Remote Sensing Assignment- Only 2 questions.

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Lab Exercise 3 – Electro-magnetic Radiation (EMR) Principles Purpose: To reinforce and apply concepts of EMR and its interaction with atmospheric constituents. Materials: Calculator Please show all calculations to receive credit for answers. Be sure that all results are given in appropriate units. 1. (A). What are the approximate/generalized wavelength ranges of the following spectral regions (in appropriate units of wavelength)? (a) Ultraviolet = 0.01 – 0.4 mm (b) Visible = 0.4 – 0.7 mm (c) Near IR = 0.7 – 1.5 mm (d) Shortwave IR = 0.7 – 3 mm (e) Thermal IR = 3-14 mm (f) Microwave > 1000 mm (B). Rayleigh scattering occurs when particles are less than 1/10th the wavelength of the striking EMR. What diameter particles (i.e. less than or equal to what particle diameters) would cause Rayleigh scattering in the following spectral regions? State diameters in microns (i.e., micrometers). (a) Ultraviolet @ 0.30 microns = 0.1 * 0.30 = 0.03 microns (b) Near IR @ 1.00 microns = 0.1 * 1.00 = 0.1 microns (c) Visible light – blue @ 0.45 microns = 0.1 * 0.45 = 0.045 microns (d) Visible light – red @ 0.65 microns = 0.1 * 0.65 = 0.065 microns (e) How much more Rayleigh scattering occurs for blue light at 0.45 microns than near infrared energy at 0.90 microns (assuming a uniform size-quantity distribution for atmospheric particles)? Blue light (1/ (0.45)4) = 24.386 mm Near infrared (1/ (0.90)4) = 1.524 mm Which is mean 24.386 – 1.524 = almost 16 blue more than red in terms of Rayleigh scattering 2. Briefly define, describe and compare the relative difference between wavelength, frequency, photon energy, and atmospheric transmission characteristics for the visible, near IR, and microwave portions of the EMR spectrum. Specify the frequency ranges for each of these three spectral regions (as you specified the wavelength ranges in #1 above). Describe photon energy and atmospheric transmission characteristics in relative terms (e.g., high, medium, low). A table is a convenient way to organize your response. Wavelength λ Frequency f Photon Energy Q Medium High Medium High Atm. Transmission Visible Low Near IR Medium Low Low High High Low Low High Microwave ‫ح‬ 3. (A) Refer to the graph below, which depicts solar spectral irradiance at the atmosphere’s edge and at sea level. Examining the general trend, which types of radiation correspond to the greatest differences between the two irradiance curves, considering your answers from questions 1 (B) and 2 (table above) regarding scattering and atmospheric transmission differences among short and long wavelengths? [Ignore the relatively short ranges in the infrared where atmospheric transmission is low and linked to specific gases on the graph.] 3. (B) Building upon your answer to question 3(A), approximate the wavelength ranges where atmospheric gases interfere strongly with transmission (deviations from the overall trend of higher scattering in shorter wavelengths)? Do you think these wavelengths are more subject to scattering, or to absorption and/or reflectance by the relatively large gas molecules specified? 4. To improve radiometric resolution, passive microwave systems normally have low spatial resolution with large instantaneous-field-of-view (IFOV; i.e., cannot resolve small targets). The natural irradiance level of microwaves is low as indicated by Planck's radiation law (Q= hf). (a) What does this law mean in terms of controls on Q (photon energy) by EMR wavelength and frequency? (b) Explain how this law relates to measurements in the microwave portion of the EMR spectrum (microwave radiometry) and why lower spatial resolution (i.e., larger IFOV) is beneficial for signal acquisition. 5. (a) If the solar irradiance at the top of the atmosphere in a given waveband is 200 Wm-2, the atmospheric transmissivity is 0.85, and the radiation is incident from zenith, what is the irradiance at the surface? Solar irradiance = > 200 w/m2 T = 0.85 Es= E * T Es = (200) (0.85) Es = 170 w/m2 (b) Given the same top of the atmosphere irradiance and waveband as in (a), what would the irradiance at the surface be if the solar zenith angle () was 45 and atmospheric transmissivity at this incidence angle were 0.80? Es = (E0) (T) (Cos0) Es = (200 w/m2) (0.80) (cos 45) ES = 113 w/m2 Electromagnetic Radiation (EMR): Properties, Sources and Atmospheric Interactions EMR as Information Link ◼ Link between surface and sensor Sun Sensor Surface Energy Transfer ◼ Conduction ◼ ◼ Transfer of kinetic energy by collision of molecules/atoms Requires direct contact Energy Transfer (cont) ◼ Convection ◼ ◼ Kinetic energy of bodies is transferred by movement of the bodies Applies only to liquids or gases (i.e., fluids) Example: boiling water Energy Transfer (cont) ◼ Radiation ◼ ◼ Energy transferred between objects in the form of electromagnetic waves/particles (light) Can occur in a vacuum (w/o a medium) EMR Properties ◼ Wave Theory ◼ ◼ ◼ ◼ EMR => continuous wave Energy transfer through media (vacuum, air, water, etc.) Properties Quantum (Particle) Theory ◼ ◼ ◼ EMR => packets of energy Photons or quanta Interaction of energy with matter Wave Theory ◼ Explains energy transfer as a wave ◼ Energy radiates in accordance with basic wave theory ◼ Travels through space at speed of light at 3 x 108 ms-1 (meters per second) Light from the Earth to the moon Sun to Earth Sun to Mercury Sun to Venus Sun to Mars Sun to Jupiter Sun to Saturn Sun to Uranus Sun to Neptune Sun to Pluto Sun to the nearest star Sun to the furthest stars Travel time 1.28 seconds 8.5 minutes 3 minutes 6 minutes 12.5 minutes 43 minutes 1 hour 2.6 hours 4 hours 5.4 hours 4.3 years 18 billion years Electromagnetic Wave Two components or fields E = electrical wave M = magnetic wave http://www.colorado.edu/physics/2000/waves_particles/ Wave Properties ◼ Amplitude ◼ ◼ ◼ Height of the wave Related to amount of energy carried by wave Wavelength ◼ ◼ Distance between successive crests or troughs of wave Measured in microns and designated by lambda (l) Wave Properties (cont) ◼ Frequency ◼ ◼ Number of wave forms or oscillations passing through a given point per unit time Measured as cycle/second (cycle s-1) or Hertz (Hz) Medium High Low Wave Properties (cont) ◼ Refraction ◼ ◼ Bending of EMR waves due to change in medium Change in medium -> change in density -> change in velocity -> change in direction air water Wave Properties (cont) ◼ Coherence ◼ ◼ Regular and systematic relationship between amplitudes of two waves - waves are in phase Incoherent if relationship between wave amplitudes is random – waves are out of phase Wave Properties (cont) ◼ Diffraction ◼ Change in direction (bending) of wave path due to coming in contact with a solid object Quantum Theory ◼ ◼ Wave theory doesn’t account for all properties of EMR Interaction of EMR with matter (atoms) ◼ ◼ ◼ Absorption Emission EMR is transferred in discrete packets (particles) ◼ Photons (quanta) Quantum Theory (cont) ◼ Planck’s Law (energy of a quantum): Q = hf Where: Q = energy of a quantum in Joules [J] h = Planck’s constant = 6.626 x 10-34 [J s] f (or ) = frequency of EMR wave [cycles s-1] or Hertz [Hz] Electromagnetic Spectrum l (mm = 10-6 m) Ultraviolet Near IR Far IR Microwave & 0.01 – 0.4 mm 0.7 – 1.5 mm 5.6 – 1000 mm Radio > 1000 mm Visible 0.4-0.7 mm Mid IR 1.5-5.6 mm Thermal-IR 3-14 mm Wavelength & Frequency Near Infrared Green Microwave Wavelength and Frequency ◼ Relationship is inverse ◼ ◼ High frequency associated with short wavelengths and high energy Low frequency associated with long wavelengths and low energy c=lxf where: c = speed of light (3 x 108 m s-1) l = wavelength f = frequency therefore: f = c/l and l = c/ f EMR Spectrum Short l, High f, High Q Long l, Low f, Low Q Wavelength and Frequency - Example ◼ Energy of Green Light l = 0.5 mm = 0.5 x 10-6 m = 5 x 10-7 m f = C/l = (3 x 108 m/s) / (5 x 10-7 m) = 0.6 x 1015 Hz = 6 x 1014 Hz Q = h f = 6.626 x 10-34 Js x 6 x 1014 cycles/s = 39.756 x 10-20 J ◼ Energy of Microwave l = 3000 mm = 3 x 10-3 m f = C/l = (3 x 108 m/s) / (3 x 10-3 m) = 1 x 1011 Hz Q = hf = 6.626 x 10-34 Js x 1 x 1011 cycles/s = 6.626 x 10-23 J Energy Laws ◼ Planck’s Radiation Law -- Total energy is a non-linear function of temperature (T) and wavelength (l) ◼ ◼ The hotter the object, the more radiation emitted The hotter the object, more radiation emitted in shorter wavelengths Sun Earth Energy Laws ◼ Stefan-Boltzmann Law quantifies total emitted radiation from a blackbody as a function of temperature M = sT4 Where M = total emitted radiation (Wm-2) s = Stefan-Boltzmann constant = 5.6697 X 10-8 Wm-2 K-4 T = temperature (K) ◼ Blackbody - a hypothetical or ideal radiator that absorbs and re-emits all energy incident upon it Energy Laws (cont) ◼ Wien’s Displacement Law ◼ ◼ Used to identify wavelength of maximum energy emission (lmax) Inversely related to temperature lmax = k/T Where k = 2898 mK T = temperature (K); K = Kelvin Energy Laws (cont) Sun: T = 6000K lmax = 0.48m Earth: T = 300K lmax = 9.7m Radiant Energy / Radiant Flux ◼ ◼ ◼ Radiant Energy (Ql) ◼ Capacity of radiation to do work Radiant Flux (F) ◼ Amount of radiant energy onto, off of or through a surface per unit time ◼ Measured in J/s or Watts (W) Radiant Flux Density ◼ Amount of radiant energy onto, off of or through a surface per unit time and area ◼ Measured in Watts per square meter (Wm-2) ◼ Irradiance = into surface; Exitance = away from surface Radiometric Quantities Name Symbol Units Concept Radiant Energy Ql Joules (J) Capacity of radiation within a spectral band to do work Radiant Flux Fl Js-1, Time rate of energy flow onto, off of or through a surface Irradiance El Wm-2 Fl incident upon a surface per unit area of that surface Radiant Exitance Ml Wm-2 Fl leaving a surface per unit area of that surface Radiance Ll Watts (W) Wm-2sr-1 Fl leaving a specific projected source area, in a given direction, w/in a specific solid angle Partitioning of Energy at Surface ◼ Radiant flux at the surface is partitioned among: ◼ Absorption ◼ Transmission ◼ Reflection Radiation Budget Equation ◼ Radiant Flux (F) incident at a surface = 1 + 2 + 3 ◼ 1) Amount of energy absorbed by the surface ◼ 2) Amount of energy reflected from the surface ◼ 3) Amount of energy transmitted through the surface Radiation Budget Equation (cont.) ◼ Dimensionless ratios: al = Fabsorbed / Fil ◼ Spectral absorptance: ◼ Spectral transmittance: tl = Ftransmitted / Fil ◼ Spectral reflectance: ◼ rl = Freflected / Fil al + tl + rl = Fil = 1 Radiation Budget Equation (cont.) ◼ Proportion of energy absorbed/transmitted/reflected will vary from target-to-target ◼ ◼ ◼ ◼ Material type Material condition For a given target, proportion absorbed, transmitted, and reflected energy will vary with wavelength Ability to distinguish between targets Atmospheric Interactions ◼ Energy detected by sensor is a function of ◼ ◼ ◼ Atmospheric influences Surface properties Atmosphere will affect EMR in three ways ◼ ◼ ◼ Absorption Transmission Scattering Constituents in the Atmosphere ◼ Responsible for absorption and scattering ◼ Water droplets/ice crystals ◼ ◼ Gas Molecules ◼ ◼ clouds CO2, water vapor, ozone Aerosols ◼ ◼ particles suspended in the atmosphere smoke, dust, sea salt, chemical pollutants Absorption ◼ ◼ Radiant energy is taken in by matter and converted into other forms of energy Atmospheric gases are selective absorbers w/ reference to wavelength ◼ Significant Absorbers: Oxygen, Nitrogen, Ozone, Carbon Dioxide, Water Vapor ◼ Ozone – ultraviolet region ◼ Water Vapor – specific bands in infrared ◼ Carbon Dioxide – thermal infrared region EMR Transmission Energy propagated directly through the atmosphere T T T A A A Atmospheric Windows ◼ Portions of the spectrum that transmit radiant energy effectively Wavelength Window Radiation Type 1.5 – 1.8 mm 2.0 – 2.4 mm 3.0 – 5.0 mm 8.0 – 14.0 mm 10.5 – 12.5 mm > 0.6 cm UV, visible, reflected IR (near) Reflected IR (shortwave) Reflected IR (shortwave) Thermal IR Thermal IR Thermal IR Microwave *0.3 – 1.1 mm *scattering may limit transmission for UV and shorter visible wavelengths Scattering ◼ Re-direction of energy ◼ No change in other properties ◼ Three primary types ◼ ◼ ◼ 1. Rayleigh (molecular) 2. Mie (aerosol) 3. Non-selective (particulates) Rayleigh (Molecular) Scattering ◼ Radiation interacts with molecules/particles whose diameter is much shorter than the wavelength ◼ ◼ Less than or equal to 1/10 the wavelength Scattering is usually performed by atmospheric gasses ◼ ◼ Oxygen Nitrogen Rayleigh Scattering (cont) ◼ Amount of scattering is inversely proportional to wavelength raised to 4th power (l-4) I a l-4 Blue light (0.4 mm) → 1/ 0.44 = 39.0625 Near infrared (0.8 mm) → 1/ 0.84 = 2.4414 → 39.0625 / 2.4414 = 16 times more scattering ◼ Accounts for: ◼ ◼ blue sky red sunset Rayleigh (cont) Mie (Aerosol) Scattering ◼ Particles with diameter about equal to wavelength d d ◼ Dust, pollutants, volcanic eruptions, smoke ◼ Affects longer wavelengths than Rayleigh scattering I a l-1 ◼ Occurs in lower 4.5 km of atmosphere Mie Scattering (cont) ◼ Direction/amount of scattering depends on physical characteristics of the aerosol: ◼ ◼ Shape / size Distribution of particles Rayleigh vs. Mie Scattering Non-Selective Scatter ◼ Diameter of particles is much larger than wavelength (on order 10 x wavelength) ◼ Cloud water droplets/ice crystals are source ◼ Affects all wavelengths, hence “non-selective” Scattering Effects ◼ Scatter can occur anywhere in information flow: Sun -> Surface -> Sensor ◼ Reduces direct illumination from sun and creates diffuse illumination ◼ Creates noise and reduces contrast in image ◼ May add to or reduce signal received by sensor ◼ Filters may be used to reduce effects of haze and scatter Path Radiance ◼ ◼ Radiation scattered into the sensor from the atmosphere Mostly shorter wavelengths Path radiance Atmospheric Effects Contrast Reduction ◼ Atmospheric effects can reduce image contrast Summary ◼ Energy Flow ◼ ◼ Wave Theory ◼ ◼ ◼ Wave properties Wavelength – Frequency relationship Quantum Theory ◼ ◼ Conduction, Convection, Radiation Energy of a photon EMR spectrum Summary ◼ Energy Levels ◼ Stefan-Boltzmann Law ◼ Wien’s Displacenment Law Summary ◼ Atmospheric Constituents ◼ Absorption ◼ Reflection/Scattering ◼ ◼ ◼ ◼ Rayleigh (molecular) Mie (aerosol) Non-selective Transmission (atmospheric windows)
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