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Spectral Imaging at Heriot Watt University. Dr Andy R Harvey School of Engineering and Physical Sciences Heriot Watt University Edinurgh, EH14 4AS Tel +0131 451 3356 [email protected] Some Heriot Watt spectral imaging solutions.

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Spectral imaging at heriot watt university l.jpg

Spectral Imaging at Heriot Watt University

Dr Andy R Harvey

School of Engineering and Physical Sciences

Heriot Watt University

Edinurgh, EH14 4AS

Tel +0131 451 3356

[email protected]


Some heriot watt spectral imaging solutions l.jpg

Some Heriot Watt spectral imaging solutions

  • Birefringent 2D Fourier-transform imaging spectrometer (FTIS)

  • Snapshot 2D foveal imaging spectrometer (OFIS)

  • Snapshot 2D imaging spectrometer (IRIS)


Birefringent f ourier t ransform i maging s pectrometer l.jpg

BirefringentFourierTransformImaging Spectrometer

Scanning mirror

Fixed

mirror

Detector array

  • Conventional FTIS offers

    • High SNR in low flux

      • MWIR, twilight

    • Very high spectral resolution

    • Wide spectral range

  • But conventional time-sequential interferometry in real-world applications is highly problematic


Birefringent ftis l.jpg

Birefringent FTIS

  • Mechanical sensitivity of conventional FTIS makes real-world applications almost impossible

  • Introduce temporal path difference with scanning Wollaston prisms

  • Inherently vibration insensitive since path difference due by birefringence within a single crystal and common path

  • Optical gearing reduces required accuracy of movement by a factor ~200


Slide6 l.jpg

Colour image

Movie of spectral image cube


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Foveal hyperspectral imaging in 2D

OpticalFibre-coupledImaging Spectrometer

  • Real-time hyperspectral imaging in 2D would require excessive information throughput

    • GVoxel/sec

  • Bottlenecks include

    • detector – 20 MVoxel/sec

    • Computer processing

  • Biological systems with this problem employ a scanning fovea….


Foveal hyperspectral imager ofis schematic l.jpg

Foveal hyperspectral imager: OFIS Schematic


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OFIS: Hardware & raw data

First fibre

Last fibre

Spatial extent

400 nm

Wavelength

  • Raw image at CCD prior to reformatting

700 nm

  • The hyperspectral fovea assembly:

  • Custom fibre optic image refromatter

  • 1D dispersive hyperspectral imager

  • CCD camera


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OFIS: Movie demonstrating real-time spectral ID with simple recognition

  • Colour image


Snapshot spectral imaging in 2d l.jpg

Snapshot spectral imaging in 2D

ImageReplicationImaging Spectrometer


I mage r eplication i maging s pectrometer iris l.jpg

Image Replication Imaging Spectrometer:IRIS

F

F

F

F

F

F

F

F

F

  • Single image multiplexed onto 2Npassband images

  • ‘100%’ optical efficiency

  • Snapshot image

    • no temporal misregistration

  • Trade spectral resolution for FoV

    • Low resolution, wide FoV

    • High resolution, small FoV

    • Gas detection

      • High spectral resolution

      • Few Bands

      • Modest FoV

  • Conceptually related to Lyot filter

  • World’s only snapshot, 2D spectral imager (almost !)

Large format

detector

Spectral

Demultiplexor


Slide13 l.jpg

IRIS snapshot spectral imager:

  • Wollaston prism polarisers replicate images

  • Each Wollaston prism-waveplate pair provides both cos2 and sin2 responses

  • All possible products of spectral responses are formed at detector


Components assembly l.jpg

Components & Assembly

  • 8 channel system

  • 3 Quartz retarders

  • 3 Calcite Wollaston prisms


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Absolute total transmission

Absolute response curves in polarised light

50

Response (%)

25

0

  • Bandpass filter & polariser dominate losses

    • Improved system: T>80%

  • Theoretical throughput is 2n times higher than for other techniques!

  • Demonstrated 96% transmission for IRIS-only components


Slide16 l.jpg

An example medical application:Blood oxymetry in the retina


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Requirements for a snapshot technique: retinal imaging

PC15

  • Improved calibration

  • Patient patience

  • Remove misregistration artefacts; imperfect coregistration arises due to

    • Distortion of eye ball with pulse

    • Variations in imaging distortion between images

  • Similar issues with other in vivo applications

    • Imaging epithelial cancers


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Blood oximetry

80

40

  • Optimal spectral band for retinal oximetry

    • Vessel thickness ~ optical depth

    • 570-615 nm

    • Eight bands approximately equally spaced


Spectral retinal imaging l.jpg

Spectral Retinal Imaging

Canon

CR4-45NM

  • Difficult imaging conditions render application of traditional HSI techniques problematic

  • IRIS enables real-time and snapshot spectral imaging


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Video sequence recorded with low-power, CW tungsten illumination


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Retinal image recorded with flash illumination


Coregistered and pca images l.jpg

574

581

592

585

607

595

603

613

Coregistered and PCA images

PC1 & PC2

PC2

PC1


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Application to microscopy:Imaging of multiple fluorophors

  • IRIS fitted to conventional epi-fluorescence microscope

  • Germinating spores of Neurospora crassa stained with

    • GFP – nucleii fluoresce at 510 nm

    • FM4-64 – membranes fluoresce at >580 nm

50

Response (%)

25

0


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MWIR IRIS

  • Consists of:

    • COTS Phoenix MWIR Camera

    • Specac Polariser

    • IRIS II Optical Telescope

Hyperspectral Working Group


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Conclusions

  • The transfer of spectral imaging from scientific to military and laboratory applications must address the needs of high SNR, accurate coregistration and logistics.

  • No single technique can satisfy all requirements simultaneously

    • ‘Horses for courses’

  • New techniques such as described here illustrate how these requirements can be satisfied

  • Similar issues occur in both military and civilian (eg medical) applications introducing significant scope for dual use.


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Additional information

Linked by previous slide buttons


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The co-registration problem

  • Co-registration required for time sequential direct and FT imaging

    • Not for snapshot/fully-staring

  • Misregistration of spectral images distorts spectral basis sets

  • Video spectrum frame rates insufficient to freeze motion from most aerial platforms

Target


The magnitude of the co registration problem l.jpg

The magnitude of the co-registration problem

  • Co-registration should be better than 1/20 - 1/50 of a pixel

  • Deployment of time sequential DIS and FTIS will normally require ‘step and track’


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Bandpass functions

  • Bandpass are overlapping bell shapes

  • Can be optimised by adjusting waveplate thickness and dispersion


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Spectral discrimination

Contiguous ‘top-hat’

  • Bell-shaped IRIS transmission functions tend to smooth spectra

    • Typically 6% reduction in separation in 8D spectral space

    • 8x improvement in SNR

IRIS


Slide31 l.jpg

Summary and novel HWU techniques in red

Direct Imaging Spectrometry

(Fourier) Transform Imaging Spectrometry

Scanning mirror

Ns

Fixed

mirror

ND(t)

FT

Nl(t)

Ny

Nx

Ny

Ny

Detector array

Nx

Nx

Ns

Nl

ND

FT

FT

Ny(t)

Ny(t)

Ny(t)

Nx

Nx

Nx

1D image x

path difference D

  • Mature

  • The traditional technique for 2D static spectral imaging

  • Low MPLX efficiency

  • Very high spectral resolution

  • Highest SNR in low-light conditions

  • The optimum technique for MWIR

  • Unsuitable for poorly controlled environments...

    • FTIS

Temporally scanned

  • No temporal coregistration problem

  • The traditional technique for 1D remote sensing

  • 2D very immature….

    • IRIS

    • OFIS

Snapshot/fully staring


Ratio of snrs in 3 5 m m band temporal scan l.jpg

Ratio of SNRs in 3-5 mm band -temporal scan

40 Hz,

10 bands

1 Hz,

10 bands

Zero range

1500 m nadir path


Ratio of snrs in 8 14 m m band temporal scan l.jpg

Ratio of SNRs in 8-14 mm band - temporal scan

40 Hz, 10 bands

1500 m nadir path

Zero range


Iris ftis snr l.jpg

IRIS:FTIS SNR

40 Hz, 10 bands

Zero range

1500 m nadir path


Lyot filter principle of operation l.jpg

Lyot filter: principle of operation

Waveplate

Polariser


Optical scaling laws l.jpg

Optical scaling laws

Polariser, retarders & Wollaston prisms

(index matched)

Field stop

Camera

Bandpass

filter

Imaging

lens

Collimating lens

Primary lens

Hamamatsu

ORCA-ER

Outputs:

Field stop size

Collimating lens rear element diameter

Splitting angles, apertures & depths of prisms

Apertures of retarders, polarisers and filters

Imaging lens focal length & front element diameter

Inputs:

FoV

Sub image size on CCD

CCD pixel size

Primary lens magnification & F#

Collimating lens back focal distance, focal length & front element diameter

Prism birefringence


Spectral retinal imaging37 l.jpg

Spectral retinal Imaging

Diabetic Retina

Normal Retina

  • By 2020 there will be 200 million visually-impaired people world wide

    • Glaucoma, diabetic retinopathy, ARMD

    • 80% of those cases are preventable or treatable

      • Screening and early detection are crucial

  • Spectral imaging provides a non-invasive route to monitoring retinal biochemistry

    • Blood oximetry, lipofuscin accumulation


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Measured & predicted spectral responses


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Imaging Concepts Group

  • Funders/Collaborators

    • AstraZeneca

    • AWE

    • BAE Systems

    • DSTL

    • EPSRC

    • NATO

    • NPL

    • QinetiQ

    • Royal Society

    • Scottish Enterprise

    • South Glos. NHST

    • SAAB

    • Thales

  • Research Group

    • Head

      • Dr Andy Harvey

    • PDRA

      • Dr Colin Fraser

      • Dr Eirini Theofanidou

      • Bertrand Lucotte

    • PhD Students

      • Alistair Goreman

      • Asloob Mudassar

      • Gonzalo Muyo

      • Sonny Ramachandran

      • Ied Abboud

      • Beatrice Graffula

    • External PhD students

      • Ruth Montgomery (NPL)

      • Robert Stead (Thales)


Research areas l.jpg

Research areas

  • Imaging Concepts Group

    • Spectral imaging

    • Retinal imaging

    • Wavefront coding

    • Aperture synthesis imaging (optical and mm-wave)

    • Optical encryption for communications

    • mm-wave imaging

    • Biophotonics

    • Insect flight dynamics


Overview l.jpg

Overview

  • Introduction to spectral imaging

  • Spectral imaging techniques at Heriot-Watt University

    • FTIS

      • Inherently robust FT imaging spectrometer

    • IRIS

      • Snapshot, ‘100%’ optical throughput imaging spectrometer

    • OFIS

      • Foveal hyperspectral imaging spectrometer

  • An example application

    • Spectral imaging of the retina

  • Conclusions


What are the issues l.jpg

What are the issues

  • High SNR required

    • >100

      • No spatial or spectral multiplexing desirable

      • Fourier-transform

        • in some conditions

  • Accurate coregistration required (<1/20 pixel)

    • Snapshot spectral imaging preferred

  • Spectral resolving power matched to requirement

    • 100s for data acquisition

    • ~10 for many applications

      • As few as two if clutter allows (eg spectral lines)

  • Detector is ‘information bottleneck’

    • 20 MVoxel/second per tap


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