Post on 26-Jun-2020
Hyperspectral remote
sensing and it's potential
Moumita Dutta, 27th August, Sensor Developement Area
Content
• Introduction
• Origin of Spectroscopy
• Spectral Signature
• Hypercube
• Spectral dispersion techniques
• Instrumentation aspects
• Hyperspectral Imagers, SAC
• Potentials
Introduction
• Panchromatic : Single Wavelength Band
• Multispectral: Multiple Broad Bands (2 to few
10s)
• Hyperspectral: Several Narrow Bands (100s)
Panchromatic Multispectral Hyperspectral
Multispectral
Hyperspectral
Multispectral v/s Hyperspectral
Multispectral
Hyperspectral
Origin of Spectroscopy
• Newton generated a rainbow with a prism and described many
characteristics of light in Opticks, 1704.
• Fraunhofer developed a spectroscope in 1814 and used the
observation of dispersed light to understand glass composition.
Kirchhoff and Bunsen, 1859 begin to understand the solar lines
first observed by Fraunhofer.
• Edwin Hubble used spectroscopy to understand the expanding
nature of our universe in 1929. Now we measure the composition
of exoplanet atmospheres.
• Spectroscopy is a powerful analytical method that enables remote
measurement for scientific discovery and other applications
Spectral Signature
• Spectral Reflectance is commonly known as
the Spectral Signature of an object. It tells how
an object reflect the light of different color.
Reflectance spectra – examples…
Leaf spectral reflectance
Absorptio
n in blue
and red
Absorption in
intermittent
wavelengths
High
reflectance in
NIR
Soil spectral reflectance
Atmospheric transmission curve
Spectroscopy
Imaging
Remote-sensing
Hyperspectral remote-sensing is about three
sciences
Diffused Scattering Surface
with Spectral Reflectance,
𝝆(𝝀)
Direct Solar Spectral
Irradiance, 𝑰𝒔(𝝀)Path Radiance, 𝑳𝒑(𝝀)
Sensor Observable
Spectral Radiance,
𝑳 (𝝀)
Diffuse Reflected
Spectral Radiance, 𝑳𝑹(𝝀)
Indirect Downwelling
Spectral Radiance,
𝑳𝒅(𝝀)
Atmospheric
Spectral
Transmission,
𝑻(𝝀)
Remote-sensing
Charged
particle
motion
Molecular
rotation
Molecular
vibrationElectronic
transition in
outer shells
O
H H
IR, hν
O
H H
microwav
e,hν
e-
radiohν
p
n
γ-ray
hν
Electronic
transition in
inner shells
Nuclear
phenomena
e
e
X-
ray
hν
p
n
e
e
UV-
VIS, hν
We shall be discussing about
visible and infrared regions only
Sensor block diagram
SAC KSS05/12/2016
Slit
Hypercube
• 3D data set in which two dimensions are spatial
and third is spectral (i.e. spectral signature) is
called a hypercube (i.e. hyperspectral cube).
• In other word it is the set of the spectral
signatures of an area array target.
Hypercube generated from
observation by
our Airborne Imaging Spectrometer
(AIMS)
Spectral Dispersion
Techniques
SAC KSS05/12/2016
Prism
Grating
Young’s double slit experiment
Path difference = d sin θn,λ
θn,λ
θn,λ
Screen
d
λ
λ
i
i
i
Path
difference
before slit =
d sin i
For constructive
interference,
d (sin θn,λ + sin i) = n λ
Double slit intensity pattern
d
Multiple slits
5 slits3 slits
Number of slits Better spectral dispersion
Grating
θn,λ
θn,λ
Screen
d
λ
λ
λ
Transmission Grating
i
Grating equation
d (sin θn,λ + sin i) = n λ,
n = order
+1 order
Grating
Normal
iθ0
B=θ-1 +β
θ+1
θ-1
A=i-β
β is such that A=B
β
β
β = (i - θ-1 )/2 Groove
Normal
0 order
-1 order
Blazed grating
Convex grating
(Pitch 50 um)
Plane grating
(Pitch 22 um)
Diamond Turning-Blazed Grating Fabrication
Convex Gratings fabricated using Single Point Diamond Turning
A very sharp diamond tool with the cutting edge of an appropriate
shape
is used for ruling or fly-cutting of the structure
Grey Scale Electron Beam Lithography for Blazed Grating Fabrication
A high-efficiency blazed grating can be formed in a single process
by control of
an electron dose for sub-micrometer patterning
Grating fabricated using E-Beam
Lithography
Grism
Transmission grating replicated onto prism
Desired order goes undeviated
Prism
Grating
Wedge Filter is also known asLinear Variable Filter (LVF)
In this spectral transmissionvary along the length of the filterdue to continuous variablethickness.
White light input
Color separated light input
Spectral transmission at various locations
Wedge filter
Fabry-Perot tunable filter
Michelson
Sagnac
DETECTOR
Property Prism Grating Wedge FT based
Spectral resolution Medium High Medium Very High
Throughput Medium High Medium High
Spectral range Narrow Broad Medium Broad
Moving parts No No No No/Yes
Simultaneous
acquisitionYes Yes No Yes
Stray light Low Low High High
Distortion High Low Medium Low
Compactness Medium Medium High Low
Comparison
Instrumentation Aspects
• Ground Sampling Distance
• Field of view
• Noise equivalent spectral radiance
• Sampling time
• Signal to Noise Ratio(SNR)
• Spectral range
• Spectral resolution
• Spectral sampling
• Smile
• Keystone
Spectral sampling and resolution
Smile
Change in dispersion
with field position
Keystone
Change in magnification
with wavelength
Smile and keystone
Smile
Wavelength
Spatial pixels
Keystone error
Signal collection
VD = Detector output signal (V)
= Spectral Radiance (mW/cm2/sr/um)
= Transmission of optics at wavelength λ
= Transmission of filter at wavelength λ
f# = f-number of the lens assembly.
Rλ = Responsivity (Volt/µJ/cm2)
Tint = Integration time (ms)
λ1, λ2 = Lower and upper bound of wavelength respectively
(um)
Hyperspectral Imagers -
SAC
Airborne imaging spectrometer
4 m spatial resolution @ 6 km
Parameters Value Units
IFOV 0.66 mrad
FOV 14.5 deg.
No. of bands 143
Spectral range 450 - 900 nm
Spectral width 3 nm
SNR 256
Quantization 10 bit
Used for aircraft
flights 1996 onwards
Airborne hyperspectral imager
4.8 m spatial resolution @ 6 km
Parameters Value Units
IFOV 0.8 mrad
FOV 11 deg.
No. of bands 512
Spectral range 465 – 995 nm
Spectral width 10 nm
Quantization 12 bit
Compact wedge filter
based spectrometer
HySI(Chandrayaan-1, IMS-1 and Youthsat)
64 bands in Chandrayaan-1 and IMS-1;
512 bands in Youthsat
Spectral range : 420 nm to 950 nm
Spectral BW : 13 to 21 nm
Lunar FeO Map by HySI
Thermal infrared imaging spectrometer
Parameters Value Units
IFOV 0.6 mrad
FOV 6.5 deg.
No. of bands 120 (10)
Spectral range 7 – 14 um
Spectral width 60 nm
Quantization 20 bit
Sagnac FTS – lab model
INTERFEROGRAM
SPECTRUM
Upcoming missions
GISAT Hysis Chandrayaan-
2
Resourcesat-3
Spectral range VNIR, SWIR
0.4 – 2.5 um
VNIR, SWIR
0.4 – 2.5 um
NIR to MWIR
0.8 – 5.0 um
VNIR
0.4 – 0.9 um
Spatial
sampling (m)
350 30 80 240
Swath (km) 160 30 20 900
Spectral
sampling (nm)
10 10 20 10
Dispersion
technique
Convex grating
Planned
launch
Q4-2019 Q4-2018 Q1-2019 Q4-2019
Dyson and Offner SpectrometersConcentric optical systems with unit magnification such as the Offner and
Dyson forms are well suited for the design of pushbroom imaging
spectrometers due to low aberrations
Offner Spectrometer
Dyson Spectrometer with a) Concave Grating, b) Convex Grating
IIRS (Chandrayaan-2)
Parameter Specifications
GSD (m)* 80 x 80
Swath (km) 20
Spectral Range (µm) 0.8 to 5
Spectral Resolution (nm) 17
SNR 500
No. of spectral bands 250
* after2x2 binned on ground
Applications:
To map lunar minerals on high spatial and spectral resolution to
understand the lunar evolution
To detect signatures of hydroxyl (OH) and water (H2O) molecules
Detection of Ice signatures on/within the faintly illuminated polar
craters
Thermal inertia mapping of lunar surface using night & day time
temperature differences.
Potential of Hyperspectral
Remote-sensing
Sounding
Vegetation health monitoringRed-Edge: 690-740nm
Red-Edge position (λ) and value (ρ): Sensitive to chlorophyll
content
Camouflage detection
Camouflage
- Spectra
Vegetation -
Spectra
Same in
Visible
Differ in
IR
Research areas
Fore optics
Spectrometer optics
Detectors
Electronics
Onboard processing
Data compression
Atmospheric correction
Classification
And many more…
Chlorophyll Molecule
Minimum Energy need
to stimulate
photosynthesis
Ring Structure (Red
Color) in the Head of the
Molecule absorbs light
SAC KSS05/12/2016
H2O, CO2 absorption line
2.74 um 6.27 um 2.66 um
7.52 um 4.26 um 14.98 um
SAC KSS05/12/2016
Young’s double slitNormal incidence
Path difference = d sin θn,λ
θn,λ
θn,λ
screen
Constructive
interference (maxima)
for path difference = n
λ
d sin θn,λ = n λ
For Constructive
Interference
d
d = distance between
the two slits
θn,λ = diffraction angle of
nth maxima
n = diffraction order of
nth maxima
λ = wavelengthλ
λ
SAC KSS05/12/2016
Young’s double slitOblique incidence
Constructive
interference (maxima)
for total path difference
= n λ
d sin θn,λ + d sin i + = n λ
For Constructive
Interference
i= incident angle
d= distance between the
two slits
θn,λ = diffraction angle of
nth maxima
n= diffraction order of nth
maxima
λ = wavelength
Path difference = d sin θn,λ
θn,λ
θn,λ
screen
d
λ
λ
i
i
i
Path
differenc
e before
slit = d
sin i
SAC KSS05/12/2016
Young’s double slitInterference Pattern
SAC KSS05/12/2016
Wedge Filter is also known asLinear Variable Filter (LVF)
In this spectral transmissionvary along the length of the filterdue to continuous variablethickness.
White light input
Color separated light input
Spectral transmission at various locations
Wedge filter
Convex grating
(Pitch 50 um)
Plane grating
(Pitch 22 um)
Diamond Turning-Blazed Grating Fabrication
Convex Gratings fabricated using Single Point Diamond Turning
A very sharp diamond tool with the cutting edge of an appropriate
shape
is used for ruling or fly-cutting of the structure
Grey Scale Electron Beam Lithography for Blazed Grating Fabrication
A high-efficiency blazed grating can be formed in a single process
by control of
an electron dose for sub-micrometer patterning
Grating fabricated
using E-Beam
Lithography
Dyson and Offner SpectrometersConcentric optical systems with unit magnification such as the Offner and
Dyson forms are well suited for the design of pushbroom imaging
spectrometers due to low aberrations
Offner Spectrometer
Dyson Spectrometer with a) Concave Grating, b) Convex Grating
IIRS (Chandrayaan-2)
Parameter Specifications
GSD (m)* 80 x 80
Swath (km) 20
Spectral Range (µm) 0.8 to 5
Spectral Resolution (nm) 17
SNR 500
No. of spectral bands 250
* after2x2 binned on ground
Applications:
To map lunar minerals on high spatial and spectral resolution to
understand the lunar evolution
To detect signatures of hydroxyl (OH) and water (H2O) molecules
Detection of Ice signatures on/within the faintly illuminated polar
craters
Thermal inertia mapping of lunar surface using night & day time
temperature differences.
Fourier Transform Interferometers principle
SAC KSS05/12/2016
05/12/2016 SAC KSS
Resolvability
Prism B dn/d B = base
Grating mN N = no. of
grooves
Etalon 2dR/(1-R) R = reflectivity
Michelson 4d/ d = total
distance
GISAT-VNIR and SWIR HySI
HyS-VNIR HyS-SWIR
Spectral
channels
158 256
Spectral
coverage (µm)
0.375 to 1.0 0.9 to 2.5
Spectral
Sampling (nm)
4.25 6.25
IGFOV (m) from
35786 km (GEO)
318 191
N-S swath (km) 163 191
Radiometry SNR > 400 SNR > 400
Bits 10 10
Convex grating based
SAC KSS05/12/2016
Sr.
No.Parameter HySI-VNIR HySI-SWIR
1GSD (m) from
630 km30 30
2Swath (km)
from 630 km30 30
3Wavelength
Range (nm)400-950 900-2500
4Spectral
Bands70 256
5
Spectral
Resolution
(nm)
8.8 6.25
6 SNR > 400 > 150
HYSIS payloads-VNIR and SWIR Spectrometers
SAC KSS05/12/2016
IIRS (Chandrayaan-2)
Parameter Specifications
GSD (m)* 80 x 80
Swath (km) 20
Spectral Range (µm) 0.8 to 5
Spectral Resolution (nm) 17
SNR 500
No. of spectral bands 250
* after2x2 binned on ground
Applications:
To map lunar minerals on high spatial and spectral resolution to
understand the lunar evolution
To detect signatures of hydroxyl (OH) and water (H2O) molecules
Detection of Ice signatures on/within the faintly illuminated polar
craters
Thermal inertia mapping of lunar surface using night & day time
temperature differences.
Youthsat LiVHySI – airglow studies
SAC KSS05/12/2016
2 2.5 3 3.5 4 4.5
5
Na
Fe
Rocket Plume Study
SAC KSS05/12/2016