tones are combinations of fundamental and overtones n i n i 3-13

14 H 2 O Molecule O O H H O λ 2 = 6.08µ H H H H λ 1 = µ λ 3 = µ Overtone : 2ν 3 λ = 1. 45µ Combination Tone : ν = ν 2 + ν 3 λ = 1.87µ 3-14

15 Crystal Field Effect Emerald and Ruby From Elachi,

16 Energy Bands From Elachi,

17 Fluorescence From Elachi,

18 Geologic Materials From Elachi,

19 Geologic Materials From Elachi,

20 Biologic Materials From Elachi,

21 Biologic Materials From Elachi,

22 Biologic Materials From Elachi,

23 Depth of Penetration The solution to the wave equation is E = Ae i( kr ωt + φ) When the medium is absorbing, the index of refraction is n = n r + in i k = n r k 0 + in i k 0 and the wave equation solution is E = Ae i( kr ωt+φ ) = ( Ae ik ( 0 n rr ωt +φ ))e k 0 n i r This equation shows an exponentially decreasing field. The penetration depth is d = 1 2n i k 0 = λ 4πn i 3-23

24 Basic Remote Sensing System Source Detector Waves Emitted Scattering Object Collecting Aperture 3-24

25 Imaging Terms Swath Width Field-of-view Dwell Time Cross-Track Direction Along-Track Direction 3-25

26 Types of Imaging Systems From Elachi,

27 Comparison of Imaging Systems Type Advantag e Disadvantag e Film fra ming came ra Electronic framing camera Scanning systems Pushbroom imagers La rge ima ge forma t High information density Cartographic accuracy Broad spectral range Data in digital format Good geometric fidelity Simple detector Na rrow fie ld-of-vie w optics Wide s we e p capa bility Ea s y to imple me nt multiple wavelenghts Long dwell time per detector Good cross-track geometric fide lity Transmission of film Pote ntia l ima ge s me a ring Difficulty in getting large arrays Wide field-of-view optics Low detector dwell time Moving parts Difficult to achieve good geometric fidelity Wide field-of-view optics From Elachi,

28 Basic Telescope Primary Secondary Focal Plane f = FD f = focal length F = focal ratio D = aperture diameter 3-28

29 Diffraction Limited Resolution Circular Aperture λ d = 2.44 D Rayleigh criterion for resolution: α = 1.22 λ D 3-29

30 Telescope Classification Te le s c o pe Prima ry Mirro r S e c o nda ry Mirro r Cassegrain Parabola Hyperbola Gregorian Parabola Ellipse Ritchey-Chritein Hyperbola Hyperbola Da ll-kirkha m Ellips e Sphe re Newtonian Parabola Flat Schmidt Aspheric Sphere Scwarzschild Sphere Sphere From Space Remote Sensing Systems: An Introduction, by H.S. Chen,

31 Types of Telescopes Newtonian Ritchey-Critien Cassegrain Schwarzschild Gregorian Dall-Kirkham Schmidt From Space Remote Sensing Systems: An Introduction, by H.S. Chen,