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

• the remote approach would be

• Both need “Calibrated”

to place a thermometer in or on the body

to hold a radiometer sensitive to thermal energy at some distance from the body

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 SpectrumRemote 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 energydominates

• 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

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

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• 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

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])

ν ~ 1/λ

c (speed of light) = λν

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 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.

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 SignatureFor 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, respectively.

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.

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.

Most remote sensing systems are designed to collect reflected radiation.

Quantifying radiationIt 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 planarsurface 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 spherical surface of radius r is A/r2

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 variableWhat durationWhat uncertainty (the paper may not discuss, you need to

think about it)What key conclusion