ee/ae 157a week 4: visible and near ir
DESCRIPTION
EE/Ae 157a Week 4: Visible and Near IR. Topics to be Covered. Space Mirrors Diffraction limited resolution, Space mirror materials, Mirror coatings, structural materials Space Detectors Photoemissive, Photoconductive, Photovoltaic, CCDs Examples of Systems Landsat MSS and TM, SPOT - PowerPoint PPT PresentationTRANSCRIPT
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EE/Ae 157a
Week 4: Visible and Near IR
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Topics to be Covered
• Space Mirrors– Diffraction limited resolution, Space mirror materials, Mirror coatings,
structural materials• Space Detectors
– Photoemissive, Photoconductive, Photovoltaic, CCDs• Examples of Systems
– Landsat MSS and TM, SPOT• Examples of Image Artifacts
– Line Dropouts, Banding, Line Offsets• Analysis Techniques
– Ratio images, Principal components, NDVI, Edge enhancements, Sharpening, Spectral unmixing, Classification
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Basic Remote Sensing System
Source
Scattering Object
Waves Emitted
Collecting Aperture
Detector
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Imaging Terms
SwathWidth
Cross-TrackDirection
Along-TrackDirection
Field-of-view
Dwell Time
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Areal Image Plane
Imaging Optics
Along-Track Direction
Cross-Track DirectionSwathWidth
PlatformMovement
ScanningMirror
ImagingOptics
“Point”Detector
Line ArrayDetectors
Imaging Optics
(a) Framing Camera (b) Scanning System
(c) Pushbroom System
Types of Imaging Systems
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Comparison of Imaging Systems
Type Advantage Disadvantage
Film framing camera Large image formatHigh information densityCartographic accuracy
Transmission of filmPotential image smearing
Electronic framing camera Broad spectral rangeData in digital formatGood geometric fidelity
Difficulty in getting large arraysWide field-of-view optics
Scanning systems Simple detectorNarrow field-of-view opticsWide sweep capabilityEasy to implement multiplewavelenghts
Low detector dwell timeMoving partsDifficult to achieve goodgeometric fidelity
Pushbroom imagers Long dwell time per detectorGood cross-track geometricfidelity
Wide field-of-view optics
From Elachi,1987
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Basic Telescope
Primary
Secondary
Focal Plane
f FDf focal lengthF focal ratioD aperture diameter
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Diffraction Limited ResolutionCircular Aperture
1.22D
Dd 44.2
Rayleigh criterion for resolution:
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Telescope Classification
Telescope Primary Mirror Secondary Mirror
Cassegrain Parabola Hyperbola
Gregorian Parabola Ellipse
Ritchey-Chritein Hyperbola Hyperbola
Dall-Kirkham Ellipse Sphere
Newtonian Parabola Flat
Schmidt Aspheric Sphere
Scwarzschild Sphere Sphere
From Space Remote Sensing Systems: An Introduction, by H.S. Chen, 1985
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Types of Telescopes
Newtonian
Cassegrain
Gregorian
Dall-Kirkham
Ritchey-Critien
Schwarzschild
Schmidt
From Space Remote Sensing Systems: An Introduction, by H.S. Chen, 1985
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Telescope Terms
• Focal Length: Effective length of the light path from the lens or mirror to the focus point
• Aperture Size: Unobstructed size of the lens or mirror• Focal plane: The area covered with sensors that
change electromagnetic energy into electrical signals• Field of View: The angle viewed by the focal plane• Pixel Field of View: The angle viewed by a single
detector in the focal plane• Field of Regard: The total angle that a scanning
telescope can image
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Diffraction Limited ResolutionCircular Aperture
1.22D
d 2.44D
Rayleigh criterion for resolution:
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Diffraction Limited ResolutionCircular Aperture
Separation < 1.22 /D Separation = 1.22 /D Separation > 1.22 /D
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Diffraction Limited ResolutionEffect of Wavelength
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Diffraction Limited ResolutionAperture Size for Constant Resolution
1
10
100
1000
10000
100000
1000000
0.1 10 1000 100000 10000000Wavelength in Microns
Aper
ture
Siz
e in
Met
ers
Ape
rtur
e S i
z e in
me t
e rs
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Diffraction Limited ResolutionEffect of Apodization
NO APODIZATION GAUSSIAN SIGMA = RADIUS
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Diffraction Limited ResolutionEffect of Apodization
NO APODIZATION GAUSSIAN SIGMA = RADIUS
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Diffraction Limited ResolutionEffect of Aperture Shape
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Diffraction Limited ResolutionEffect of Surface Errors
NO ERRORS WAVELENGTH / 10
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Diffraction Limited ResolutionEffect of Surface Errors
NO ERRORS WAVELENGTH / 10
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Diffraction Limited ResolutionEffect of Surface Errors
NO ERRORS WAVELENGTH / 6.66
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Improving Angular Resolution Through Aperture Synthesis
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Improving Angular Resolution Through Aperture Synthesis
LIGHT ADDED IN PHASE
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Improving Angular Resolution Through Aperture Synthesis
LIGHT ADDED OUT OF PHASE
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Aperture SynthesisEffect of Aperture Spacing
Spacing = 4 diameters Spacing = 8.5 diameters
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Space Mirror Materials
Material Density Modulus of Elacticity(N/ cm2 x 106)
Coefficient of ThermalExpansion(1/ oC x 10-6)
Fused Silica 2.20 7.0 0.55
ULE 2.21 6.74 0.03
Cer-Vit 2.50 9.23 0.1
Zerodur 2.52 9.20 0.05
Beryllium 1.86 28.0 12.4
Aluminum 2.70 6.9 23.9
Invar 8.0 14.8 1.3
Graphite Epoxy 1.72 6.89 1.0
From Space Remote Sensing Systems, by H.S. Chen
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Space Mirror Coatings
0
10
20
30
40
50
60
70
80
90
100
0.2 0.4 0.6 0.8 1Wavelength in microns
% R
eflec
tanc
e
AluminumSilverGoldCopper
Adapted From Space Remote Sensing Systems, by H.S. Chen
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Space Structural Materials
Characteristics Al Be Gr/ Ep Gr/ Al Gr/ Mg
Light Weight Fair Fair Good Good Good
High Modulus Fair Good Fair Good Good
No Outgassing Fair Fair Poor Poor Poor
Conductivity Fair Fair Poor Fair Fair
Cost Low Medium Medium High High
From Space Remote Sensing Systems, by H.S. Chen
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Detectors
• Electro-optical detectors transforms wave energy into electrical energy
• The two most common types are thermal and quantum detectors• Thermal detectors rely on the increase in temperature in heat
sensitive material due to absorption of incident radiation• Implementations include bolometers and thermocouplers• Thermal detectors are slow, have low sensitivity, and their
response is independent of wavelength• Thermal detectors are not commonly used in modern remote
sensing systems
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Detectors
• Quantum detectors use the direct interaction of the incident photons with the detector material, which produces free charge carriers
• They are typically classified into three categories: photoemissive, photoconductive, and photovoltaic
• Quantum detectors have fast response and high sensitivity, but have a limited spectral response
• Quantum detectors are characterized by a parameter
D* A f
NEP
A Detector areaf Bandwidth
NEP Noise equivalent power
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D*
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Photoemissive Detectors
• In photoemissive detectors, the incident radiation leads to electron emission from a photosensitive intercepting surface
• The emitted electrons are accelerated and amplified• These detectors are primarily used at shorter wavelengths, since
the incoming photons must have sufficient energy to overcome the binding energy of the electrons
• Cesium has a cut-off wavelength of 0.64 microns• Composites, such as silver-oxygen-cesium have longer
wavelength (1.25 microns) cut-off wavelength• An example of this type of detector is the Photomultiplier tube
(PMT)• Landsat multi-spectral scanner (MSS) used PMT detectors for
three of the four bands
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Photoconductive Detectors
• In photoconductive detectors, photons with incident energy greater than the forbidden band energy gap in the semiconductor material produces free-charge carriers
• This causes the resistance of the photosensitive material to vary inversely proportional to the number of incident photons
• Exciting electrons across the forbidden band requires substantially less energy than electron emission, and consequently photoconductive detectors can operate at longer wavelengths
• Back-biased silicon photodiodes operate in the photoconductive mode
• Photodiodes can respond within a few nanoseconds• Landsat MSS band 4 used a photodiode as a detector.
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Photovoltaic Detectors
• In the case of photovoltaic detectors, the incident energy is focused on a p-n junction, modifying the electrical properties, such as the backward bias current
• Unbiased silicon photodiodes operate in the photovoltaic mode• Because this mode has no dark current, it has distinct advantages
for low-level dc radiation signals• The photovoltaic response time is typically limited to a few
microseconds
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Detector Landscape> 1 mm100-1000 um10-100 um1-10 um0.1-1 um10-100 nm1-10 nm
mmWaveSub-mmFIRMIRNIRVisUV
TECHNOLOGIES
SC Calorimeter CCD
Micro Channel Plate
CMOS
InGaAs
Si: As
QWIP
InSb
SC Bolometer
HEB
SIS
Schottky
InP HEMTGaN Ge: GaSi: Sb
HgCdTe
CCD Calorimeter
Uncooled Bolo
Commercial and defense applications in terrestrial imaging and sensing• strong technical infrastructure• synergistic funding
Commercial and defense applications in comms and radar
Primarily driven by space based astrophysics• weak infrastructure• limited funding • great science
SAFIR
• strong technical infrastructure
• synergistic funding
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Charge Coupled Device (CCD) Detectors
• CCD devices control the movement of signal electrons by the application of electric fields
• Most CCD devices can operate in either the photoconductive or the photovoltaic modes
• In monolithic CCDs the photon detection and multiplexing are performed on the same chip. It is best suited for VLSI technology, and have lower production costs
• In hybrid CCDs these operations are performed by two separate chips. Splitting these operations means that each can be optimized separately
• CCD detectors are easily integrated into arrays• Most modern remote sensing systems use CCD detectors.
Examples include SPOT, MOMS and Galileo
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CCD Readout
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CCD Timing
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Example: Kodak CCDs
Device Pixels (HxV) Pixel Size (H x Vµm) KAF-0261E 512 x 512 20.0 x 20.0 KAF-0401E(/LE) 768 x 512 9.0 x 9.0 KAF-1001E 1024 x 1024 24.0 x 24.0 KAF-1301E(/LE) 1280 x 1024 16.0 x 16.0 KAF-1401E 1320 x 1037 6.8 x 6.8 KAF-1602E(/LE) 1536 x 1024 9.0 x 9.0 KAF-3200E(ME) 2184 x 1472 6.8 x 6.8 KAF-4301E 2084 x 2084 24.0 x 24.0 KAF-6303E(/02LE) 3088 x 2056 9.0 x 9.0 KAF-16801E(/LE) 4096 x 4096 9.0 x 9.0
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Landsat 7 Orbit
Parameter Value
Orbit Altitude, km 705.3
Orbit Period, min 98.9
Orbit Inclination, deg 98.2
Repeat Cycle, days 16
Orbit Type Sun synchronous
Image Time 10:00 a.m local time
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Landsat ETM+Parameter Value
Telescope Diameter, cm 40.6 Telescope Type Ritchey-Chritien f-number 6 Swath Width, km 185 Scan Frequency, Hz 7.0 Scan Angle, deg 7.7 Number of lines per scan 16 Ground Resolution, m 15/ 30 / 60 Bandpass, m Band 1 0.45-0.52 Band 2 0.52-0.60 Band 3 0.63-0.69 Band 4 0.76-0.90 Band 5 1.55-1.75 Band 6 10.4-12.5 Band 7 2.08-2.35 Band 8 0.52-0.90 (15m) Quantization Level 256 (8 bits)
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Landsat TM Optical System
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SPOT
Parameter Value
Orbit Altitude, km 822Orbit Type Sun synchronousImage Time 10:30 a.m.Swath Width, km 60Imager Type PushbroomNumber of detectors per line 6000 / 3000Detector Type CCD ArraysGround Resolution, m 10 / 20Bandpass, m Band 1 0.50-0.59 Band 2 0.61-0.69 Band 3 0.79-0.90 Band 4 0.50-0.90
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SPOT vs LANDSAT
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Analysis TechniquesColor Combinations
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Analysis TechniquesColor Combinations
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ASTER Parameters( m)Subsystem Band
No.Spectral Range Spatial
Resolution (m)
Quantization Levels(bits)
VNIR 1 0.52-0.60 15 8
2 0.63-0.69
3N 0.78-0.86
3B 0.78-0.86
SWIR 4 1.60-1.70 30 8
5 2.145-2.185
6 2.185-2.225
7 2.235-2.285
8 2.295-2.365
9 2.360-2.430
TIR 10 8.125-8.475 90 12
11 8.475-8.825
12 8.925-9.275
13 10.25-10.95
14 10.95-11.65
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ASTER DATA OF CUPRITE, NV
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ASTER Color Combinations
1 - 2 - 3 1 - 3 - 6
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Analysis TechniquesRatio Images
• Ratio images are formed by dividing the data value in one band by that of another band
• Ratio images are used to emphasize differences in spectral reflectance of materials. For example, vegetation shows a maximum reflectance in TM Band 4 and a lower reflectance in band 2. The ratio image 4/2 enhances the vegetation signature
• Ratio images minimize the difference in illumination conditions, and suppress the effects of topography
• A disadvantage is that ratio images suppress differences in albedo; materials with different albedos but similar spectral properties may not be distinguishable in ratio images
• Another disadvantage is that noise is emphasized in ratio images
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Ratio Images
0
10
20
30
40
50
60
70
80
90
100
0.50 1.00 1.50 2.00 2.50
Wavelength in Microns
% R
efle
ctan
ce
KaoliniteAluniteHematiteMontmorilloniteJarositeGoethiteBuddingtoniteMuscovite
1
2
3
4
5
6
7
8
9
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Ratio Images: Band 4/7
Highlights presence of clays due to Al-OH bending mode absorption feature in band 7
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Ratio Images: Band 3/1
Highlights presence of iron oxides
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Ratio Images: Band 4/3
Highlights presence of iron oxides
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Ratio Images: Color Combination
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Analysis TechniquesNDVI
• The normalized difference vegetation index (NDVI) is defined as
• Higher values of NDVI indicate higher concentration of green vegetation
• NDVI maps are typically calculated using biweekly combinations of images to reduce the effects of cloud cover
NDVI nir red
nir red
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Analysis TechniquesNDVI
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Analysis TechniquesIntensity/Hue/Saturation Transformation
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Analysis TechniquesIntensity/Hue/Saturation Transformation
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Analysis TechniquesSensor Combinations
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Analysis TechniquesPrincipal Components
• Typically, images from individual bands are highly correlated on a pixel by pixel basis
• The principal component transformation arranges images in order of the amount of variance in the data across the image
• This mathematical transformation is similar to calculating the eigenvalues and eigenvectors of the image on a pixel by pixel basis
• Most of the variance is typically in the first few principal components, with the last few dominated by noise
• The first PC image is typically dominated by topographic effects• By displaying three PC images as red, green and blue, spectral
variations are typically enhanced
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Analysis TechniquesPrincipal Components
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Analysis TechniquesPrincipal Components
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Analysis TechniquesPrincipal Components
1 – 2 - 3 PC1 – PC2 – PC3
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Analysis TechniquesEdge Enhancements
• Edge enhancement filters are used to enhance linear features in images
• Geologists use linear features to map faults, while geographers use linear features to identify man-made structures such as roads
• Edges can be enhanced using non-directional or directional filters• An example of a non-directional filter is the Laplace kernel
• Directional edge enhances are used to identify linear features in specific directions:
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Analysis TechniquesEdge Enhancements
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Analysis TechniquesSupervised Classification
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Analysis TechniquesUnsupervised Classification
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Spectral Unmixing
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Spectral Unmixing