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Spectral Imaging at Heriot Watt University

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 a.r.harvey@hw.ac.uk. Some Heriot Watt spectral imaging solutions.

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Spectral Imaging at Heriot Watt University

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  1. 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 a.r.harvey@hw.ac.uk

  2. Some Heriot Watt spectral imaging solutions • Birefringent 2D Fourier-transform imaging spectrometer (FTIS) • Snapshot 2D foveal imaging spectrometer (OFIS) • Snapshot 2D imaging spectrometer (IRIS)

  3. 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

  4. 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

  5. Colour image Movie of spectral image cube

  6. 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….

  7. Foveal hyperspectral imager: OFIS Schematic

  8. 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

  9. OFIS: Movie demonstrating real-time spectral ID with simple recognition • Colour image

  10. Snapshot spectral imaging in 2D ImageReplicationImaging Spectrometer

  11. 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

  12. 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

  13. Components & Assembly • 8 channel system • 3 Quartz retarders • 3 Calcite Wollaston prisms

  14. 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

  15. An example medical application:Blood oxymetry in the retina

  16. 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

  17. Blood oximetry 80 40 • Optimal spectral band for retinal oximetry • Vessel thickness ~ optical depth • 570-615 nm • Eight bands approximately equally spaced

  18. Spectral Retinal Imaging Canon CR4-45NM • Difficult imaging conditions render application of traditional HSI techniques problematic • IRIS enables real-time and snapshot spectral imaging

  19. Video sequence recorded with low-power, CW tungsten illumination

  20. Retinal image recorded with flash illumination

  21. 574 581 592 585 607 595 603 613 Coregistered and PCA images PC1 & PC2 PC2 PC1

  22. 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

  23. MWIR IRIS • Consists of: • COTS Phoenix MWIR Camera • Specac Polariser • IRIS II Optical Telescope Hyperspectral Working Group

  24. 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.

  25. Additional information Linked by previous slide buttons

  26. 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

  27. 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’

  28. Bandpass functions • Bandpass are overlapping bell shapes • Can be optimised by adjusting waveplate thickness and dispersion

  29. 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

  30. 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

  31. Ratio of SNRs in 3-5 mm band -temporal scan 40 Hz, 10 bands 1 Hz, 10 bands Zero range 1500 m nadir path

  32. Ratio of SNRs in 8-14 mm band - temporal scan 40 Hz, 10 bands 1500 m nadir path Zero range

  33. IRIS:FTIS SNR 40 Hz, 10 bands Zero range 1500 m nadir path

  34. Lyot filter: principle of operation Waveplate Polariser

  35. 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

  36. 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

  37. Measured & predicted spectral responses

  38. 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)

  39. 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

  40. 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

  41. 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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