Lecture 2 Remote Sensing:
Quantum Physics Underlying
Professor Menglin S. Jin
Department of Meteorology
San Jose State University
This diagram for remote sensing
Brief history
• Since the 1960s, most remote sensing has been
conducted from satellites
• Prior to that remote sensing is associated mainly
with aerial photography, using cameras mounted
in aircraft that fly at various altitudes (with scale
emcompassed)
• Aircraft remote sensing continues through today
but is usually directed towards specific tasks and
missions.
"Remote" and "Proximal" Sensing
• “Remote” sensing involves making
measurements and collecting data for (and
from) objects, classes, and materials that
are not in contact with the sensor (sensing
device) whereas the “Proximal” sensing
includes making direct contact with these
targets
if the objective is to measure a
person's bodily temperature
• the proximate approach would be
to place a thermometer in or on the body
• the remote approach would be
to hold a radiometer sensitive to thermal energy
at some distance from the body
•
Both need “Calibrated”
its response as a sensor must be transformable into a good
approximation of the actual temperature by determining the response
using a target whose temperature range is specifically known.
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?
This diagram for remote sensing
Spectral Signature
For any given material, the amount of solar radiation that it reflects, absorbs, transmits,
or emits varies with wavelength
a general example of a
reflectance plot for
some (unspecified) vegetation type
(bio-organic material)
• Spectral Signature is important property of
matter makes it possible to identify
different substances or classes and to
separate them by their individual spectral
signatures, as shown in the figure below.
Spectral Signature
Vegetation: NDVI
• NDVI - Normalized Difference Vegetation Index
• Video
• Negative values of NDVI (values approaching -1)
correspond to water. Values close to zero (-0.1 to 0.1)
generally correspond to barren areas of rock, sand, or
snow. Lastly, low, positive values represent shrub and
grassland (approximately 0.2 to 0.4), while high values
indicate temperate and tropical rainforests (values
approaching 1).
Vegetation Spectral Signature
where RED and NIR stand for the spectral reflectance measurements acquired in the red and near-infrared regions, respectively.
These spectral reflectances are themselves ratios of the reflected over the incoming radiation in each spectral band individually,
hence they take on values between 0.0 and 1.0. By design, the NDVI itself thus varies between -1.0 and +1.0.
The pigment in plant leaves, chlorophyll, strongly absorbs visible light
(from 0.4 to 0.7 µm) for use in photosynthesis.
The cell structure of the leaves, on the other hand, strongly reflects
near-infrared light (from 0.7 to 1.1 µm).
The more leaves a plant has, the more these wavelengths of light are affected, respectivel
Class Participation
Calculate NDVI in these two trees.
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.
Quantifying radiation
It is necessary to understand the energy quantities that are typically
used in remote sensing
 Radiant energy (Q in joules) is a measure of the capacity of an EM
wave to do work by moving an object, heating, or changing its state.
 Radiant flux (Φ in watts) is the time rate (flow) of energy passing
through a certain location.
 Radiant flux density (watts/m2) is the flux intercepted by a planar
surface of unit area.
 Irradiance (E) is flux density incident upon a surface.
 Exitance (M) or emittance is flux density leaving a surface.
 The solid angle (Ω in steradians)
subtended by an area A on a
2
spherical surface of radius r is A/r
 Radiant intensity (I in watts/sr) is the flux per unit solid angle in a
given direction.
 Radiance (L in watts/m2/sr) is the intensity per unit projected area.
 Radiance from source to object is conserved
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)
• 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
Extra Credit (1 point)
• Read “The Green Wave” from Our Changing Planet
• Read “Snow Cover: The most dynamic feature on the Earth Surface”
• Write hale-page comments for each pepr. Answer the following key
question:
What instrument, orbit
What variable – how they retrieve this variable
What duration
What uncertainty (the paper may not discuss, you need to think
about it)
What key conclusion