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Electromagnetic Radiation (EMR)

Electromagnetic Radiation (EMR). Remote Sensing Uses EMR! Today’s learning objectives:. What is light and how do we describe it? What are the physical units that we use to describe light? Be able to convert between them and use scientific notation!

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Electromagnetic Radiation (EMR)

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  1. Electromagnetic Radiation(EMR)

  2. Remote Sensing Uses EMR!Today’s learning objectives: • What is light and how do we describe it? • What are the physical units that we use to describe light? • Be able to convert between them and use scientific notation! • How do we calculate wavelength, frequency, and energy?

  3. Learning objectives (cont.): • What is the electromagnetic spectrum, and how do we describe it for RS? • What are the additive primary colors, how do they combine, and why is this important? • How does the atmosphere affect light, and why does this matter? • How do clouds affect RS data?

  4. All objects warmer than absolute zero emit EMR • Temperature of object determines the quality (peak wavelength) of emitted light • Objects reflect EMR emitted by other objects • Key basis of remote sensing because earth’s surface materials interact in unique ways with EMR

  5. How do we describe EMR? • Wavelength • Frequency • Energy

  6. Structure of electromagnetic radiation

  7. Wavelength

  8. Wavelength Units • Meters (m) • Centimeters (cm) • Millimeters (mm) • Micrometers (µm) • Nanometers (nm) • Angstroms (Ǻ)

  9. Frequency • The number of waves that pass through an imaginary plane in a specific amount of time (e.g., 1 second)

  10. Frequency Units • Hertz (Hz) • Kilohertz (KHz) • Megahertz (MHz) • Gigahertz (GHz) • Etc.

  11. c = wavelength x frequency (𝜆 x 𝜐) c = 2.98 x 108 m/sec (the speed of light) = 186,000 miles/sec Velocity of Light (c) [Worksheet Questions]

  12. Energy (Q) = Energy of a quantum (joules) h = Planck’s constant (6.626 * 10-34J*s/cycle) = Frequency (Hz = cycles/sec) So… Energy is proportional to frequency Energy is inversely proportional to wavelength

  13. Worksheet Question: • What are the units of energy in the equation ?

  14. Frequency-Wavelength-Energy Activity Wavelength, Frequency and Energy with a rope. Web Demo – Electromagnetic Waves

  15. The EMR Spectrum

  16. Electromagnetic Spectrum

  17. Visible Light • Wavelengths that dominate radiation given off by the sun • Most animals evolved to “see” these wavelengths • Captured by your digital camera

  18. Additive Primaries (Color Theory) (Add together in different proportions to make all other colors) Red + Blue = Magenta Red + Green = Yellow Blue + Green = Cyan Red + Blue + Green = White

  19. Why is Color Theory Important? • Your computer screen uses the 3 additive primaries to display all possible colors • To interpret remotely sensed imagery you must be able to interpret color [Worksheet Questions]

  20. Assigning Bands to Primary Colors • Computer monitor uses red, green, and blue to create color images • You assign your choice of satellite band to each primary color • Brightness of each color is determined by each pixel value in each band • Result is a color image with each pixel’s color determined by combination of RGB of different brightness.

  21. Infrared Portion of the Spectrum

  22. Infrared Radiation • Near Infrared (NIR) 720 – 1300 nm • Mid Infrared (MIR or SWIR) 1300 – 3000 nm • Far Infrared (FIR or Thermal) > 3000 nm

  23. Other parts of the Spectrum UV Radar

  24. Atmospheric Effects • Absorption (and transmittance) • Scattering (will discuss later in the semester)

  25. Absorption • Prevention or attenuation of the transmission of radiant energy through the atmosphere • Especially important: Ozone (O3), Carbon Dioxide (CO2), Water vapor (H2O)

  26. Transmission (“opposite of aborption) • Transmittance (t) = Transmitted/Incident • Varies with wavelength • Atmospheric transmittance varies depending on atmospheric conditions for each wavelength

  27. Absorption Ozone Hole Thermal IR – Greenhouse Effect

  28. Ozone • Absorbs strongly in the UV (short wavelengths) • Protects us from skin cancer!

  29. Carbon Dioxide • Absorbs in mid and far infrared • Greenhouse effect!

  30. Water Vapor • Very strong absorber in 5.5-7.0 um range • Very strong absorber > 27 um • Variable in time and space

  31. Clouds! • Most EMR wavelengths can’t penetrate clouds • Big problem in remotely sensed imagery—tropics especially • Temporal compositing to get rid of clouds • Cloud shadows a problem too

  32. Riverton Landsat Image July 15 1999 Cloudy!

  33. Summary • All of this is important because it determines in part how objects of interest interact with EMR which is what we use in remote sensing. The better we understand these interactions, the better we are at using the remote sensing tool!

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