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…

THANK YOUduttamoumita@sac.isro.gov.in

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