Definitions of Remote Sensing • The science and art of obtaining useful information about an object, area or phenomenon through the analysis of data acquired by a device that is not in contact with the object, area, or phenomenon under investigation. – Lillesand, Thomas M. and Ralph W. Kiefer, “Remote Sensing and Image Interpretation” , John Wiley and Sons, Inc, 1979. • The measurement and analysis of electromagnetic radiation reflected from, transmitted through, or absorbed and scattered by the atmosphere, the hydrosphere and by material at or near the land surface, for the purpose of understanding and managing the Earth’s resources and environment. – Larry Morley – Teledetection International Our definition • Understanding the physical, chemical, and biological properties of the earth and planets from the way electromagnetic radiation interacts with their surface materials and atmospheres as viewed from above. Electromagnetic Radiation (EMR) • Velocity in vacuum: c = ~3.0 x 108 m per second. • Does not require a specific medium to travel thru. • Has both electrical and magnetic properties Dual Nature of EMR (Light) Wave-like – Wavelength – Frequency (i.e., number of waves per second) – It ‘radiates’, reflects, refracts • Particle-like – Interacts with matter as discrete packets of energy (photons) Visible Light UV IR • Wavelengths of 400-700 nm (nanometers) • Bounded by UV and IR The Electromagnetic Spectrum • A means of ordering EMR according to – Wavelength (λ) – m, cm, m, nm – Frequency () – Hertz (Hz) = 1 cycle per second – Energy (eV) • Covers – – – – Gamma rays X-rays Ultraviolet Visible Light – – – – Reflected IR Thermal IR (Heat) Microwaves Radiowaves Development of Electromagnetic Theory 1666 Isaac Newton “And so the true Cause of the Length of that Image was detected to be no other, than that Light is not similar or Homogenial, but consists of Difform Rays, some of which are more Refrangible than others”. He coined the term ‘spectrum’. From Voltaire's Eléments de la Philosophie de Newton, 1738 1704-1730 Isaac Newton Issac Newton publishes four editions of Opticks. Query 29: “Nothing is more requisite for producing all the variety of Colours, and degrees of Refrangibility than that the Rays of Light be Bodies of different Sizes, the least of which may take violet the weakest and darkest of the Colours, and be more easily diverted by refracting Surfaces from the right Course; and the rest as they are bigger and bigger, may make the stronger and more lucid colours, blue, green, yellow, and red, and be more and more difficultly diverted”. "Are not the Rays of Light very small Bodies emitted from shining Substances?" "Even the Rays of Light seem to be hard bodies; for otherwise they would not retain different properties in their different Sides." He develops the particle theory of light (also called the corpuscular theory of light). He is able to give plausible explanations for properties of light such as color, reflection, and refraction. He was not able to explain everything about light, diffraction bands outside the geometrical shadow (discovered by Grimaldi in 1665) being one and Newton's rings being another. An extremely important prediction implicit in Newton's particle theory is that, as light moves from air to water, it SPEEDS up. A wave theory of light existed in Newton's day. Its leading champion was Christiaan Huygens, but the theory was incomplete. It only addressed a small fraction of the phenomena Newton discussed and was difficult to understand. So, due to the wave theory's poor explanatory power and Newton's great authority within the science world, the particle theory of light reigned supreme. 1800: William Herschel discovers the infrared portion of the spectrum. A year later, Johann Ritter discovered ultraviolet light. 1814-1823: Joseph von Fraunhofer discovered the dark lines in the sun's spectrum (Fraunhofer lines). In 1821, he reported results of using a defraction grating to measure the wavelengths of Na lines. 1815-1819: Augustin Fresnel independently rediscovered interference and begins to study (and extend mathematically) the wave theory of light; completely refutes particle theory of light. 1826-1849: John Herschel and W.H. Fox Talbot demonstrated, when a substance is heated and its light passed through a spectroscope, that each element gave off its own set of characteristic bright lines of color. Birth of the ‘Emission Spectrum’. Spectrum of White Light Spectrum of Excited Hydrogen Gas Line Spectra of Other Elements 1849-1850: Lóon Foucault worked to measure the speed of light and test Newton’s particle theory. He was able to compare the two values needed to test Newton's particle theory for light: light travels SLOWER in water than it does in air. In 1862, Foucault determined the speed of light to be 298,000 ± 500 km/sec. 1862: Anders Jonas Ångström identified three lines in the visible portion of the hydrogen emission spectrum, a red line, a blue-green line and a violet line. James Clerk Maxwell (1831 - 1879)1873: James Clerk Maxwell, a Scottish physicist, succeeded in unifying electricity and magnetism. I. Gauss' law for electricity II. Gauss' law for magnetism III. Faraday's law of induction IV. Ampere's law 1888: Heinrich Hertz demonstrated the existence of the electromagnetic radiation that Maxwell predicted by building an apparatus to produce radio waves. 1899: Max Planck discovered a new fundamental constant, which is named Planck's constant (h), and is, for example, used to calculate the energy of a photon. 1900: Planck discovered the law of heat radiation, “Planck's law of black body radiation”. This law became the basis of quantum theory, which emerged ten years later in cooperation with Albert Einstein and Niels Bohr. Development of Quantum Physics 1900 to 1930 – Development of ideas of quantum mechanics • Also called wave mechanics • Highly successful in explaining the behavior of atoms, molecules, and nuclei • Quantum Mechanics reduces to classical mechanics when applied to macroscopic systems • Involved a large number of physicists – Planck introduced basic ideas – Mathematical developments and interpretations involved such people as Einstein, Bohr, Schrödinger, de Broglie, Heisenberg, Born and Dirac • • Definition: A wave is a traveling disturbance Any form of change or disturbance that propagates (travels) from one region to another can be thought of as a wave Examples Sound waves Ocean waves Earthquakes Light Defining a wave Frequency = # wave crests passing per second Wavelength (λ) Speed Wavelength (λ) speed = wavelength (λ) x frequency(f) An electromagnetic wave is a disturbance propagated as a variation in the local electric and magnetic fields Electromagnetic (EM) radiation • Visible light is just one ‘form’ of EM radiation • For all waves: speed = wavelength (λ) x frequency(f) • For all EM waves: speed = speed of light (c) c=λf The Electromagnetic Spectrum The long and the short of it • Electromagnetic (EM) waves – longest wavelength - radio waves – shortest wavelength - gamma rays – and between these X-rays, UV, visible light, IR, microwaves Increasing wavelength • Important point about waves – they transmit information in the form of energy Definition: Energy - a measure of the ability to do work - units are Joules (J) = N m = m2 kg/s2 Energy has many forms - energy of motion - EM wave energy - chemical energy - heat energy - nuclear energy James Prestcott Joule (1818 -1889) Radiant Energy • The amount of energy E carried by an electromagnetic wave is related to its frequency: E=hf where h = Planck’s constant = 6.626 x 10-34 Jsec So, the greater the frequency the greater the amount of energy carried by the wave • Many applications are interested in energy transfer: • For example: – the temperature of a planet is governed by the amount of EM energy it receives from the Sun – Hence: • We would like to know exactly how much EM energy the sun radiates into space James Watt (1736 - 1819) Terminology: Luminosity (L) = total amount of EM energy radiated at all wavelengths into space per second: units = Joules / sec. = Watts Flux (F) = energy received per square meter per second: units = Watts/m2 • The problem is – We want to determine the total energy output (the luminosity L), but it can’t be measured directly The solution Measure the flux (the energy received at a detector per second per square meter) and find a relationship between flux and luminosity. • Take a source of EM radiation e.g., the sun – energy is radiated into space in all directions – at a distance d from the star the radiated energy will be spread out over the surface of a sphere of radius d Sphere of radius d d Sun of luminosity L L = radiative energy at source (luminosity) Energy passing through surface per m2 per sec = FLUX • A really useful result: Energy flux a distance d from source of luminosity L: F= L 4π d 2 where 4π d2 is the surface area of a sphere of radius d A fundamental result • Question: what is the Sun’s luminosity • Idea for answer: – measure Sun’s energy flux at the Earth’s orbit Get F = 1370 W/m2 Procedure: F = Sun’s energy flux at Earth’s orbit we know the Earth is 1 AU from the Sun theory: F = L / 4π (1 AU)2 Symbol means Sun related value • The picture: Sphere of radius 1 AU about Sun Sun 1 AU Earth’s orbit (where we measure the flux) • Hence result: L ≈ 4 x 1026 Watts 100 Watt light bulb = Sun’s Luminosity (energy output) x 4,000,000,000,000, 000,000,000,000 That’s a lot of light bulbs! OK, let us do a calculation • Volunteer? – Calculate Sun’s energy flux at the orbit of Mars and compare it to that at Earth. d(Mars) = 1.52 x 1 AU L(Sun) = 3.85 x 1026 Watts Hint: watch your units The Sun - fundamental properties • Mass - from Kepler’s 3rd law – M = 1.99 x 1030 kg • Radius - from angular diameter + distance 8 R = 6.96 x 10 m – • Luminosity - from flux at Earth’s orbit – L = 3.85 x 1026 Watts Some other useful quantities Radiant energy is defined as the energy carried by electro- magnetic radiation and expressed in the unit of joule (J). Radiant flux is radiant energy transmitted as a radial direction per unit time and expressed in a unit of watt (W). Radiant intensity is radiant flux radiated from a point source per unit solid angle in a radiant direction and expressed in the unit of Wsr-1. Irradiance is radiant flux incident upon a surface per unit area and expressed in the unit of Wm-2. Exitance is radiant flux leaving a surface per unit area (outgoing power). Radiant emittance is radiant flux radiated from a surface per unit area, and expressed in a unit of Wm-2. Radiance is radiant intensity per unit projected area in a radial direction and expressed in the unit of Wm-2 sr-1. Blackbody Radiation • An object at any temperature is known to emit electromagnetic radiation – Sometimes called thermal radiation – Stefan’s Law describes the total power radiated P= AeT Stefan’s constant 4 emissivity – The spectrum of the radiation depends on the temperature and properties of the object Blackbody • Blackbody is an idealized system that absorbs incident radiation of all wavelengths • If it is heated to a certain temperature, it starts radiate electromagnetic waves of all wavelengths • Cavity is a good real-life approximation to a blackbody Planck Curve and Blackbody (Thermal) Radiation A backbody radiates proportional to its temperature. Spectral radiance, I( ,T), is determined by Planck’s Radiation Law: I , T = 2 π h c2 5 1 exp hc/ kT −1 h = Planck’s constant = 6.6260755 = 10-34 joule-second k = Boltzmann’s constant = 1.3807 x 10-23 joule-kevins Planck’s law defines the nature of blackbody radiation. Real objects are not blackbodies so a correction for emissivity should be made. Emissivity ranges between 0 and 1 depending on the dielectric constant of the object, surface roughness, temperature, wavelength, look angle. The temperature of the black body which radiates the same radiant energy as an observed object is called the brightness temperature of the object. Many natural surface materials are well approximated by blackbodies in the infrared region. For instance, water has a thermal infrared emissivity of .98. The spectral emissivity and spectral radiant flux for three objects that are a black body, a gray body and a selective radiator. Wien’s Displacement Law: k λmax= T k = 2898 µm K, and T is the absolute temperature in degrees Kelvin It is obtained by differentiating the spectral radiance. It shows that the product of wavelength (corresponding to the maximum peak of spectral radiance) and temperature, is approximately 3,000 (µmK is the best for measurement of objects with a temperature of 300K. This law is useful for determining the optimum (peak) wavelength for temperature measurement of objects with a temperature of T. For example, about 10µm is the best for measurement of objects with a temperature of 300K. Stefan-Boltzmann Law: Mλ=σT4 Where σ is the Stefan-Boltzmann constant, 5.6697 x 10-8 W-2K-4 Gives the total amount of emitted radiation from a blackbody Units: (Same as radiant emittance) W m-2 Proportional to T4 Obtained by integrating the area under the Planck Curve.
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