Hyperspectral Imaging Systems

1
Hyperspectral Imaging Systems Atmospheric
Effects
Dr. Richard B. Gomez, Instructor School of
Computational Sciences George Mason University
2
Outline

• Atmospheric Structure
• Atmospheric Composition
• Solar/Earth Energy Distribution
• Atmospheric Radiative Transfer Theory

3
Basic Forms of Electromagnetic Energy

• Electromagnetic energy exists in many forms
  besides visible light (400 - 700 nm). Among
  these are
• Radio Waves gt100 m
• Microwaves 0.1
  100 cm
• Millimeter Waves 1 mm 0.1 cm
• Terahertz Frequencies 30 ?m 3 mm
• Thermal Infrared (Heat) 3 5
  ?m 8 14 ?m
• Ultraviolet Rays 3
  400 nm
• X Rays
  0.03 3 nm
• Gamma Rays
  lt 0.03 nm

4
Radiometric Quantities
Quantity Radiant Energy (J) - Q Radiant Energy
Density (J/m3) U Radiant Flux (W) Power
P Radiant Exitance (W/m2) M Irradiance (W/m2)
E Radiance (W/m2-sr) L Radiant Intensity (W/sr)
- I
Definition ?Pdt dQ/dV dQ/dt dP/dA dP/dA d2P/dAd?
dP/d?
5
Atmospheric Structure
CompositionNitrogen 78 Oxygen 21 Argon
nearly 1 Others HydrogenHelium, Neon, Carbon
Dioxide, etc.
The greatest error and uncertainty in deriving
reflectance factors usually comes from the
atmospheric modeling component.
6
Image from the NASA Langley Research Center,
Atmospheric Sciences Division. http//asd-www.larc
.nasa.gov/erbe/ASDerbe.html
7
Background Radiometry
Image courtesy of Digital Imaging and Remote
Sensing (DIRS) Laboratory
8
Atmospheric Continuum
9
Background SEBASS Sensor
10
Background Thermal Spectrum
11
What is a Photon?

• ?E h?
• ?E E2 E1 Energy of photon in joules (J).?
  Frequency of the photon in hertz. h Planck's
  constant 6.625 1034 joule-seconds
• Wavelength ? c/? hc/?E
• A light wave that is emitted with a single
  quantum of energy ?E h? is called a photon

12
Photomultiplier
13
Absorption of Light
14
Molecular Spectra
15
Space Reference System
16
Solid Angle
d? dS / r2 (in steradians, sr)
17
Basic Components of Remote Sensing System

• Energy Source
• Transmission Path
• Target
• Background
• Sensor

18
Hyperspectral Imaging General Concept
19
Hyperspectral Reflectance Measurements
20
Key Characteristics of Surface Reflectance

• Range of values of ?
• Spatial variability
• Degree of departure from Lambertian surface
• Surface polarization
• Water and vegetated surfaces have
  non-Lambertianreflectance characteristics for
  reasons due to specular reflection,
  backscattering, shadows, backgrounds, or other
  reasons

21
Object Information Derived From Signal
22
Atmospheric Effects (Richards Fig. 2.1)
23
Spectral Windows
Photons falling between the spectral windows
highlighted above are severely attenuated (either
absorbed, scattered, or both) by the Earths
atmosphere.
24
Upward Radiance Components

• L L1 L2 L3 L4
• L1 Path Radiance independent of surface
  reflectance
• L2 Attenuated Signal depends only on the
  surface reflectance in the field of view.
  Provides Surface information.
• L3 Scattered by atmosphere to the surface and
  reflected to the sensor. Affected by
  non-Lambertian surface light.
• L4 Light radiance reflected by the surface
  with at least one scattering in the atmosphere
  before reaching the sensor. Affectedby
  nonuniform surface and by non-Lambertian surface
  reflectionthat may be out of the field of view
  of the sensor.

25
Total Radiance Available to Sensor

• Total Irradiance EG at Earths Surface
• EG E??T?cos ? ?? ED (Path Irradiance
  ED)
• The Radiance LT Due to Global Irradiance EG of
  the pixel
• LT (R/?)E??T?cos ? ?? ED
• Total Radiance Available to Sensor
• LS (RT?/?)E??T?cos ? ?? ED LP
• See John Richards Book Page 42, Equation 2.4

26
John Richards Equations
27
John Richards Reflectance Calculation
28
Atmospheric Compensation (continue)
29
Luminescence

• Luminescence the emission of light by a
  substance. It occurs when an electron returns to
  the electronic ground state from an excited state
  and loses it's excess energy as a
  photonDifferent Types
• Fluorescence
• Phosphorescence
• Chemiluminescence
• Bioluminescence

30
Atmospheric Compensation
31
Remote Sensing Electromagnetic Spectrum
Solar Radiance Back-Scattered from Earths Surface
VIS
Black Body Radiation of the earth (300K)
Energy
3u
10u
0.3u
1u
1 mm
500
a
0.4u
0.7u
VIS
MIR
FIR
MW
SWIR
100
NIR
Blocked
Transmission
500
50
100
5
1
10
500
20
300
5
3
10
2
1.0
0.5
1.5
0.3
0
b
(mm)
um
Wavelength
Human Eye
Photography
Radiometers Imaging Systems
Radars
Passive microwave Radiometers
Laser Sensors
32
Atmospheric Transmission Characteristics
33
Surface Emissions
34
Absorption Lines
When light from a luminous source passes through
a gas, the gas may extract certain specific
energies from the continuous spectrum. We then
see dark lines where the energy has been removed.
These dark lines are called absorption lines.
35
Linewidth Full Width at Half Maximum
Airborne Hyperspectral Systems
36
Solar/Earth Energy Distribution
37
Plancks Radiation Law
L? gives the energy per unit volume per unit
wavelength interval for blackbody radiation
38
Kirchhoffs Law

• Absorptivity ? Emissivity ?
• A good Absorber is a good emitter of radiation
• This law generally holds for each spectral
  component of the blackbody field and for each
  direction of the incident radiation

39
Stefan-Boltzmann Law
The total power per unit area radiated by a
blackbody is given by the Stefan-Boltzmann law.
? Stefan-Boltzmann constant.
40
Beers Law
41
Limitations of the Beer-Lambert law

• The linearity of the Beer-Lambert law is limited
  by chemical and instrumental factors. These
  include
• absorptivity coefficients deviations at high
  concentrations (gt0.01M) due to electrostatic
  interactions between molecules in close proximity
  
• scattering of light due to particulates in the
  path
• fluorescence or phosphorescence of the sample
• changes in refractive index at high
  concentrations
• non-monochromatic radiation
• stray light

42
Wien's Displacement Law
The wavelength at which a blackbody has its peak
emittance is given by Wien's Displacement
Law
43
RTE for Plane-Parallel Atmospheres Model
44
Instrument Response Accountability
45
Source Function
46
Solar Backscatter Radiance
Reflection characteristics of a surface are
provided by the bidirectional reflectance
distribution function (BRDF)
47
Weighing Function
p atmospheric pressure? optical thickness
48
EOSAEL