1. Electromagnetic Radiation (EMR)

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

4. 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?

5. Maxwell’s Equations describe light as perpendicular electrical and magnetic waves. The changing magnetic wave creates the electrical wave, and vice versa.
What is light?
Direction of Electromagnetic Wave Propagation
From Wikipedia

6. All objects warmer than absolute zero emit EMR
Temperature of an object determines the quality and intensity (wavelength distribution) of emitted light
Objects reflect EMR emitted by other objects
Key basis of remote sensing because earth’s surface materials reflect light in unique ways.

8. How do we describe EMR?
Wavelength
Frequency
Energy

9. Wavelength

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

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

12. Frequency Units Hertz (Hz) = 1 wave cycle/second Kilohertz (KHz)
Megahertz (MHz)
Gigahertz (GHz)
Etc.
Work problem #1a and #1c on your lab worksheet!

13. Work problems #2 and #3 on today’s lab worksheet!
Velocity of Light (c)
c = wavelength x frequency (𝜆 x 𝜐)
c = 2.98 x 108 m/sec (the speed of light)
= 186,000 miles/sec
So…what happens to wavelength as frequency increases?
Work problems #2 and #3 on today’s lab worksheet!

14. Energy (Q) Light is a form of energy that propagates through space.
𝑄=ℎ∗ν = Energy of a quantum (joules)
h = Planck’s constant (6.626 * J*s/cycle)
ν = Frequency (Hz = cycles/sec)
So…
Energy is proportional to frequency
Energy is inversely proportional to wavelength
Work problems #4 and #5 on your lab worksheet!

15. The EMR Spectrum

16. Electromagnetic Spectrum

18. Visible Light Wavelengths that dominate radiation given off by the sun
Most animals evolved to “see” these wavelengths
In fact, eyes only sensitive to red,
green, and blue (using “cones”)
Captured by your personal multi-
spectral sensor (= digital camera)

19. Additive Primaries (Color Theory)
(Can add together in different proportions to make all other colors we see)
Red + Blue = Magenta
Red + Green = Yellow
Blue + Green = Cyan
Red + Blue + Green = White
Note that magenta doesn’t have a wavelength. Had a video about it, but it won’t work with powerpoint.
Do all of these colors have their own wavelength??

20. The Odd Case of Magenta
YouTube video about magenta

21. 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

22. Assigning Bands to Primary Colors
Computer monitor uses red, green, and blue to create color images
You choose 3 satellite bands and color each with one of the 3 primary colors
Brightness of each color is determined by each pixel value (DN) in in the band to which the color is assigned, and the three colors mix.
Result is a color image with each pixel’s color determined by combination of RGB of different brightness.
Work problem #6 on your lab worksheet!

23. Infrared Portion of the Spectrum

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

26. Other parts of the SpectrumUV
Radar

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

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

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

31. Absorption
Ozone Hole
Thermal IR – Greenhouse Effect

32. Ozone Absorbs strongly in the UV (short wavelengths)
Protects us from skin cancer!
Image: NH Dept. of Env. Services: Link

33. Carbon Dioxide Absorbs in mid and far infrared Greenhouse effect!
The Keeling Curve

34. Water Vapor Very strong absorber in 5.5-7.0 um range
Very strong absorber > 27 um
Variable in time and space
GOES water vapor

35. 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

36. Riverton Landsat Image
July
Cloudy!

37. 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!