Physical Principles of Remote Sensing: Electromagnetic Radiation -

Outline Properties of electromagnetic radiation The electromagnetic spectrum The Planck, Stefan-Boltzmann, and Wien’s equations Spectral emissivity Radiant temperature vs. kinetic temperature -

Energy Transfer Energy is “the ability to do work” -

Electromagnetic Radiation EMR is the source for most types of remote sensing Passive - Active

Electrical (E) and magnetic field (B) are orthogonal to each other Direction of each field is perpendicular to the direction of wave propagation. -

Electromagnetic Waves Described by: Wavelength Frequency Amplitude -

Frequency vs. Wavelength l = distance of separation between two successive wave peaks = number of wave peaks passing in a given time The product of wavelength and frequency is a constant: c=n l c = speed of light in a vacuum = 3.0 108 ms-1 -

Electromagnetic Radiation Its harmonic wave form can be described according to the Maxwell equations: Where, E is the electric field Eo is a constant vector = angular frequency (2pn), n = c/l, l = wavelength c = speed of light in a vacuum (300,000,000 ms-1) k = wavenumber (2p/l) x = distance along the x-axis t = time The value of E at any point depends only on x and t -

Energy vs. Frequency When considering the particle form of energy, we call it a photon The energy of a photon, Q, is proportional to its frequency, n: Q = h n n = c/l Q = hc/l h = Planck’s constant = 6.63 10-34 Js c = speed of light = 3.0 108 ms-1 Thus, Q ~ 1/l -

The EM Spectrum -

Blackbody Radiation All objects whose temperature are above absolute zero Kelvin (-273.15oC) emit radiation at all wavelengths A “blackbody” is one that is a perfect absorber and perfect emitter (hypothetical, though Earth and Sun are close) -

Stefan-Boltzmann equation: Planck’s equation: Stefan-Boltzmann equation: Wien’s displacement equation: c speed of light 3.0  108 ms-1 h Planck constant 6.63  10-34 Js k Boltzmann constant 1.38  10-23 JK-1  Stefan-Boltzmann constant 5.67  10-8 Wm-2K-4 -

What do they mean?? Planck equation gives the radiance of an object at a given temperature at any wavelength Stefan-Boltzmann equation describes the total amount of energy being radiated Wien’s equation describes the wavelength of maximum radiation -

Planck Equation, continued Planck’s equation describes how heat energy is transformed into radiant energy According to Planck’s law, an object will emit radiation in all wavelengths but not equally This is the basic law for radiation measurements in all parts of the EM spectrum -

Examples using Wien’s Displacement Equation Tsun= 5800K Peak of Sun’s radiation = 2898mmK / 5800K = 0.5 mm Tearth = 288K Peak of Earth’s radiation = 2898mmK / 288K = 10 mm -

Graybody Radiation an object that is not a perfect absorber/emitter it reflects part of incident radiation emissivity, e, is the ratio of graybody exitance to blackbody exitance -

Emissivity Describes the actual energy absorption and emission properties of real objects (“graybodies”) Is wavelength dependent (so, it’s actually a “colored body”) Emissivity establishes the radiant temperature Trad of an object -

Blackbody Radiation Stefan-Boltzmann equation: Radiant exitance (M) is proportional to physical temperature This assumes a “blackbody” which is a perfect emitter and a perfect absorber -

Radiant Temperature vs. Kinetic Temperature Two objects can have the same kinetic temperature but different radiant temperatures Object Emissivity Kinetic Temperature Radiant Temperature Blackbody 1.0 300 Water, distilled 0.99 299.2 Basalt, rough 0.95 296.2 Basalt, smooth 0.92 293.8 Obsidian 0.86 288.9 Mirror 0.02 112.8 -

Terminology (yuck!) Radiant flux f, units W Irradiance (flux density), E units Wm-2 (called Exitance, M when it is away from the surface) Radiance, L, units Wm-2 sr-1 Note, all can be functions of wavelength with additional units mm-1 -

Sun’s Radiant Energy Distribution Name of Spectral Region Wavelength Range, mm Percent of Total Energy Gamma and X-rays < 0.01 Negligible Far Ultraviolet 0.01 - 0.2 0.02 Middle Ultraviolet 0.2 - 0.3 1.95 Near Ultraviolet 0.3 - 0.4 5.32 Visible 0.4 - 0.7 43.5 Near Infrared 0.7 - 1.5 36.8 Middle Infrared 1.5 - 5.6 12.0 Thermal Infrared 5.6 - 1000 0.41 Microwave > 1000 Radio Waves -

More about radiation Energy emitted from the Sun is enormous: 6.4  107 Wm-2 It is reduced to 1370 Wm-2 at the top of the Earth’s atmosphere because of the Earth-Sun distance (1/r2) An object at 288K emits only 390 Wm-2 -

Solar Emittance Curve Radiation leaving the surface of the sun Solar radiation at sea level -

For terrestrial remote sensing, the most important source is the sun Reflected solar energy is used 0.3 - 2.5 mm The Earth is also an energy source >6 mm for self-emitted energy -

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