Download Lecture 2 EMS - San Jose State University

Lecture 3 Quantum PhysicsUnderlying Theory for Remote
Sensing
Professor Menglin S. Jin
Department of Meteorology
San Jose State University
diagram for remote sensing–solar
radiation
Electromagnetic Spectrum
Remote sensing relies on measurements in the
electromagnetic spectrum (except sonar)
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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
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Time of the measurements lead to selecting a specific band
Type of detector/sensor partially determined by the spectral bands
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
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
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
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
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.
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
wave
ν ~ 1/λ
c (speed of light) = λν
the distance between two adjacent peaks on a wave is its wavelength λ
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 ν
(in units of cycles per second, whose SI version is Hertz [1 Hertz = 1/s-1])
Wave
• The wave amplitudes of the two fields are
also coincident in time and are a measure
of radiation intensity (brightness)
Planck's general equation
• E=hv
• The amount of energy characterizing a photon is
determined using Planck's general equation
• h is Planck's constant (6.6260... x 10-34 Joulessec), 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.
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)
Planck Equation
• Wavelength is the inverse of frequency
C= λv
V= c/λ
c is the constant that expresses the speed of light
•we can also write the Planck equation as
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)
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
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
• For more, read the Chapter on The Nature
of Electromagnetic Radiation in the
Manual of Remote Sensing, 2nd Ed
How these physics related to
remote sensing?
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:
Electromagnetic Spectrum: Transmittance,
Absorptance, and Reflectance
• (1) Transmittance (τ) - 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 (α) - 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 (ρ) - 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: τ + α + ρ = 1.
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.
Important Concepts
• Another formulation of radiant intensity is
given by the radiant flux per unit of solid
angle ω (in steradians - a cone angle in
which the unit is a radian or 57 degrees,
17 minutes, 44 seconds)
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
WRT remote Sensing
• What really measured by remote sensing
detectors are radiances at different
wavelengths leaving extended areas
Radiative Transfer
What happens to radiation
(energy) as it travels from the
“target” (e.g., ground, cloud...) to
the satellite’s sensor?
Processes:
transmission
reflection
scattering
absorption
refraction
dispersion
diffraction
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!)
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.
Reflection from smooth surface
light ray
angle of
incidence
angle of
reflection
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.
Molecular scattering
(or other particles)
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/λ4
• Scattering by particles similar to or larger than
the wavelength of light is typically treated by the
Mie scattering
Scattering from irregular surface
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.
incident
radiation
substance (air, water,
ice, smog, etc.)
absorption
transmitted
radiation
window
Atmosphere
Window
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.
Refraction in two different media
less dense
medium
more dense
medium
Gradually changing medium
low density
ray
wave
fronts
high density
Dispersion
• the process in which radiation is separated
into its component wavelengths (colors).
The “classic” example
prism
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.
light
shadow
region
Solid object
Atmospheric Constituents:
empty space
molecules
dust and pollutants
salt particles
volcanic materials
cloud droplets
rain drops
ice crystals
Optical phenomena
light
process
+
atmospheric
constituent
optical
phenomena
atmospheric
structure
Atmospheric Structure
temperature gradient
humidity gradient
clouds
layers of stuff - pollutants, clouds
• 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.