Chapter 2 - Electromagnetic Radiation Principles Flashcards
Interactions that EMR from Sun has before becoming data
Radiated by atomic particles (the sun)
Travels through vacuum of space at speed of light
Interacts with Earth’s atmosphere
Interacts with Earth’s surface
Interacts with the atmosphere again
Reaches remote sensor, interacts with various optics, filters, film emulsions, or detectors
Energy
Ability to do work, often transferred from one body/ place to another 3 ways of transfer: - conduction - convection - radiation
Conduction
Transfer of kinetic E by collision (one hot object transfers heat to another hot object this way
Convection
Transfer of E by physically moving the bodies (hot air rising)
Radiation
Transfer of E by emission of electromagnetic waves
Electromagnetic Radiation (Structure, creation)
Perpendicular fluctuation fields - one electric and one magnetic, created when an electron jumps down an energy level after excitation
Frequency
Number of wavelengths that pass a point per unit time
wavelength-free equation
c = 𝛌v
Blackbody
Theoretical construct that absorbs & radiates energy at maximum possible rate per unit area at each wavelength 𝛌, for given T
Total emitted energy from a blackbody
M𝛌 = σT^4
where σ = Stephan Boltzmann constant
T = abs temp in K
Dominant wavelength emitted from blackbody
𝛌max = k/T where k = 2898 µm K T = abs temp in K
Excitation
Electrons can move “up” an energy level if an energy input threshold is reached (otherwise no work is accepted)
Potential energy is increased, after ~10^-8 seconds, electron falls back down & gives off radiation - possibly to an intermediate rung first
Quantum leap/jump
The movement of an electron to a different rung (electron changes states discretely without ever being in between). If an electron makes stops at intermediate rungs, the energy of small jumps sum to the energy of what the large jump would be
Energy of a specific wavelength
Q = hv = hc/𝛌
wavelength is inversely proportional to 𝛌
Photoelectric effect
Matter can be heated to such high T that electrons that normally move in non-radiating orbits break free & the atoms are ionized. When a free electron drops in to fill the vacant energy level, then radiation is given off in a continuous spectrum.
Radiant energy
Capacity of radiation within a spectral band to do work
Refraction
“Bending” of light as it passes through material of different optical density. Angle is predicted by snell’s law.
Index of refraction
Measure of optical density of a substance - ratio of the speed of light in a vacuum, c, to the speed in the substance, cn
n = c/cn
Snell’s law
n1sinθ1 = n2sinθ2
Relates the angle of traveling ray with indexes of refraction of multiple materials
Types of scattering
Rayleigh, Mie, Nonselective
Rayleigh scattering
(Molecular scattering) - diameter of matter are many times smaller than the wavelength of incident EMR - impossible to predict direction of photon reemission
Rayleigh scattering cross-section algorithm
Describes the amount of Rayleigh scattering
τm = 8π^3(n^2-1)^2/(3N^2λ^4)
n = index of refraction
N = # of air molecules per unit volume
Amt is inversely proportional to λ^4, so longer wavelengths are effected much less.
Explain why sunset is red, sky is blue
Sky is blue b/c Rayleigh scattering affects blue light more than red -> most red light makes it to the ground, while some blue light is “trapped” in atmosphere
Sunset is red/ orange because there’s more atmosphere to pass through and most blue light is scattered completely before it can reach your eye
Mie scattering
(Non-molecular/ aerosol scattering) - main scattering agent for visible light -> dust & smoke & other particles
amt of scattering > Rayleigh scattering
Pollution increases mie scattering (& makes sunsets beautiful)
Nonselective scattering
Occurs when particles are 10x greater than λ
All λ are affected
Water & Ice that make up clouds and fog scatter all λ & appear white
Turning on headlights in fog & blinding yourself is an ex of nonselective scattering
Absorption
Radiant energy is absorbed & converted to other types of E (heat, other λ).
Dif substances have dif absorption bands
Extinction coefficient
combined effect of absorption and scattering - inversely related to transmission
Atmospheric transmission coefficients
How much energy penetrates the atmosphere (one coef. for entering, and one for exiting)
Reflectance
When radiation “bounces off” an object (reradiation of photons in unison by atoms or molecules approx 1/2 λ deep)
Angle of incidence and reflectance are approx equal
vertical, ray of incidence, and ray of reflectance lie in the same plane
Specular reflectance
reflected radiation is smooth - average profile height of surface is several times smaller than the λ of the radiation striking the surface
Diffuse reflection
Does not yield mirror image - light scattered in all directions. A perfect diffuse surface = Lambertian surface
Radiant Flux (φ)
Time rate of flow of energy off of, or through a surface, measured in Watts (W)
Characteristics of φ as it hits terrain (incident φ) gives important info about terrain
Radiation budget equation
φi = φreflected + φabsorbed + φtransmitted
AT A GIVEN λ
incident radiant flux at λ equals the flux reflected, absorbed, and transmitted through the surface it strikes
Hemispherical reflectance (ρλ)
Dimensionless ratio of the radiant flux reflected from a surface to the φ incident to it
ρλ = φreflectedλ / φiλ
Hemispherical absorptance (αλ)
Dimensionless ratio of absorbed radiant flux to incident
αλ = φabsorbedλ / φiλ
related to hem. reflectance, transmittance by
αλ = 1 - (ρλ + τλ)
by combining equations with radiation budget equation
Hemispherical transmittance (τλ)
Dimensionless ratio of transmitted radiant flux thru surface to incident φ
τλ = φtransmittedλ / φiλ
Percent reflectance (ρλ%)
ρλ% = (φreflectedλ / φiλ) * 100%
Looking at percent reflectance curves (how much radiant flux is reflected in each wavelength for different substances) gives analysts an idea of which bands to use to distinguish between substances (wherever the largest contrast exists).
Radiant flux density
Avg radiant flux intercepted divided by area
Irradiance
(Eλ) amount of radiant flux incident upon a surface per unit area - a type of radiant flux density
Eλ = φλ/ A
gives no info about angle of incidence
Exitance
Amount of radiant flux leaving a surface per unit area - type of radiant flux density
Mλ = φλ/ A
gives no info about angle of incidence
Radiance
(Lλ, most precise remote sensing radiometric measurement) radiant intensity per unit of projected source area in specified direction
Lλ = (φλ/Ω) / Acosθ
Solid Angle, Ω
measured in steradians, like a 3 dimensional cone/tube that represents volume of captured radiant flux
Path 1
Contains spectral solar irradiance Eoλ that was attenuated very little before illuminating terrain in IFOV
Is a function of atmospheric transmittance where 0 < Tθo < 1
Path 2
Spectral diffuse sky irradiance Edλ, that never reaches earth’s surface (target study area) -> some is scattered into the IFOV of the sensor (Rayleigh scattering of blue light contributes a lot to this
Path 3
Contains energy from the sun that has undergone various types of scattering before illuminating the area of study.
Spectral composition & polarization will be different than path 1
Path 4
Contains radiation that was reflected or scattered by terrain near the area of study ρλn
Path 5
Contains energy that was reflected from nearby terrain into atmosphere, then scattered or reflected onto study area
Total solar irradiance reaching earth’s surface, Egλ
Egλ = integral from λ1 to λ2 of EoλTθocosθo + Edλ where Eoλ = spectral solar radiance at top of atmosphere Tθo = atmospheric transmittance, 0-1 θo = solar zenith angle Edλ = Spectral diffuse sky irradiance
Total amount of radiance exiting target study area LT
LT = 1/pi * integral from λ1 to λ2 of
ρλTθv(EoλTθocosθo + Edλ)
where ρλ is the avg surface target reflectance
Path radiance, Lp
Radiance from all other paths that is an intrusive component
Ls = LT + Lp
where Ls is the radiant flux entering the sensor, LT is the radiance exiting the study area and Lp is path radiance
