Lecture 3 Quantum Physics- Underlying Theory for Remote Sensing

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Lecture 3 Quantum Physics- Underlying Theory for
Remote Sensing

• Professor Menglin S. Jin
• Department of Meteorology
• San Jose State University

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diagram for remote sensingsolar radiation
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Electromagnetic Spectrum

• Remote sensing relies on measurements in the
• electromagnetic spectrum (except sonar)
• Remote sensing of the ground from space
• Need to see through the atmosphere
• The ground must have some feature of interest
  in that spectral region
• Studying reflected light requires a spectral
  region where solar energy
• dominates
• Radar approaches mean we need frequencies that we
  can generate
• Also need to ensure that we are not affected
  by other radio sources
• Atmosphere should be transparent at the
  selected frequency
• Time of the measurements lead to selecting a
  specific band
• Type of detector/sensor partially determined by
  the spectral bands

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THE QUANTUM PHYSICS UNDERLYING REMOTE SENSING

• Quanta, or photons (the energy packets first
  identified
• by Einstein in 1905), are particles of pure
  energy
• having zero mass at rest

• the demonstration by Max Planck in 1901, and more
  
• specifically by Einstein in 1905, that
  electromagnetic
• waves consist of individual packets of energy
  was
• in essence a revival of Isaac Newton's
• (in the 17th Century) proposed but then
• discarded corpuscular theory of light

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THE QUANTUM PHYSICS UNDERLYING REMOTE SENSING

• light, and all other forms of EMR, behaves both
  as waves and as particles. This is the famous
  "wave-particle" duality enunciated by de Broglie,
  Heisenberg, Born, Schroedinger, and others mainly
  in the 1920s

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THE QUANTUM PHYSICS UNDERLYING REMOTE SENSING

• How is EMR produced?
• Essentially, EMR is generated when an electric
  charge is accelerated, or more generally,
  whenever the size and/or direction of the
  electric (E) or magnetic (H) field is varied with
  time at its source

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PHOTON
The photon is the physical form of a quantum,
the basic particle of energy studied in quantum
mechanics (which deals with the physics of the
very small, that is, particles and their
behavior at atomic and subatomic levels). The
photon is also described as the messenger
particle for EM force or as the smallest bundle
of light. This subatomic massless particle,
which also does not carry an electric charge,
comprises radiation emitted by matter when it is
excited thermally, or by nuclear processes
(fusion, fission), or by bombardment with other
radiation (as well as by particle collisions).
It also can become involved as reflected or
absorbed radiation. Photons move at the speed
of light 299,792.46 km/sec (commonly rounded
off to 300,000 km/sec or 186,000 miles/sec).
Consult http//en.wikipedia.org/wiki/Photon for
more details
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Photon

• Photon particles also move as waves and hence,
  have a "dual" nature. These waves follow a
  pattern that can be described in terms of a sine
  (trigonometric) function, as shown in two
  dimensions in the figure below.

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photon travels as an EM wave

• having two components, oscillating as sine waves
  mutually at right angles, one consisting of the
  varying electric field, the other the varying
  magnetic field

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wave
? 1/?
c (speed of light) ??
the distance between two adjacent peaks on a wave
is its wavelength ?
The total number of peaks (top of the individual
up-down curve) that pass by a reference
lookpoint in a second is that wave's frequency ?
(in units of cycles per second, whose SI version
is Hertz 1 Hertz 1/s-1)
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Wave

• The wave amplitudes of the two fields are also
  coincident in time and are a measure of radiation
  intensity (brightness)

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Planck's general equation

• Ehv
• The amount of energy characterizing a photon is
  determined using Planck's general equation
• h is Planck's constant (6.6260... x 10-34
  Joules-sec), v (read as nu), representing
  frequency
• A photon is said to be quantized, any given one
  possesses a certain quantity of energy
• Some other photon can have a different energy
  value
• Photons as quanta thus show a wide range of
  discrete energies.

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Planck's general equation

• Photons traveling at higher frequencies are
  therefore more energetic.
• If a material under excitation experiences a
  change in energy level from a higher level E2 to
  a lower level E1, we restate the above formula
  as

where v has some discrete value determined by (v2
- v1)
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Planck Equation

• Wavelength is the inverse of frequency

C ?v V c/?
c is the constant that expresses the speed of
light

• we can also write the Planck equation as

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Class wake-up activity

• Calculate the wavelength of a quantum of
  radiation whose photon energy is 2.10 x 10-19
  Joules use 3 x 108 m/sec as the speed of light c
• A radio station broadcasts at 120 MHz (megahertz
  or a million cycles/sec) what is the
  corresponding wavelength in meters (hint convert
  MHz to units of Hertz)

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polychromatic vs. monochromatic

• A beam of radiation (such as from the Sun) is
  usually polychromatic (has photons of different
  energies)
• if only photons of one wavelength are involved
  the beam is monochromatic.
• the distribution of all photon energies over the
  range of observed frequencies is embodied in the
  term spectrum

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photoelectric effect measure photon energy level

• the discovery by Albert Einstein in 1905

• His experiments also revealed that regardless
• of the radiation intensity, photoelectrons are
• emitted only after a threshold frequency is
  exceeded

• for those higher than the threshold value
  (exceeding
• the work function) the numbers of photoelectrons
• released re proportional to the number
• of incident photons

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• For more, read the Chapter on The Nature of
  Electromagnetic Radiation in the Manual of Remote
  Sensing, 2nd Ed

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• How these physics related to
• remote sensing?

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Electromagnetic Spectrum Transmittance,
Absorptance, and Reflectance

• Any beam of photons from some source passing
  through medium 1 (usually air) that impinges upon
  an object or target (medium 2) will experience
  one or more reactions that are summarized below

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Electromagnetic Spectrum Transmittance,
Absorptance, and Reflectance

• (1) Transmittance (t) - some fraction (up to
  100) of the radiation penetrates into certain
  surface materials such as water and if the
  material is transparent and thin in one
  dimension, normally passes through, generally
  with some diminution.
• (2) Absorptance (a) - some radiation is absorbed
  through electron or molecular reactions within
  the medium a portion of this energy is then
  re-emitted, usually at longer wavelengths, and
  some of it remains and heats the target
• (3) Reflectance (?) - some radiation (commonly
  100) reflects (moves away from the target) at
  specific angles and/or scatters away from the
  target at various angles, depending on the
  surface roughness and the angle of incidence of
  the rays.

the Law of Conservation of Energy t a ? 1.
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Most remote sensing systems are designed to
collect reflected radiation.
When a remote sensing instrument has a
line-of-sight with an object that is reflecting
solar energy, then the instrument collects that
reflected energy and records the observation.
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Important Concepts

• Another formulation of radiant intensity is given
  by the radiant flux per unit of solid angle ? (in
  steradians - a cone angle in which the unit is a
  radian or 57 degrees, 17 minutes, 44 seconds)

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Important Concepts

• radiance is defined as the radiant flux per unit
  solid angle leaving an extended source (of area
  A) in a given direction per unit projected
  surface area in that direction

L Watt m-2 sr-1
where the Watt term is the radiant flux
Radiance is loosely related to the concept of
brightness as associated with luminous bodies
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WRT remote Sensing

• What really measured by remote sensing detectors
  are radiances at different wavelengths leaving
  extended areas

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Radiative Transfer

• What happens to radiation (energy) as it travels
  from the target (e.g., ground, cloud...) to the
  satellites sensor?

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Processes

• transmission
• reflection
• scattering
• absorption
• refraction
• dispersion
• diffraction

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transmission

• the passage of electromagnetic radiation through
  a medium
• transmission is a part of every optical phenomena
  (otherwise, the phenomena would never have
  occurred in the first place!)

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reflection

• the process whereby a surface of discontinuity
  turns back a portion of the incident radiation
  into the medium through which the radiation
  approached the reflected radiation is at the
  same angle as the incident radiation.

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Reflection from smooth surface
light ray
angle of reflection
angle of incidence
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Scattering

• The process by which small particles suspended in
  a medium of a different index of refraction
  diffuse a portion of the incident radiation in
  all directions. No energy transformation
  results, only a change in the spatial
  distribution of the radiation.

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Molecular scattering (or other particles)
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Rayleigh Scattering vs Mie Scattering

• Rayleigh scattering (named after the British
  physicist Lord Rayleigh) is the elastic
  scattering of light or other electromagnetic
  radiation by particles much smaller than the
  wavelength of the light, which may be individual
  atoms or molecules .
• the Rayleigh scattering intensity for a single
  particle is 1/?4
• Scattering by particles similar to or larger than
  the wavelength of light is typically treated by
  the Mie scattering

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Scattering from irregular surface
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Absorption (attenuation)

• The process in which incident radiant energy is
  retained by a substance.
• A further process always results from absorption
• The irreversible conversion of the absorbed
  radiation goes into some other form of energy
  (usually heat) within the absorbing medium.

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incident radiation
substance (air, water, ice, smog, etc.)
transmitted radiation
absorption
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window
Atmosphere Window
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Refraction

• The process in which the direction of energy
  propagation is changed as a result of
• A change in density within the propagation
  medium, or
• As energy passes through the interface
  representing a density discontinuity between two
  media.

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Refraction in two different media
less dense medium
more dense medium
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Refraction in two different media
less dense medium
Dt
more dense medium
Dt
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Gradually changing medium
low density
ray
wave fronts
high density
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Dispersion

• the process in which radiation is separated into
  its component wavelengths (colors).

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The classic example
white light
prism
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Diffraction

• The process by which the direction of radiation
  is changed so that it spreads into the geometric
  shadow region of an opaque or refractive object
  that lies in a radiation field.

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light
shadow region
Solid object
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Atmospheric Constituents

• empty space
• molecules
• dust and pollutants
• salt particles
• volcanic materials
• cloud droplets
• rain drops
• ice crystals

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Optical phenomena
light
atmospheric constituent
optical phenomena

process
atmospheric structure
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Atmospheric Structure

• temperature gradient
• humidity gradient
• clouds
• layers of stuff - pollutants, clouds

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Optical phenomena
light
atmospheric constituent
optical phenomena

process
atmospheric structure
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White clouds

• scattering off cloud droplets 20 microns

Question is this Mie scattering or Rayleigh
scattering? Why?
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Dark clouds

• scattering and attenuation from larger cloud
  droplets and raindrops

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Blue skies

• scattering from O2 and N2 molecules, dust
• violet light is scattered 16 times more than red

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Molecular scattering (nitrogen and oxygen)

• blue scatters more than red

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Hazy (milky white) sky

• Scattering from tiny particles
• terpenes (hydrocarbons) and ozone

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Orange sun (as at sunset or sunrise)

• Scattering from molecules
• This is the normal sunset we see frequently

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Red sun (as at sunset or sunrise)

• Scattering from molecules, dust, salt particles,
  volcanic material
• At 4 elevation angle, sun light passes through
  12 times as much atmosphere as when directly
  overhead

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• The change of sky colour at sunset (red nearest
  the sun, blue furthest away) is caused by
  Rayleigh scattering by atmospheric gas particles
  which are much smaller than the wavelengths of
  visible light. The grey/white colour of the
  clouds is caused by Mie scattering by water
  droplets which are of a comparable size to the
  wavelengths of visible light.