ELECTROMAGNETIC RADIATION (EMR) AND REMOTE SENSING

ELECTROMAGNETIC RADIATION (EMR) AND REMOTE SENSINGGABRIEL PARODI & DIANA CHAVARRO-RINCON

What is remote sensing?

Remote sensing is a way of collecting and analysing data to get information about an object without the instrument used to collect the data being in direct contact with the object.

Normal photography is an example of remote sensing

EarthEmission processes

Thermal emission

Atmospheric emission

Reflection processes

Reflectedradiation

LEARNING SEQUENCE IN THIS LECTURE

TOA

Clouds Scattered radiation*

Atmospheric absorption

scatteredradiation**

transmittedradiation

SunEM radiation

Source

ReflectionAtmosphere

1

2

3

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1. EMR AND REMOTE SENSING: PROPAGATION

https://www.youtube.com/watch?time_continue=266&v=lwfJPc-rSXwVideo courtesy of ScienceAtNASA

1. ELECTROMAGNETIC RADIATION

Wave–Particle duality

Light is Electromagnetic (EM) radiation It can be modeled in 2 ways:  by waves by photons (energy bearing particles)

WAVE‐PARTICLE DUALITY

Source: https://toutestquantique.fr/en/

𝐸 = Electric vector𝑀 = Magnetic vector

𝑐 = speed of light

EM RADIATION: WAVE MODEL

EMR travels as waves Waves are characterized

by 2 fields: Electric and Magnetic

The 2 fields oscillate in time The 2 fields oscillate in

space perpendicularly to each other and to the direction of travel

Waves travel with speed of light:

WAVELENGTH AND CYCLE

Frequency 𝒇 is the number of cycles passing a fixed point per second

Frequency is inversely proportional to wavelength (c = speed of light)

units: 𝜆 in metres m𝑓 in s Hz hertz

Particle theory: EM radiation is composed of particles called photons.

Particle theory is useful for describing the amount of energy measured by the sensor

(Planck-Einstein relation)

𝑄 – amount of energy per photon Jℎ – Planck’s constant, ℎ 6.626 ⋅ 10 J s

The photon energy is proportional to the frequency

EM RADIATION: PARTICLE MODEL

Combination of models

and 𝑄 have inverse relationship (since ℎ and 𝑐 are constant). The photon energy is proportional to the frequency (inversely proportional to )

Q = [Joule = watt . sec]h = [Joule . sec]f = [sec-1]c = [meter . sec -1]λ = [meter]

THE EM SPECTRUM

THE EM SPECTRUM

EarthEmission processes

Thermal emission

Atmospheric emission

Reflection processes

Reflectedradiation

LEARNING SEQUENCE IN THIS LECTURE

TOA

Clouds Scattered radiation*

Atmospheric absorption

scatteredradiation**

transmittedradiation

Sun

Source2

SOURCES OF EMRTHE BLACK BODY CONCEPTEM FOR REAL OBJECTS

All matter above T = 0 K radiates electromagnetic radiation IN ALL WAVELENGTHS. Max Planck investigated how much…

SOURCES OF EM RADIATION

Earth’s surface ~ 27 ºC = ? K

Sun’s surface ~ 6000K = ? ºC

27ºC +273 = 300K

6000K -273 =5763ºC

0 K= -273ºC

0ºC= 273 K

PLANCK’S RADIATION LAW

A body absorbs part of the EMR that hits it. A black body (BB) is an ideal radiator that absorbs all

incoming radiation. Planck’s law for a black body

𝐿 Spectral radiance W sr m µm h Planck constant 6.62606896.10 34 J.s 𝑘 Boltzmann’s constant - c speed of light

k 1.38 ⋅ 10 J K 𝑇 Absolute temperature in Kelvin K

Stefan-Boltzmann law: Total emitted radiation M in all wavelength (area under the curve):

Wien’s displacement law:Wavelength with maximum radiation

BLACK BODY RADIATION CURVES 

Stefan-Boltzmann constant:𝜎 5.67 ⋅ 10 W m K

Wien’s displacement constant:𝑏 2898 μm 𝐾]

Real objects also reflect and transmit a part of incident radiation Energy is conserved

𝛼 𝜏 𝜌 1 Applies in all wavelengths Real objects absorb less than black body In equilibrium object re-emits all absorbed radiation So, in equilibrium, what is absorbed is being emitted for both real

and blackbodies!!

REAL OBJECTS 

0t

arAbsorptivity []: absorbed radiation /

incident radiation

Transmissivity []: transmitted radiation/ incident radiation

Reflectivity []: reflected radiation/ incident radiation

𝛼 𝜌 1

Real object emits less radiation than black body with the same temperature, 𝐿 𝜆, 𝑇

How much less: described by emissivity 𝑳 𝝀, 𝑻 𝛜 𝛌 ⋅ 𝑳𝑩𝑩 𝛌, 𝐓 Emissivity of black body: 𝜖 𝜆 1 Emissivity of real objects: 𝜖 𝜆 1 Radiation measured by a sensor is a sum of radiation reflected

and emitted by the Earth Not possible to separate directly Emitted radiation: spectrum depends only on 𝑇 and 𝜖 If 𝜖 is known, 𝑇 can be derived from 𝐿 Otherwise, 𝑇 cannot be determined from 𝐿

EMISSIVITY

EarthEmission processes

Thermal emission

Atmospheric emission

Reflection processes

Reflectedradiation

LEARNING SEQUENCE IN THIS LECTURE

TOA

Clouds Scattered radiation*

Atmospheric absorption

scatteredradiation**

transmittedradiation

Sun

Reflection

3

INTERACTION OF EMR WITH SURFACE: REFLECTION

REFLECTION IN NATURE

Used for photosynthesis

(a) Specular reflection from a smooth surface

(b) Diffused reflection froma rough surface

()

REFLECTIONS FROM THE SURFACE

SPECULAR REFLECTION ‐ EXAMPLE

Energy reaching the surface: irradiance [W m‐2]

Energy reflected by the surface: radiance [W m‐2]

Reflectance curve: fraction of irradiance that is reflected as a function of wavelength

radianceirradiance

Reflectance curves are material specific: spectral signature

SPECTRAL REFLECTANCE CURVES

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REFLECTANCE BY SENSORS: DIFFERENT AT DIFFERENT HEIGHTS?

YES!!!

BOA orSUR

Canopy

TOA𝐿 ↓

𝐿 ↓

𝐿 ↓

𝐿 ↑

𝐿 ↑

𝐿 ↑ _𝐿 ↑ _

𝜌 𝜆 _𝐿 ↑ _𝐿 ↓ 𝜌 𝜆 _

𝐿 ↑ _𝐿 ↓

𝜌 𝜆𝐿 ↑𝐿 ↓

𝜌 𝜆𝐿 ↑𝐿 ↓

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SEQUENCE OF REFLECTANCE ESTIMATION IN THE LAB

1 2

3

65

7 ?

8 ?

Factors Contributing

to leaf reflectance

Leafpigment Water content

Scattering by leaf cells

Absorption by free water in plant tissue

UV I n f r a r e d

FIR/TIRNIR MIR

Wavelengths (m)

50

40

20

00.1 0.4 0.5 0.6 0.7 1.35 1.4 1.9 3

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Visiblerange

Absorption for photosynthesis

SPECTRAL REFLECTANCE  ‐ HEALTHY VEGETATION

SOME TYPICAL REFLECTANCE CURVES

EXAMPLE IN IMAGES

Visible Infrared

Visible IR

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EXAMPLE

EFFECT OF SUN ILLUMINATION ANGLE

Same amount of radiation in equal solid angle, different footprint area

Smaller footprint area – larger irradiance

Affects measured radiance, must be considered for reflectance calculations

Multi temporal studies: take into account season, date and time!

1θ 2θ

EFFECT OF RELIEF

𝜃- local incidence angle

- Depends on slope- Shadows

OTHER TYPICAL SPECTRAL REFLECTANCE CURVES

EarthEmission processes

Thermal emission

Atmospheric emission

Reflection processes

Reflectedradiation

LEARNING SEQUENCE IN THIS LECTURE

TOA

Clouds Scattered radiation*

Atmospheric absorption

scatteredradiation**

transmittedradiation

Sun

Atmosphere

4

4. INTERACTION OF ERM WITH THE ATMOSPHERE

Gases mainly absorb EM radiationknown concentrations and location (cycles) enable to predict influence (per 𝜆)

Aerosols mainly scatter EM radiationvariable and difficult to model (human and natural changing influence)

Either way the satellite senses less than what reached the Earth’s atmosphere!

ATMOSPHERIC INTERACTIONS

ENERGY INTERACTIONS WITH THE ATMOSPHERE

Visible

THE SOLAR SPECTRUM (ABSORPTION)

ATMOSPHERE (ABSORPTION)

SELECTIVE (RAYLEIGH) SCATTERING

SCATTERING TYPE: REFERENCE GRAPH

Source: many authors

The same particle hit by a different wavelengths produces different kind of scattering in the wavelength.

SUMMARY

RS is based on detecting EMR EMR is described as waves & photons EM spectrum Blackbody radiance & emissivity Interaction with the atmosphere Absorption and scattering Atmospheric windows Interaction at the surface Spectral reflectance curves (‘spectral signatures’)

THANKS