Electromagnetic Radiation Electromagnetic Spectrum Radiation Laws Atmospheric Absorption Radiation Terminology

Remote sensing images record the interaction of electromagnetic energy with the earth’s surface. The most familiar form of electromagnetic radiation is VISIBLE LIGHT. Electromagnetic Radiation

Source Magnetic Component Electric Component Two properties of electromagnetic radiation: (1) wave, and (2) particle.

Wavelength, Amplitude and Frequency = c/ is the wavelength c is the speed of light (~ 3×10 8 m/s) is the frequency (s -1 = Hz) red = 3x10 8 /4.6x10 14 = 650 x m = 650 nm Energy levels Amplitude wavelength

Frequencies used in remote sensing unitFrequency (cycles per second) Hertz (Hz)1 Kilohertz10 3 (=1,000) Megahertz (MHz)10 6 (=1,000,000) Gigahertz (GHz)10 9 (=1,000,000,000)

0.03 nm 300 nm380 nm720 nm 15.0  m 30 cm VISIBLE LIGHT Violet: nm Blue: nm Green: nm Yellow: nm Orange: nm Red: nm Angstrom = m nm = m  m = m cm = m km = 1000 m

Radiation Laws 1. Plank’s law: Wavelength (  m) c is the speed of light, 3×10 8 m/s is the frequency T is temperature, in Kevin h is Plank constant K is Boltzmann constant Wavelength = c /

3. Wien’s law:  2,897.8/T  wavelength (  m) at which radiance is at a maximum Radiation Laws Wavelength (  m) As objects become hotter, the wavelength of maximum emittance shifts to shorter wavelengths.

3. Stefan-Boltzmann’s law: W =  T   W = total emitted radiation (watt m -2 )  Stefan-Boltzmann constant (5.6697x10 -8 W m -2 K -4 ) T = temperature in Kelvin (T in Kelvin = o C) Radiation Laws Hot blackbodies emit more energy per unit area than do cold blackbodies

Radiation Laws 4. Kirchhoff’s law:  = M/M b  emissivity: a measure of the effectiveness of an object as a radiator of electromagnetic energy (0-1) M = emittance of a given object M b = emittance of a blackbody Blackbody: object that absorbs all incident radiation; none is reflected

Radiation Laws 5. Radiant Energy Conservation Law Absorption + Reflection + Transmission = Incoming Radiation Absorptivity + Reflectivity + Transmissivity = 1 absorption reflection transmission Incoming

Radiation Laws 6. Cosine’s Law E s = E p cos  00 90  cos   EsEs EpEp

Interaction with the Atmosphere Scattering Near InfraredRedGreenBlue Wavelength (  m) Intensity of Scattering (%) Scattered Rayleigh Scattering Blue sky and red-orange sunset

Scattering Rayleigh Mie Nonselective Specks of dust, N 2 and O 2 molecules Larger water droplets, large particles of airborne dust Dust, pollen, smoke, water droplets Scattering decreases the quality of an image ~  Weakly wavelength dependent Wavelength independent Strongly wavelength dependent

Electromagnetic Radiation IoIo IsIs IDID I = IDID Io Io IsIs + +

Interaction with the Atmosphere Absorption Ozone (O 3 ) Carbone Dioxyde (CO 2 ) Water Vapor (H 2 O) of radiation occurs when the atmosphere prevents, or strongly attenuates, transmission of radiation or its energy through the atmosphere.

Interaction with the Atmosphere Atmospheric Windows Transmittance (%) Far Infrared Mid IR Near Infrared Visible Absorption UV & Visible  m  m mid- Infrared  m  m Thermal IR  m   m Microwave mm mm Near infrared

Typical values for the Earth as a whole. Absorbed by Ozone 3 Reflected from Clouds 25 Absorbed by Dust, Gases 19 Absorbed by Ground 45 Reflected from Ground 8 Solar Radiation (Shortwave)

Terrestrial Radiation (Longwave > 10  m) Radiated from Ground 113 Radiated to Ground 98 Radiated to Space by Atmosphere 49 Through Atmospheric Windows 15 Evaporation 22 Turbulence Transfer 8

Interaction with Surfaces Reflection Specular reflectionDiffuse (isotropic) reflection Water bodies Mirror-type surface “Lambertian surface”

Interaction with Surfaces Transmission Transmittance t = Transmitted radiation Incident radiation Non-selective selective

Radiation Terminology (1) 1.Radiant Energy: total radiation energy in Joules (J) 2. Radiant flux: Radiant energy per unit time in J/s 3. Radiant flux density: Radiant flux per unit surface area in J/(m 2 · s) or W/m 2

Radiation Terminology (2) 4. Irradiance: Radiant flux density. It refers to fluxes to or from a flat surface in all directions in J/(m 2 · s) 5. Radiance: Radiant flux density per unit solid angle. It refers to fluxes to or from a surface in a specific direction in J/(m 2 · s · sr) 6. Spectral Radiance: Radiant flux per unit wavelength in J/(m 2 · s · sr ·  m)

Radiation Terminology (3) 7. Albedo: Reflected radiant flux density (irradiance) divided by incoming radiant flux density (irradiance) from a flat surface in all directions unitless 8. Reflectance: Reflected radiance in one direction (in J/(m 2 · s · sr) times  (in sr) divided by incoming radiant flux density (in J/(m 2 · s)) unitless

Interaction with the Atmosphere Refraction GLASS AIR  ’’ n = c/c n n sin  = n’sin  ’ sin  ’ = n/n’ sin 