Electromagnetic Radiation

Physical Principles of RemoteSensing

Outline for 4/3/2003

• Properties of electromagnetic radiation

• The electromagnetic spectrum

• Spectral emissivity

• Radiant temperature vs. kinematictemperature

Energy Transfer

• Energy is “the ability to do work”• Energy transfer:

– Conduction: transfer of kinetic energy bycontact between atoms or molecules

– Convection: transfer of kinetic energy byphysically moving the mass that containsthe energy

– Radiation: propagation via waves/particlesthrough a vacuum (or through a medium)

Electromagnetic Radiation

• EMR is the source for most types of remote sensing– Sun and Earth are both passive sources of EM radiation

– Lasers and radar are active sources

• Generated by transformation of energy from otherforms– Kinetic: thermal motion of atoms and molecules (heat)– Electrical: radio frequency (dipole antenna)– Magnetic: electron tube (microwave)– Radioactive: decay of radioactive substances– Chemical/Laser: molecular excitation

Electrical (E) and magnetic field (B) are orthogonal toeach other

Direction of each field is perpendicular to the directionof wave propagation.

Electromagnetic Waves

Described by:

• Wavelength

• Amplitude

Electromagnetic Radiation

• Its harmonic wave form can be describedaccording to the Maxwell equations:

†

†

Ex = E0 cos(wt - kz)Where,E is the electric fieldw= angular frequency (2pn), n = c/l, l = wavelengthc = speed of light in a vacuum (300,000 kms-1)k = wavenumber (2p/l)z = distancet = time

Frequency vs. WavelengthThe product of wavelength and frequency is a

constant: n l=c

l = distance of separation between twosuccessive wave peaks

n = number of wave peaks passing in a giventime

c = speed of light in a vacuum (300,000 kms-1)

Energy vs. Frequency

When considering the particle form of energy,we call it a photon

The energy of a photon is proportional tofrequency:

Q = h n n = c/lQ = hc/l

where, h = Planck’s constant = 6.626 10-34 Js

Thus, Q ~ 1/l

The EM Spectrum

Polarization

E and B fields are perpendicular to eachother but their orientation can change

• If both remain in their respective planes, theradiation is called “plane polarized”

• If they rotate around the axis of propagation,the radiation is called “circularly polarized” or“elliptically polarized”

• If their orientation changes randomly, it iscalled “randomly polarized” or unpolarized

Polarization• Plane polarized light can be either

– vertically polarized (E0 is perpendicular to theplane of incidence)

– horizontally polarized (E0 is parallel to the plane ofincidence)

• Solar radiation is unpolarized (random) butcan become polarized by reflection, scattering,etc.

• Lasers and radars produce polarizedradiation

Spectral Emittance

• All bodies whose temperature are aboveabsolute zero Kelvin (-273.2 oC) emit radiationat all wavelengths

• A “blackbody” is one that is a perfect absorberand perfect emitter (hypothetical, thoughEarth and Sun are close)

• Planck’s Law describes how heat energy istransformed into radiant energy

• This is the basic law for radiationmeasurements in all parts of the EM spectrum

Planck’s Blackbody Equation

†

Ml =C1

l5 eC2 lT -1[ ]Ml = spectral radiant exitance (emittance), units are W m-2 mm-1

l = wavelengthT = the blackbody’s temperature in Kelvin (K)C1 = 3.74151 ¥ 108 W m-2 mm4

C2 = 1.43879 ¥ 104 mm K

Blackbody Radiation

• According to Planck’s law, a blackbody willemit radiation in all wavelengths but notequally

• Stefan-Boltzmann Law:Emittance is proportional to physical temperatures = 5.670 10-8 W m-2 K-4

• Graybody:Object that reflects part of incident radiatione < 1.0

†

†

M = sT 4

†

M = esT 4

Emissivity

• Describes the actual absorption andemission properties of real objects(“graybodies”)

• Is wavelength dependent• Emissivity = graybody emittance/blackbody emittance

• Emissivity establishes the radianttemperature Trad of an object

Radiant Temperature vs.Kinematic Temperature

• Two objects can have the same kinematictemperature but different radiant temperatures

112.83000.02Mirror

288.93000.86Obsidian

293.83000.92Basalt, smooth

296.23000.95Basalt, rough

299.23000.99Water, distilled

3003001.0Blackbody

Radiant

Temperature

KinematicTemperature

EmissivityObject

Wien’s Law

lmax = a /T

a = 2898 mm K• The wavelength of peak emittance is inversely proportional to the

kinematic temperature

• Sun’s temperature = 6000 K2898/6000 = 0.48 mm

• Earth’s temperature = 300 K2898/300 = 9.6 mm

Sun’s Radiant Energy Distribution

Negligible> 1000Radio Waves

Negligible> 1000Microwave

0.415.6 - 1000Thermal Infrared

12.01.5 - 5.6Middle Infrared

36.80.7 - 1.5Near Infrared

43.50.4 - 0.7Visible

5.320.3 - 0.4Near Ultraviolet

1.950.2 - 0.3Middle Ultraviolet

0.020.01 - 0.2Far Ultraviolet

Negligible< 0.01Gamma and X-rays

Percent of TotalEnergy

WavelengthRange, mm

Name of SpectralRegion

Solar Emittance Curve

Emission spectrum of a 6000K blackbody

Radiation leaving the surface of the sun

Solar radiation at sea level

• For terrestrial remote sensing, the mostimportant source is the sunReflected solar energy is used0.3 - 2.5 mm

• The Earth is also an energy source>6 mm for self-emitted energy