Electromagnetic Radiation (EMR) Principles

Remote Sensing and Electromagnetic Radiation • The first requirement for remote sensing is to have an energy source to illuminate the target. This energy for remote sensing instruments is in the form of electromagnetic radiation. • Remote sensing is concerned with the measurement of EMR returned by the Earth's natural and man-made features that first receive energy from the sun or an artificial source such as a radar transmitter. • Different objects return different types and amounts of EMR. • Objective of remote sensing is to detect these differences with the appropriate instruments. • Differences make it possible to identify and assess a broad range of surface features and their conditions Electromagnetic Energy • Electromagnetic energy (radiation) is one of many forms of energy. It can be generated by changes in the energy levels of electrons, acceleration of electrical charges, decay of radioactive substances, and the thermal motion of atoms and molecules. • All natural and synthetic substances above absolute zero (0 Kevin, -273°C) emit a range of electromagnetic energy. • Most remote sensing systems are passive sensors, relying on the sun to generate all the EM energy needed to image earth's atmosphere and land surfaces. Active sensors (like radar) transmits energy in a certain direction and records the portion reflected back by features within the signal path. Three Modes of Energy Transfer • Conduct: molecular interaction, direct transfer with physical contact, most important in solids, of intermediate importance in liquids and least important in gases • Convection: transfer by physical fluid movement, possible only in liquids and gases • Radiation: self propagation from one body to another in the absence of an intervening material medium; if the medium exists, it must be sufficiently transparent. Electromagnetic Radiation • Nuclear reactions within the sun produce a full spectrum of EM radiation which is transmitted through space without major changes in its character until it reaches the atmosphere. • In the absence of matter (vacuum condition), EMR travels at the speed of light (3x108 m/s); • In matter the traveling speed is slower. The dense the matter, the slower the speed.

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Modern physics view EMR as having dual nature, enabling it to be independently described as a wave or a particle.

Wave Model of EMR (Maxwell's equations) • ERM is carried by a series of continuous waves that are equally and repetitively spaced in time (harmonic waves); • Electromagnetic radiation consists of two fluctuating fields: an electrical field (E) which varies in magnitude in a direction perpendicular to the direction in which the radiation is traveling, and a magnetic field (M) oriented at right angles to the electrical field. Both these fields travel at the speed of light (c). • Paired fields are perpendicular to each other, and both are perpendicular to direction of wave propagation (transverse waves). Each has a sinusoidal shape because their plots resemble sine curves. • Wave nature of EMR is characterized by wavelength and frequency Wave Nature of EMR • Wavelength (λ): linear distance between 2 successive wave crests or troughs • Frequency (ν or f): number of wave crests or troughs (cycles) that pass a fixed point per second • Wavelength and frequency are related to the velocity of an electromagnetic wave (speed of light). Wavelength and frequency have an inverse relationship. The shorter the wavelength, the higher the frequency. The longer the wavelength, the lower the frequency. c = vλ • Wavelength is measured in metres (m) or some factor of metres such as nanometres (nm, 10-9 metres), micrometres (μm, 10-6 metres) or centimetres (cm, 10-2 metres). In remote sensing, most energy in the visible and infrared portions of the electromagnetic spectrum is measured in micrometers (10-6 m). • Frequency refers to the number of cycles of a wave passing a fixed point per unit of time. Frequency is normally measured in hertz (Hz), equivalent to one cycle per second, and various multiples of hertz (KHz, MHz, GHz). Particle Model of EMR • Emphasizes behavior of EMR as if EMR were composed of a collection of discrete, particle-like objects called quanta or photons, in which electromagnetic energy is transferred at the speed of light. • Photons carry particle-like properties such as energy and momentum from source, but have no mass. Particle Nature of EMR (Planck's Radiation Law) • Energy of a quantum is given as: hc Q = hv =

λ

Q: energy of quantum [Joules - J] h - Planck's constant [6.26x10-34 J s]

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Energy of a photon varies directly with frequency, the higher frequency, the stronger energy Energy of a photon varies inversely with wavelength. The shorter the wavelength, the higher the energy. For this reason, shorter wavelengths are easier to sense than very long ones such as passive terrestrial microwave emissions.

Polarized EMR • The electric and magnetic vibrations associated with a quantum can be in any orientation at right angles to the direction of propagation. If the fields for all quanta are lined up in one direction by some means, the radiation becomes polarized plane. • Horizontal or vertical polarization. Blackbody Model • Blackbody is a hypothetical, ideal radiator that totally absorbs and re-emits all energy incident upon it, namely, perfect absorber and emitter of radiation • Emittance is a function only of temperature. Emitted EMR Energy (Stefan-Boltzmann Law) • For a blackbody, the total energy emitted from an object over all wavelengths varies as a function of the fourth power of surface temperature: M = σT 4 where M is total radiant exitance from the surface of a material; σ is Stefan-Boltzmann constant (5.67x10-8 Wm-2K-4); T is absolute temperature of the emitting material (K). • The hotter the object, the more energy it gives off. The emitted energy increases very rapidly (non-linear) with increases in temperature. • In nature, true blackbodies do not exist. Most objects emit radiation with less than 100% efficiency. These objects are known as graybodies. For a graybody: M = εσT 4 ε is a dimensionless factor, known as emmissivity (0,1) Dominant Wavelength (Wien's Displacement Law) • The wavelength of maximum radiance for a blackbody of a given temperature: A λm = T where λm is the wavelength of maximum emittance (μm); A is a constant (2898 (μm K); T is temperature (K) A Color temperature Tc =

λm

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Wavelength at which the largest portion of energy is emitted depends on temperature. Blackbody curves for hotter temperatures "peak" at lower wavelengths than do the curves for cooler temperature objects. Hotter objects emit more energy at lower wavelengths than do cooler objects

Electromagnetic Spectrum (EMS) • EMS represents the continuum of electromagnetic energy from extremely short wavelengths (cosmic and gamma rays) to extremely long wavelengths (microwaves). • White light can be separated into its spectral colors by a glass prism. • Spectrum is often arbitrarily segmented into major divisions. There are no natural breaks in the EMS. • Spectrum is artificially separated and named as various spectral bands (divisions) for the description convenience. UV • Causes fluorescence and is good in some geological and vegetation applications. • Not much is done with UV for remote sensing since these shorter wavelengths are easily scattered by the atmosphere making spaceborne and some airborne sensors impractical. Visible • Small portion of the EMS that humans are sensitive to 1) Blue (.4-.5 micrometers) 2) Green (.5-.6 micrometers) 3) Red (.6-.73 micrometers) Infrared Spectrum • There are three logical zones in the IR spectrum. 1) Near infrared: reflected, can be recorded on film emulsions (0.7 1.3μm). 2) Mid infrared: reflected, can be detected using electro-optical sensors (1.3 - 3.0μm). 3) Thermal infrared: emitted, can only be detected using electro-optical sensors (3.0 - 5.0 and 8 - 14 μm). Microwave • Radar sensors, wavelengths range from 1mm to 1m. Sources of EMR • All matter in the universe that is warmer than 0K (-273C) emit continuously emits electromagnetic radiation; all objects with even the slightest sub-molecular motion radiate some energy. • 0K is defined as the temperature at which molecular motion stops. All objects in every day life emit EMR. • The amount and type of the emitted energy depends on the temperature of object. The hotter objects emit more energy with shorter wavelengths.

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Remote Sensing uses electromagnetic energy from both natural and man-made sources, although most remote sensing instruments are designed to detect solar radiation and terrestrial radiation.

Solar radiation (shortwave radiation) • EMR emitted from sun which passes through the atmosphere and is reflected in varying degrees by Earth's surface and atmosphere. • Detectable only during daylight. • Sun's visible surface (photosphere) has temperature - 6000K. • Energy radiated from gamma to radio waves. • 99% of sun's radiation fall between 0.2 - 5.6μm; 80% - 0.4 - 1.5μm (visible and reflected infrared. • Atmosphere quite transparent to incoming solar radiation. • Maximum radiation occurs 0.48μm (visible). • About 1/2 of solar radiation passes through the atmosphere and absorbed in varying degrees by surface. Terrestrial radiation (longwave radiation) • Energy emitted from the Earth and atmosphere. • Detectable both day and night. • Earth's ambient temperature (the "average" temperature given off by the soil, water, vegetation, and built environment) is about 300 Kelvin. • Earth radiates 160,000 times less than the sun • Essentially all energy is radiated at (invisible) thermal infrared wavelengths between 4-25μm. • Maximum emission occurs at 9.7μm. Reading Assignment: Chapter 2 in Jensen, J.R. 2000. Remote Sensing of the Environment: An Earth Resource Perspective. Second Edition, Upper Saddle River, NJ, Prentice Hall. 592 pp.