Electromagnetic Radiation Electromagnetic Spectrum Radiation Laws Atmospheric Absorption

Electromagnetic Radiation Electromagnetic Spectrum Radiation Laws Atmospheric Absorption Radiation Terminology

Electromagnetic Radiation 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 Magne ti c Com ponen Source t t n ne po ic ctr C om e El Two properties of electromagnetic radiation: (1) wave, and (2) particle.

Wavelength, Amplitude and Frequency l = c/n l is the wavelength c is the speed of light (~ 3× 108 m/s) n is the frequency (s-1 = Hz) wavelength Energy levels Amplitude lred = 3 x 108/4. 6 x 1014 = 650 x 10 -9 m = 650 nm

Frequencies used in remote sensing unit Frequency (cycles per second) Hertz (Hz) 1 Kilohertz 103 (=1, 000) Megahertz (MHz) 106 (=1, 000) Gigahertz (GHz) 109 (=1, 000, 000)

0. 03 nm Angstrom = nm = 10 -9 m m = 10 -6 m cm = 10 -2 m km = 1000 m 30 0 10 -10 m nm 38 0 72 nm 0 15 nm . 0 m 30 cm VISIBLE LIGHT Violet: 400 - 446 nm Blue: 446 - 500 nm Green: 500 - 578 nm Yellow: 578 - 592 nm Orange: 592 - 620 nm Red: 620 - 700 nm

Radiation Laws 1. Plank’s law: c is the speed of light, 3× 108 m/s is the frequency T is temperature, in Kevin h is Plank constant K is Boltzmann constant 0 5 10 15 Wavelength (mm) 20 Wavelength = c/

Radiation Laws 3. Wien’s law: l = 2, 897. 8/T l = wavelength (mm) at which radiance is at a maximum As objects become hotter, the wavelength of maximum emittance shifts to shorter wavelengths. 0 5 10 15 Wavelength (mm) 20

Radiation Laws 3. Stefan-Boltzmann’s law: W = s. T 4 W = total emitted radiation (watt m-2) s = Stefan-Boltzmann constant (5. 6697 x 10 -8 W m-2 K-4) T = temperature in Kelvin (T in Kelvin = 273. 15 + o. C) Hot blackbodies emit more energy per unit area than do cold blackbodies

Radiation Laws 4. Kirchhoff’s law: e = M/Mb e = emissivity: a measure of the effectiveness of an object as a radiator of electromagnetic energy (0 -1) M = emittance of a given object Mb = 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 Incoming reflection absorption transmission Absorptivity + Reflectivity + Transmissivity = 1

Radiation Laws 6. Cosine’s Law Es= Ep cos Ep 1. 0 cos 0 0 90 Es

Interaction with the Atmosphere Intensity of Scattering (%) Scattering 100 Rayleigh Scattering 80 60 40 20 0 Scattered 0. 4 0. 5 0. 6 Blue Green 0. 7 0. 8 0. 9 Red Near Infrared Wavelength (mm) Blue sky and red-orange sunset

Scattering Rayleigh Specks of dust, N 2 and O 2 molecules ~ l -4 Strongly wavelength dependent Mie Dust, pollen, smoke, water droplets Weakly wavelength dependent Nonselective Larger water droplets, large particles of airborne dust Wavelength independent Scattering decreases the quality of an image

Electromagnetic Radiation Io Is I = Is + Io + ID ID

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

Interaction with the Atmosphere Absorption Atmospheric Windows Transmittance (%) UV & Visible 0. 30 -0. 75 mm Near infrared 0. 77 -0. 91 mm mid- Infrared 1. 55 -1. 75 mm 2. 05 -2. 40 mm Mid IR Far Infrared Near Infrared Visible Thermal IR 8. 0 -9. 2 mm 10. 2 -12. 4 mm Microwave 7. 5 -11. 5 mm 20. 0 + mm

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

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

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

Interaction with Surfaces Transmission Transmittance t = Transmitted radiation Incident radiation Non-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 · sr) 6. Spectral Radiance: Radiant flux per unit wavelength in J/(m 2 · sr · m)

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

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