Electromagnetic Radiation Principles

1. Electromagnetic Radiation Principles

2. Interactions of EM Energy Space Sensor Sun Atmosphere Earth’s Surface Adapted from Buenemann by Campbell 2010

3. Modes of Energy Transfer Adapted from Buenemann by Campbell 2010

4. EMR Models • Models that describe how EMR is created, how it propagates through space, and how it interacts with other matter • Two models: • Wave Model • Conceptualizes EMR as an electromagnetic wave that travels through space at the speed of light • Particle Model • Conceptualizes EMR as an electromagnetic wave that consists of discrete packets of energy, or quanta Adapted from Buenemann by Campbell 2010

5. The Wave Model • Speed of EMR in space = Speed of light= 3  108 m/sec (= 300,000,000 m/sec) • Magnetic and electric fields • Wavelength, Frequency, Amplitude Adapted from Buenemann by Campbell 2010

6. Wave Components • Relationship between wavelength (l), frequency (n), and speed of light (c): • Frequency • Wavelength (measured in Hertz – cycles/second) (measured inmm, nm, etc.) Adapted from Buenemann by Campbell 2010

7. Blackbodies and More • Blackbody: • Radiates energy at the maximum possible rate per unit area at each wavelength for any given temperature • Absorbs all the radiant energy incident on it (i.e., no reflected or transmitted energy) Adapted from Buenemann by Campbell 2010

8. Blackbodies and More • Total emitted radiation from a blackbody: Ml = total emitted radiation from a blackbody (Watts/m2) s = Stefan-Boltzmann constant (5.6697  10-8 Wm-2K-4) T = absolute temperature (degrees Kelvin) • What does this mean? • The hotter the object, the greater the amount of energy emitted by that object • Amount of energy emitted corresponds to sum/integral of the area under its curve (Stefan-Boltzmann law) Adapted from Buenemann by Campbell 2010

9. Blackbodies and More • Dominant wavelength of radiation emitted by a blackbody: T = absolute temperature (degrees Kelvin) lmax = dominant wavelength k = constant (2898 mm  K) • What does this mean? • The hotter the object, the shorter its dominant wavelength or the higher its dominant frequency (Wien’s displacement law) Adapted from Buenemann by Campbell 2010

10. EM Spectrum of Radiant Energy • Wavelength or frequency interval= band, channel, or region • Spectral resolution of sensors Adapted from Buenemann by Campbell 2010

11. The Particle Model • Quantizes EM radiation as particles (photons) and energy as packets of energy (quanta; light quantum = unit of light = photon) rather than in terms of its wavelike properties • Atom: • smallest possible particle of a chemical element • consists of: • electrons: negatively charged • protons: positively charged • neutrons: no charge • consists of: • heavy nucleus: protons and neutrons • light electron cloud Adapted from Buenemann by Campbell 2010

12. The Particle Model • Interaction between nucleus (+) and electrons (-) keeps electron in orbit • Motion of electron is within a certain range from the nucleus but the orbital paths are variable • Think of the orbital paths as energy levels resembling an unevenly spaced ladder! • Why uneven? • Because electrons do no necessarily use consecutive rungs of the ladder: they follow selection rules • That is, electrons may jump from one orbit to another without traversing positions in between = quantum leap / quantum jump Adapted from Buenemann by Campbell 2010

13. The Particle Model • To climb to a higher level, the electron must perform work, which requires a quantum of energy • If energy is added, the electron absorbs it, which causes it to get excited and jump to a higher level • An electron falls to a lower level when it moves to a de-excited state • When this happens, the electron gives off whatever energy is left over in the form of a single packet of EMR, a particle-like unit of light (= photon or light quantum) • Amount of energy released is independent of the number of jumps used to get from a higher to lower level Adapted from Buenemann by Campbell 2010

14. The Particle Model Creation of Light from Atomic Particles Adapted from Buenemann by Campbell 2010

15. The Particle Model • Quantum theory of EM radiation (Niels Bohr & Max Planck) • As discussed: energy is transferred in discrete packets called quanta or photons • Relationship between frequency of radiation as expressed by wave theory and the quantum: Q = energy of a quantum (Joules) h = Planck constant (6.626  10-34 J sec) c = speed of light l = dominant wavelength Adapted from Buenemann by Campbell 2010

16. The Particle Model • What does that mean? • Energy of quantum = inversely proportional to its wavelength • The longer the wavelength, the lower its energy content • What does that mean for remote sensing? • Longer-wavelength energy (e.g., TIR) is more difficult to detect than shorter-wavelength energy (e.g., RGB) --- a sensor may have to swell longer on an IFOV to measure the longer wavelength energy Adapted from Buenemann by Campbell 2010

17. The Particle Model • Substances have different colors due to: • differences in their energy levels • differences in their selection ruls • So, energy in different substances releases energy as photons of different wavelengths • In atoms and molecules .. • orbital changes produce the shortest-wavelength radiation • molecule vibrational motion changes produce near- and/or middle-infrared radiation • rotational motion changes produce long-wavelength infrared or microwave radiation Adapted from Buenemann by Campbell 2010

18. The Particle Model • Photoelectric effect • When matter is heated to such high temps that an electron breaks free from its orbit, leaving a free electron and an ion • If an atom becomes ionized and a free electron drops in to fill a vacant energy level, then the radiation given off is unquantized and a continuous spectrum or radiation at all wavelengths (rather than band or series of bands) is produced Adapted from Buenemann by Campbell 2010

19. Energy of Quanta Adapted from Buenemann by Campbell 2010

20. Energy-Matter Interactions • 100% solar energy input- 25% scattered and reflected – 20% absorbed= 55% transmitted to Earth Adapted from Buenemann by Campbell 2010

21. What happens to the Sun’s Radiant Energy Once It Enters The Earth’s Atmosphere? • Scattering • Absorption • Reflectance • Refraction • Transmission Adapted from Buenemann by Campbell 2010

22. Absorption • Ability of an object to assimilate energy from EM energy that strikes it • Causes temperature of object to increase, and radiation to be converted from one form of energy to another (i.e. change in wavelength) • Good radiators are also good absorbers.Poor radiators are also poor absorbers. • Dark objects tend to be better absorbers than light objects Adapted from Buenemann by Campbell 2010

23. Reflectance • Ability of an object to return waves in the general direction from which they came • Neither the object nor the waves are modified (vs. absorption) • If a wave is reflected, it cannot be absorbed, and vice versa • Good absorbers are poor reflectorsPoor absorbers are good reflectors. • Dark objects tend to be better absorbers than light objects Adapted from Buenemann by Campbell 2010

24. Reflectance vs. Absorption • White clothes of one skier reflect much solar energy, keeping him cool, while dark clothes of the other skier absorb much solar energy, keeping him warm. Adapted from Buenemann by Campbell 2010

25. Types of Reflectance • Specular reflection (~ smooth surface) • When the average profile height of the surface from which the radiation is reflected is several times smaller than the wavelength of the incident radiation • Near-perfect specular reflector: e.g., calm water • Diffuse reflection (~ rough surface) • When the average profile height of the surface from which the radiation is reflected is relatively large compared to the wavelength of the incident radiation • Near-perfect diffuse reflector: e.g., white paper • Perfect diffuse reflector: e.g., Lambertian surface Adapted from Buenemann by Campbell 2010

26. Types of Reflectance • Lambertian surface = Surface for which the radiant flux leaving the surface is constant for any angle of reflectance to the surface Adapted from Buenemann by Campbell 2010

27. Scattering • Deflection and redirection of radiation by particulate matter and gases in the atmosphere • Involves a change in the direction of light waves but no change in wavelengths • Amount of scattering is affected by: • Wavelength of the radiation • Size, shape, and composition of the molecule or particulate Adapted from Buenemann by Campbell 2010

28. Types of Scattering • Rayleigh scattering / Molecular scattering • When the effective diameter of the matter are many times smaller (< 0.1) than the wavelength of incident EM radiation • Mie scattering / Nonmolecular scattering • When the diameters of particles are roughly equal (0.1-10) to the wavelength of incident EM radiation • Nonselective scattering • When particles are greater (> 10) than the wavelength of incident EM radiation Adapted from Buenemann by Campbell 2010

29. Types of Scattering Adapted from Buenemann by Campbell 2010

30. Why is the Sky Blue? • Air molecules are tiny, and therefore good Rayleigh scatterers • Ordinary sunlight (visible light) is composed of a spectrum of colors that ranges from violet and blue (.47 um) to orange and red (0.63 um) • Pure air scatters violet and blue (shorter wavelengths) three to four times more effectively than reds and oranges (longer wavelengths) • Were it not for the fact that human eyes are more sensitive to blue light than to violet light and were it not for the fact that there are more blue than violet wavelengths in sunlight, selective scattering would make a clear sky appear violet instead of blue! Adapted from Buenemann by Campbell 2010

31. Why is Impure Sky Gray or White? • Airborne pollutant are fairly large (.5 to 1 um), and therefore poor Rayleigh scatterers • Airborne pollutants scatter all wavelengths of the visible light L.A. – White sky L.A. – Blue sky Adapted from Buenemann by Campbell 2010

32. Why are Sunsets Orange-Red? • Path of sunlight through the atmosphere is much longer at sunrise or sunset than during the day  Increased amount of violet and blue light is scattered out of the beam along the way  Light that remains at sunrise or sunset is reddened Example: a beam of sunlight that produces a red sunset over the Appalachians is, at the same time, contributing to the deep blue of a late afternoon sky over the Rockies Adapted from Buenemann by Campbell 2010

33. Transmission • Process whereby radiant energy passes completely through a medium • Transmissivity of objects varies considerably among mediums • Ex.: earth materials – poor transmitters; water – good transmitter • Transmissivity of objects varies depending on the wavelength • Ex.: glass – good transmitter for shortwave radiation but not for longwave radiation Adapted from Buenemann by Campbell 2010

34. Refraction • Bending effect (change of speed and direction) on EM waves that occurs when solar radiation passes from one medium to another, i.e., from virtually empty space into the atmosphere • Comparable to the process by which a crystal disperses the component colors of the light passing through it • Some effects: • Rainbows • Flattening of the sun at sunset Adapted from Buenemann by Campbell 2010

35. Refraction & Rainbows • Produced as raindrops refract and reflect light in such a way that the shortest wavelengths (violets + blues) appear on the inside of the bow, and the longest wavelengths (oranges + reds) on the outside of the bow Adapted from Buenemann by Campbell 2010

36. Refraction & Flattening of the Sun • Distorted appearance of the Sun nearing sunset over the ocean is produced by refraction of the Sun’s image in the atmosphere – the sun is actually below the horizon Adapted from Buenemann by Campbell 2010

37. What happens to the Sun’s Radiant Energy Once It Reaches The Earth’s Surface? Some of the radiant energy is: • Scattered • Absorbed • Reflected • Refracted • Transmitted Some of the energy is transferred via: • Conduction • Convection • Latent heat Adapted from Buenemann by Campbell 2010

38. Radiant Budget Equation • Radiant flux (F) • Amount of energy onto, off of, or through a surface per unit time (in Watts (W)) • Provides fundamental information about terrain • Radiation budget equation Fil = incident radiant flux in specific wavelength (l) rl = amount of energy reflected from the surface al = amount of energy absorbed by through the surface tl = amount of energy transmitted through the surface (based on radiant energy incident to the surface from any angle in a hemisphere) Adapted from Buenemann by Campbell 2010

39. Radiometric Concepts • Hemispherical reflectance (rl) • Dimensionless ratio of radiant flux reflected from a surface to the radiant flux incident to it • Hemispherical transmittance (tl) • Dimensionless ratio of radiant flux transmitted through a surface to the radiant flux incident to it • Hemispherical absorptance (al) • Dimensionless ratio of radiant flux absorbed by a surface to radiant flux incident to it Adapted from Buenemann by Campbell 2010

40. In a Nutshell … • The Laws of Thermodynamics in a Nutshell • 1st Law (Conservation of Energy): Energy is conserved; it can be neither created nor destroyed. • 2nd Law (Entropy):In an isolated system, natural processes are spontaneous when they lead to an increase in disorder, or entropy. • 3rd Law (Absolute Zero):The entropy of a system approaches a constant (zero) when the thermodynamic temperature approaches absolute zero (0 K). • Previous and following equations assume that energy is conserved! Adapted from Buenemann by Campbell 2010

41. Radiometric Concepts • Percent reflectance (prl) of a surface: • Used to describe spectral reflectance characteristics of surface target features • Hemispherical reflectance  100 Adapted from Buenemann by Campbell 2010

42. Radiometric Concepts • Problem: • RS images are “snapshots” only • rl, tl, and al radiometric quantities = too generalized • No information about the amount of incoming / outgoing radiant flux • No information about the direction of incoming / outgoing radiant flux • More precise radiometric quantities needed • Radiant flux density • Radiance Adapted from Buenemann by Campbell 2010

43. Radiometric Concepts • Radiant flux density • Average radiant flux density • Irrandiance & Exitance (W/m2) • Irradiance: radiant flux incident per unit area • Exitance: radiant flux leaving per unit area • Pro: Information on size of study area included • Con: Still no info about the direction of radiant flux

44. measured in steridians (sr) Radiometric Concepts • Radiance (Ll or Fl) – measured in W m-2 sr-1 • Radiant flux per unit solid angle (in certain wavelenths) leaving a real surface area in a given direction per unit of projected surface area in that direction Sensor only measures the radiant flux that is “funneled” through the 3D-cone – solid angle field of view

45. Steridian • Standard International unit (alternative to degrees squared) of the solid angular ‘area’ subtended by a 2D-surface about the origin in 3D-space • Frequently used where a flux through a 3D-surface is involved • Used for normalizing whatever is being described to the angular area subtended by a sphere • Fractional area of the surface relative to the sphere = projected surface area (r2)/ total area of the sphere (4pr2) • Projected surface area = real surface area  cos u(u = angle b/w the radiation direction and the surface normal) • Solid angle in steradians = fractional area 4p (Sphere subtends 4p steradians about the origin) • Solid angle in degrees squared = fractional area by 4  1802/π = 129,600/π Adapted from Buenemann by Campbell 2010

46. What happens to the Radiant Flux Once It Leaves The Earth’s Surface? • Scattering • Absorption • Reflectance • Refraction • Transmission + • Counter-radiation Adapted from Buenemann by Campbell 2010

47. What happens to the Radiant FluxOnce It Reaches the Sensor System? • Various things, e.g.: • Interactions with camera filter, lens, emulsion, etc. • Recording of photons in different bands, depending on the satellite sensor • Etc. Adapted from Buenemann by Campbell 2010

48. Radiometric Variables • Problem with simple radiation (Ll) concept: • Noise is introduced along the way … • Radiant energy recorded at sensor is not a true function of the amount of radiance leaving the terrain within the instantaneous field of view (IFOV) at a specified solid angle • So …: • More measurements … more variables … Adapted from Buenemann by Campbell 2010

49. Radiometric Variables • Eo = solar irradiance at the top of the atmosphere (W m-2) • Eol = spectral solar irradiance at the top of the atmosphere (W m-2 mm-1) • Ed = diffuse sky irradiance (Wm-2) • Ed l= spectral diffuse sky irradiance (Wm-2 mm-1) • Eg = global irradiance incident on the surface (Wm-2) • Eg l= spectral global irradiance incident on the surface (Wm-2 mm-1) • t = normal atmospheric optical thickness • Tu = atmospheric transmittance at an angle u to the zenith • uo = solar zenith angle • uv = view angle of the satellite sensor (or scan angle) • m = cos u • rl = average target reflectance at a specific wavelength • rln = average reflectance from a neighboring area at a specific wavelength • Ls = total radiance at the sensor (W m-2 sr-1) • Lt = total radiance from the target of interest toward the sensor (W m-2 sr-1) • Li = intrinsic radiance of the target (W m-2 sr-1) • Lp = path radiance from multiple scattering (W m-2 sr-1) Adapted from Buenemann by Campbell 2010

50. Target and Path Radiance • Radiance (Lt) from paths 1, 3, and 5 contains valuable intrinsic spectral information about the target of interest • Path radiance (Lp) from paths 2 and 4 includes diffuse sky irradiance or radiance from neighboring area, which introduces radiometric noise in the RS data Adapted from Buenemann by Campbell 2010