Basic Science in Remote Sensing Basic radiation physics EMR in the atmosphere (to be explained later) Interactions of EMR with targets (to be explained later) spectral reflectance geometric effects

Electromagnetic Radiation

What is Light?

What is Color?

Units and Dimensions The SI System Dimension Unit Symbol(s) Length Meter/Micrometer (m, μm) Area Square meter (m2) Volume Cubic meter (m3) Mass gram (g) Time second (s) Temperature Kelvin (K) Energy joule (j) Velocity meter/second (m/s) Power Joule/second (watt) (j/s) Energy Flux Density watt/square meter (W/m2)

What is EMR? - Two theories Wave theory Particle theory

Wave theory In the 1860s, James Clerk Maxwell (1831-1879) conceptualized electromagnetic radiation as an electromagnetic wave that travels through space at the speed of light, c, which is 3 x 108 m/s. EM wave consists of two fluctuating fields -Electric -Magnetic - Hence the terminology electromagnetic. - The two vectors are at right angles to one another - Both are perpendicular to the direction of travel.

Electromagnetic Wave

Terms Crest : The highest point of the wave. Trough : The lowest point of the wave. Amplitude : The height of the wave as measured to the trough or crest. Wavelength : The distance between two nearest identical points on the wave in meters. Frequency : The number of peaks passing a point in a set period of time.

Electromagnetic Waves

Electromagnetic Waves A wave is characterized by two principal measures: wavelength (λ) -- distance (in µm) between successive wave crests frequency (ν) is the number of peaks passing a point per second (in Hertz, hz)

Wavelength vs. Frequency Low Long Short High

Wavelength and Frequency Wavelength and frequency are related by the following formula:

Electromagnetic Spectrum: Distribution of Radiant Energies

Gamma Rays Very high energy, more penetrating than X-rays. Generated by nuclear processes. Some applications in medical imaging

X-Rays High energy, penetrates everything but hardest metals Extensive use in medical imaging

Ultraviolet Radiation Beneficial in small amounts, damaging in large Absorbed by ozone layer

Visible Spectrum Humans eyes sensitive to this region 0.4 µm < λ < 0.7 µm Humans eyes sensitive to this region Majority of solar radiation in this region widely used in remote sensing

Infrared Spectrum 0.7 µm < λ < 100 µm Two major subdivisions: Reflective (also near and middle IR) – 0.7 µm < λ < 3.0 µm Thermal (also emissive, radiative) -- 3 µm < λ < 100 µm Very widely used in remote sensing

Microwave Radiation 1 mm – 1 m Regions are indicated by a letter code: P, L, X, etc. Widely used in remote sensing -- Radar

Particle Model of EMR (Quantum theory) Energy is transferred in discrete packets called quanta or photons. The relationship between the frequency of radiation expressed by wave theory and the quantum is: Q = h ν where Q - energy of a quantum measured in Joules (J), h - Planck constant (6.626 x 10-34J s-1), ν - frequency of the radiation.

Two Very Important Things about EMR: Radiation Laws Two Very Important Things about EMR: EMR is emitted by any object with a temperature greater than absolute zero (i.e. > 0 Kelvin) (K = ˚C + 273) The hotter the object, the more EMR it emits, at higher energy, and at shorter wavelengths.

Planck’s Blackbody Radiation Law A blackbody is a hypothetical, ideal radiator that totally absorbs and reemits all energy incident upon it at a given temperature. We may think of the Sun as a 6,000 ˚K blackbody. Planck’s law gives the relationship between energy emitted, temperature, and wavelength of blackbody.

Planck’s Law h = Planck’s constant Planck's law describes the spectral radiance of electromagnetic radiation at all wavelengths emitted in the normal direction from a black body at temperature T. h = Planck’s constant k = Boltzmann constant (=1.38*10-23 JK-1) c = speed of light (constant) T = temperature (in K) λ = Wavelength

Blackbody Radiation Curves Planck’s law solved for λ at constant T Blackbody radiation curves for several objects including the Sun and the Earth which approximate 6,000 ˚K and 300 ˚K (27 ˚C) respectively.

Developments from Planck’s Law: Wien’s Displacement Law Notice that the peak of the Blackbody curve shifts to shorter wavelengths as temperature increases. This peak represents the wavelength of maximum emittance – dominant wavelength (λmax).

Wien’s Displacement Law Note: Rλ(T) is the Planck function Solution: This value is called Wien’s constant

Examples of Wien’s Law λmax of Earth (temperature = 300 K) λmax = 2898 μm*K/300 K λmax = 9.7 μm λmax of Sun (temperature = 6000 K) λmax = 2898 μm*K/6000 K λmax = 0.48 μm