Interactions of energy with atmosphere & earth surface features

Agricultural College, Bapatla

Class seminar on

By M. Sunil Kumar BAD-14-06

1

– Propagates through the vacuum of space

– Interacts with the Earth's atmosphere & surface

– Reaches the remote sensor (interacts with various

optical systems, filters, emulsions, or detectors)

Electromagnetic energy interactions

Electromagnetic Radiation Models

WaveWave model model

ParticleParticle model model

An electromagnetic wave is composed of electric and magnetic An electromagnetic wave is composed of electric and magnetic vectors that are orthogonal to one another and travel from the vectors that are orthogonal to one another and travel from the source at the speed of light (3 x 10source at the speed of light (3 x 1088 m s m s-1-1))

Energy is transferred in discrete packets called quanta or photonsEnergy is transferred in discrete packets called quanta or photonsQ = hQ = h νν

Q = Energy of a quantum measured in Joules (J)Q = Energy of a quantum measured in Joules (J) h = Planck constant (6.626 x 10h = Planck constant (6.626 x 10-34-34 J s J s-1-1)) ν ν = Frequency of the radiation= Frequency of the radiation

Energy interactions in the atmosphere

Refraction & transmission

Scattering

Absorption

Atmospheric Layers and ConstituentsMajor subdivisions of the atmosphere and the types of molecules and aerosols found in each layer.

1. Atmospheric refraction

θ 1

θ 2

θ 3

O ptically less dense atm osphere

O p tically m ore dense a tm osphere

O p tically less dense a tm osphere

P ath o f energy in

hom o geneous a tm osphere

3 n

n 1 = index o f re fraction for th is layer o f the a tm osphere

n 2

Inc iden t rad ian t energy

N orm al to the su rface

P ath o f rad ian t energy affected by a tm ospheric re fraction

A tm osp h eric R efraction

θ 1

θ 2

θ 3

O ptically less dense a tm osphere

O ptically m ore dense a tm osphere

O p tically less dense a tm osphere

P ath o f energy in

hom o geneous a tm osphere

3 n

n 1 = index o f re frac tion for th is layer o f the a tm osphere

n 2

Inciden t radian t energy

N orm al to the su rface

P ath o f rad ian t energy affected by atm ospheric re frac tion

A tm osp h eric R efraction Snell's law: The ratio of the sines of the angles of incidence and refraction is constant for all incidences in any given pair of media for electromagnetic waves of a definite frequency

The incident radiant energy is bent from its normal trajectory as it travels from one atmospheric layer to another. Snell's law (n1 sin θ1 = n2 sin θ2 = n3 sin θ3 ) can be used to predict how much bending will take place based on a knowledge of the angle of incidence and the optical density of each atmospheric level.

ni = c/ci

ni = index of refractionc = speed of light in a vacuumci = speed of light in a substance

Snell's law: The ratio of the sines of the angles of incidence and refraction is constant for all incidences in any given pair of media for electromagnetic waves of a definite frequency

The incident radiant energy is bent from its normal trajectory as it travels from one atmospheric layer to another. Snell's law (n1 sin θ1 = n2 sin θ2 = n3 sin θ3 ) can be used to predict how much bending will take place based on a knowledge of the angle of incidence and the optical density of each atmospheric level.

ni = c/ci

ni = index of refractionc = speed of light in a vacuumci = speed of light in a substance

Transmission The EMR varies along a frequency spectrum with infinite bounds The frequency range measurable and usable by remote sensors vary within

more than 9 orders of magnitude ~[0.1 mm – 100 m] The physical principles of interaction of the EMR with targets are different

over each spectral range. The human eye is generally sensitive between the [0.4-0.8] nm

range. The ultraviolet zone is mostly opaque, hence is generally unusable

for the RS applications There is no spectral window where the atmosphere is 100%

transparent The ionosphere is 100% opaque at frequencies below 10 Mhz

Atmospheric transmission

The atmosphere absorbs and scatters the EMR. Atmospheric opacity is largely due to absorption by molecules like H

20, O

2, O

3,

water etc Light is also scattered by the atmospheric molecules and particles

2. Scattering

Types of scattering

Rayleigh scattering

Mie scattering

Nonselective scattering

Deflection of a ray from a straight path

1. Rayleigh scattering Also called molecular

scattering; Consists of scattering from

atmospheric molecules; Dominant at elevations of 9 to

10 km above the surface; Follows a wavelength

dependency of ~ It is the Rayleigh scattering

that causes the blue color of the sky and the red color at sunset.

2. Mie scattering

• Particles' diameters are equivalent to

the wavelength d ≈ l

- Water vapor and dust are major causes

of Mie scattering

- Mie scattering tends to influence longer

wavelengths.

- It is common in lower atmosphere

where large particles are more

abundant, and dominates under

overcast could conditions.

3. Nonselective scattering

Particles are much larger than the wavelength d>>l

Water droplets (5-100 μm) and larger dust particles

Non-selective scattering is independent of wavelength

All wavelength are scattered equally (A could appears

white)

It scatters all visible and near to mid IR wavelengths.

Effects of scattering

• It causes haze in remotely sensed images

• It decreases the spatial detail on the images

• It also decreases the contrast of the images

3. Absorption• Absorption is the process by which radiant energy is absorbed and

converted into other forms of energy

• The atmosphere prevents, or strongly attenuates, transmission of

radiation through the atmosphere

• An absorption band is a range of wavelengths (or frequencies) in

the EMS within which radiant energy is absorbed by substances

such as water, CO2, O2, O3, & N2O.

• O3: absorbs ultraviolet radiation high in atmosphere

• CO2: absorbs mid and far infrared (13-17.5 μm) in lower

atmosphere

• H2O: absorbs mid-far infrared (5.5-7.0, >27 μm) in lower

atmosphere

Absorption bands

Absorption bands by different gases in different layers of atmosphere

Transmission, reflection, scattering, & absorption

Atmospheric windows (transmission bands )

The wavelength ranges in which the atmosphere is particularly transmissive

Atmospheric Windows • The windows:

UV & visible: 0.30-0.75 mm

Near infrared: 0.77-0.91 mm

Mid infrared: 1.55-1.75mm, 2.05-2.4 mm

Far infrared: 3.50-4.10 mm, 8.00- 9.20 mm, 10.2-12.4 mm

Microwave: 7.50-11.5 mm, 20.0+mm

• X-Rays and UV are very strongly absorbed and Gamma Rays

and IR are somewhat less strongly absorbed.

• The atmospheric windows are important for RS sensor design

Energy interactions with earth surface features

Energy Interactions with Earth Surface Features

• Reflection

• Absorption

• Transmission

• Emission

• All EM energy reaches earth's surface must be reflected, absorbed, or transmitted

• The proportion of each depends on:

– The spectral reflectance properties of the surface materials

– The surface smoothness relative to the radiation wavelength

–Wavelength

– Angle of illumination

Energy Interactions with Earth Surface Features

Energy Interactions with Earth Surface Features

- Light ray is redirected as it strikes a nontransparent surface

- Albedo - Spectral reflectance R (λ): the average amount of incident radiation reflected by an object at some wavelength interval

R (λ) = ER (λ) / EI (λ) x 100

Where

ER(λ) = reflected radiant energy

EI (λ) = incident radiant energy

1. Reflection

Specular versus diffuse reflectance

- Specular reflectors are flat surfaces that manifest mirrolike

reflections. The angle of reflection equals the angle of incident.

- Diffuse (or Lambertian) reflectors are rough surfaces that reflect

uniformly in all the directions

- If the surface is rough, the reflected rays go in many directions,

depending on the orientation of the smaller reflecting surfaces

- Diffuse contain spectral information on the color of the reflecting

surface, whereas specular reflections do not.

- In remote sensing we are often interested in measuring the diffuse

reflectance of objects.

Specular versus diffuse reflectance

Per cent reflection of earth surface features

2. Transmission

• Radiation passes through a substance without significant attenuation

• Transmittance (t):

transmitted radiation t = --------------------------- incident radiation

3. Absorption

Reflection + Transmission + Absorption = 100%

4. Emission

Spectral Characteristics of Features

Spectral reflectance curves for vegetation, soil, & water

Identification of Surface Materials Based on Spectral Reflectance

Spectra of vegetation• Chlorophyll absorbs blue and red, reflects green • Vegetation has a high reflection and transmission at NIR

wavelength range• Reflection or absorption at MIR range, the water

absorption bands•

Spectra of vegetation

Absorption is dominant process in visibleScattering is dominant process in near infraredWater absorption is increasingly important with increasing wavelength in the infrared.

Spectra of soil

What are the important properties of a soil in an RS image

-Soil texture (proportion of sand/silt/clay)

-Soil moisture content

-Organic matter content

-Mineral contents, including iron-oxide and carbonates

-Surface roughness

Dry soil spectrum

20

60

100

Percent Reflectance

0.5 0.7 1.1 1.30

Wavelength (µm)

80

40

0.9 1.5 1.7 1.9 2.1 2.3 2.5

Silt

Sand

10

30

50

70

90

• Coarse soil (dry) has relatively high reflectance• Increasing reflectance with increasing wavelength through the visible, near and mid infrared portions of the spectrum

Soil moisture and texture

• Soil moisture decreases reflectance

• Clays hold more water more ‘tightly’ than sand.

• Thus, clay spectra display more prominent water

absorption bands than sand spectra

Soil moisture and texture

20

60

Percent Reflectance

0.5 0.7 1.1 1.30

40

0.9 1.5 1.7 1.9 2.1 2.3 2.5

22 – 32%

10

30

50

Sand

20

60

0.5 0.7 1.1 1.30

Wavelength (µm)

40

0.9 1.5 1.7 1.9 2.1 2.3 2.5

35 – 40% 10

30

50 2 – 6%

0 – 4% moisture content

5 – 12%

Clay

a.

b.

Percent Reflectance

SandSandSand

ClayClayClay

Soil Organic Matter

Organic matter is a strong absorber of EMR, so more organic matter leads to darker soils (lower reflectance curves).

Iron Oxide

Recall that iron oxide causes a charge transfer absorption in the UV, blue and green wavelengths, and a crystal field absorption in the NIR (850 to 900 nm). Also, scattering in the red is higher than soils without iron oxide, leading to a red color.

Surface Roughness

• Smooth surface appears black.

• Smooth soil surfaces tend to be clayey or silty,

often are moist and may contain strong absorbers

such as organic content and iron oxide.

• Rough surface scatters EMR and thus appears

bright.

Spectra of water

• Transmission at visible bands and a strong absorption at NIR bands

• Water surface, suspended material, and bottom of water body can affect the spectral response

Thank You!